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Jazzer The Cat 08-24-2009 09:40 AM

Choosing a new tire
I have been wanting to do this post for quite some time, as these are common questions and come up quite often when someone wants new shoes on his/her ride. There are many different issues to consider when purchasing new tires. Many posts begin with "I have an 'XX Mustang and need new tires, which ones should I buy?" Not an unreasonable question, but just doesn't provide enough information.

Tires should be chosen by the drivers needs, but decided by the budget!

What kind of driving do I do?
Is this a daily driver (DD) car just used to go back/forth to work and around town? Do you drive in wet weather often or very cold weather with some snow fall? Do you like a bit more spirited driving and enjoy fast accelleration from a stop? Do you like to head out to the coast and enjoy a nice tip-toe through the twisties?

What is my budget?
This is a huge variable and cannot be decided by anyone else. You can certainly shop around for good prices and will find them at site sponsors such as the main page here in Tires & Wheels section. The amount of money you wish to spend will, in some cases, limit your choices as to new tires. If you desire higher performance on the street or track, you will likely need to spend more than you did on your previous set of tires. You know... the ones showing small pieces of steel thread poking out the sides currently? On to the decision process.....

I have often said that tires are the single most important item keeping ones car on the road, next to the driver. Without proper traction, you cannot maintain control of your car reguardless of the conditions. For a wonderful place to start, head over to...

WAIT!.... I also need wheels because mine are too skinny :(

Welcome to the wonderful world of not only choosing new tires, but which style of wheel to choose. What finish do I want? What width will fit on my car? What diameter should I get? And what in the world is this off-set and back-space stuff I keep seeing? Good golly miss Molly.... I JUST WANT NEW WHEELS!!! You thought you were gonna get off easy, didn't ya?:icon_duck:

How wide can I go?*
'79~'93 Mustang (Fox Body):
Up to 9" wheel all the way around and recommend a 17" in most cases. A 245/45 is a good fit and will have lots of choices for tires. There may be a slight bit of rubbing on fender liner, but nothing major. There is a possibility of rubbing on quad-shocks out back, but have two options. It can be rotated 180 or just removed. If removed, recommend upgrade LCA's to prevent wheel hop.

'94~'98 Mustang (SN95):
Up to 9" in front and 10.5" out back. You have more decisions to consider here as additional fenderwell clearance affords you some options in both width and diameter of wheel. There is a possibility of rubbing on quad-shocks out back, but have two options. It can be rotated 180 or just removed. If removed, recommend upgrade LCA's to prevent wheel hop.

'99-'04 Mustang (New Edge, although technically an SN95)(non-Cobra):
Up to 9" in front and 10.5" out back. You have just a bit more fenderwell clearance than the '94~'98 years, so a slightly larger tire will fit up front.There is a possibility of rubbing on quad-shocks out back, but have two options. It can be rotated 180 or just removed. If removed, recommend upgrade LCA's to prevent wheel hop.

'99-'04 Mustang (New Edge)(IRS Cobra):
Up to 9" in front and 10.5" out back, but have some clearance issues with a bolt on the rear suspension on a 17" wheel. I have been told directly that an 18" wheel will clear this IRS bolt and fit fine, but you should do some research to confirm exact wheel you choose.

'05~'09 Mustang (S197):
Up to 9.5" in front and 10" out back. I have seen this thread by a machinist that widened a set of +24mm 9"x18" wheels to 12". Some minor mods on a 335/30/18" tire and a bit of rubbing with a 1/4" spacer. I think this is REALLY pushing it and would recommend you stick with something smaller. You have a lot more fenderwell clearance in diameter than the '99~'04 years, so a 20" wheel will fit fine.

I ain't no vegetarian... I want FAT MEATS!: OK, the first thing you need to do is confirm the clearance of your fenders. The fronts are going to be tough to figure out and just way too many variables to address here, but the backs are pretty easy (relatively speaking!). You need to park your car at a 45 angle up a steep incline so that the rear axle is articulated as far as possible. This is when one wheel is way up in the fender and the other is way out of the fender. It would be best if the one side is touching the bump stop so you know it is maxed out. Now measure the physical clearance between the tire and EVERYHING that is along its path as it moves up into the fender (measure these clearances on BOTH SIDES OF THE CAR) This includes e-brake cables, suspension components, nuts/bolts, exhaust and such. You will likely need to drive it back down a bit to check clearance as the tire passes the fender lip. Take all the measurements and do the math as to how much wider/taller you can go with wheel/tire compared to the existing ones.

Let's say you have 1" clearance along the back side of tire before it would run into ANYTHING and 1" clearance between the tire and fender lip as it passes by. Now let's say, you have 2" clearance between the top of tire and the top/inside of fender before it would rub. So.... if you currently have an 8" wheel with a tire that is 25" diameter you could, in theory, run a 10" wheel with the same off-set and a 29" tire and not rub. I say theory, because the top/inside of the fender is rounded and if you go 1" wider you will LOSE additional clearance due to this arc and will not allow the addition of a 4" larger diameter tire... make sense? Now, this is NOT an exact science, by any stretch of the imagination and some fender work would be needed to avoid rubbing because that new wheel/tire consumes EXACTLY the remaining room you measured for clearance. If some rubbing occurs, or is LIKELY to occur, one can do a bit of hammering up inside the well and roll the fenders. I will not get into rolling the fenders accept to say that an S197 has them already done from the factory and the SN95 does not. This rolling will allow sugnificant clearance and avoid tire damage, so do some research if you plan to go big! It is very important that one address both off-set and back-space when installing aftemarket tires/wheels and especially if going wider, so keep reading :D

Off-set*: This is the location of the mounting flange relative to the inside/outside of the wheel. This measurement is going to given in milimeters and will be +xxmm all of the time on an above year Mustang wheel option. The off-set is going to be VERY important when looking into these wider sizes and generally need to install wheels meant for your model year. In some cases, you can install other model year, but may not give you the "look" you desire. The following image is credited to RS Racing wheel tech page:

Back-space*: This is another way of measuring the off-set of a given wheel and is MUCH easier to determine for defining clearance of a given wheel. The following conversion chart is again from RS Racing wheel tech page:

* there are additional issues to address for proper wheel fitment and usually don't come into play unless you have modified/upgraded your brakes or changed spindles. Larger rotors and brake calipers can effect how and if certain wheels will fit your ride and spindles can effect bolt pattern and off-set/back-space. If you have done suspension modifications, they may also play a part in fitment. If you have such upgrades, post them as a new thread so you can get specific info as to which wheels may/may not fit your ride.

OK, hopefully gave some better understanding of wheels and how they fit. We are now moving onto tires and what all that stuff is about :D

UTQG: Universal Tire Quality Grade is a tool to help one understand the life expectancy of a given tire. Of course, there are LOTS of variables, so it's far from perfect, but a nice guide none the less. VERY generally speaking, the higher this number, the harder the tire. The harder the tire, the higher the expected mileage. The higher the mileage, the less Summer traction you will have. The lower this number, the reverse of the above. Again, VERY general statement, but will help you to understand something about fundamental differences between different tires.

Tread pattern: Thousands of variations and just about every manufacturer offers something for a given road condition. Some are designed to limit noise while others are designed to dispurse water and tend to be quiet comfortable ride and usually a very high UTQG.
Goodyear Assurance All-Season radial 620 UTQG:

A tire that will typically offer improved performance for more spirited driving year-round, is a High Performance All-Season tire. This will often have less tread blocks and offer a bit more noise due to a slightly stiffer sidewall and more contact area of tire to the road surface. Note the "V" pattern to help dispurse water and many tread blocks have small slices rather than complete separations to improve contact patch and reduce tread squirm.
Goodyear Eagle F1 All-Season 420 UTQG:

For much more Summer performance driving, you get into even less tread blocks and softer compounds to grip the road better such as High or Extreme Performance Summer tires. VERY few tread blocks, by comparison, and softer compound for improved performance required by sports cars and very spirited driving conditions. The sidewall is going to be stiffer on this one to improve turn-in response and support what is usually a lower profile tire.
Direzza Sport Z1 Star Spec 200 UTQG:

Track and competition DOT rated tires are really designed for Summer only and do not do well, or are downright dangerous, in wet conditions (freezing conditions can destroy them!). These are often a mono-block design (semi-slick) or have no real pattern at all! Note the continuous thread that is actually a solid tire with some grooved carved, but not separate blocks. This offers more structure to the tire as the compound is VERY soft and would be less stable if tread was deeper or made up of many blocks. This is a vast improvement in contact patch and offers improved braking as well. The sidewall is going to be very stiff on this tire to help hold its shape under the higher lateral loads of extreme driving conditions. Expect them to be QUITE noisy and figure these will sound like a low-flying crop duster plane while coming to a stop, such as my R888's (not kidding!)
Advan A048 60 UTQG:

Here we have a tire that would be a bad choice for ANYTHING but the track and certainly a horrible tire in wet conditions on the street. As about a stiff a sidewall as one will find and just short of that solid rubber tire found on the little red wagon of your youth!
Hoosier A6 40 UTQG:

The above two tires are not only VERY soft compound and would offer limited mileage, but will have less tread depth from the start. Most passenger car tires are around 11/32" tread depth while the last two are typically about 6/32" (racers will have them shaved to 4/32" or less!). This is not only a much more expensive tire to purchase, it will offer a lot less mileage due to nearly half as much tread depth. I recommend if one wants the extreme performance offered by the last two catagories, you purchase a second set of wheels and have them for Summer ONLY or track ONLY use. These tires can also be heat-cycled to improve performance and insure proper wear.

Drag radials: The last of our tire catagories is Drag Radials (DR's). These are also VERY soft, even more so than the last two were, but have a fundamental difference that makes them less desirable for street use. These will have a very soft sidewall, rather than a stiff one so can offer poor support and greatly reduced traction while cornering. This soft sidewall offers very little support during a lateral load, so can be unsafe at speed. When you turn a corner in one of these baby's, you will be less stable due to tall sidewall that has limited structural rigidity. On the other hand, this tall/soft sidewall will perform better on the 1/4 mile. The soft sidewall allows the tire to attain the "wrinkle" one sees on 1/4 mile launch cars. This wrinkle allows energy to be stored in the tires so they will, in effect, sling-shot the car off the line. This flexable sidewall helps keep the tires from breaking traction and aids them in restoring traction more quickly. These should ONLY be purchased for a second set of wheels, usually a 15" on a Mustang, and should be installed ON SITE of a speed event. This allows the highest sidewall on a reasonable overall diameter tire for clearance. That is about the depth of my knowledge on them, so maybe missed some things.
Micky Thompson ET Drag:

Those three numbers I keep seeing: When you see 255/45/17" on a tire, you have the following information:
255 = The width of the tire given in milimeters. Divide by 25.4 to convert to inches.
45 = This is the aspect ratio or profile hight. This is the percentage of hight vs. width of tire.
17" = This determines the size of wheel in which the tire will fit. Will NOT fit any other wheel diameter.
This information will give you the overall diameter of the tire and important for a couple reasons and discussed below. See Performance Probe for some additional info on the breakdown of these numbers.

Tire width: This is the first number and will determine the width of wheel required to run a given tire. Each tire will have a range of wheel widths approved for a given tire width. Example: A 255/45/17" tire size is made by many manufacturers and just about all of them will require an 8"~9.5" wheel to fit properly and under warranty. This is not always true, so will need to research a specific tire you want to run, as there are some exceptions.

Profile hight: This is the middle number (AKA series) and can be very important in the determination of which tire to purchase when going with a wider wheel:
245/45/17" tire = 25.7"
245/50/17" tire = 26.8"
Both of these tires are approved for any 8"x17" wheel, but the 50 series is just over 1" larger diameter.

Wheel diameter: This is the third number and determines the wheel diameter required. A 255/50/17" tire cannot be run on anything other than a 17" wheel, period!

Overall diameter: This is the sum-total of the three numbers and determines the hight of the tire. If you buy aftermarket wheels that are larger/smaller diameter or wider than OEM, you must address this issue or may have some clearance and speedometer accuracy issues. This does not necessarily mean a wider/larger diameter wheel will have rubbing or speedometer issues. Here are some extreme examples:
195/65/15" = 25"
255/35/18" = 25"
The second tire is not only over 2" wider, but requires a 3" larger diameter wheel and is the exact same overall diameter tire. These tires will have the identical gap as seen between the fender and tire and the speedometer will not be effected. This does NOT mean you can just exchage these sizes and not rub, as you have width considerations to make, but best to run OEM diameter tires. See How wide can I go? above.

Profile hight vs. wheel diameter: In order to retain proper clearance and avoid rubbing, if you go larger diameter wheel, you need to go lower profile tires. This is not a 100% hard/fast rule, but don't recommend much larger diameter than stock and discussed in more detail in Speedometer accuracy below.

Profile hight vs. ride quality: It is safe to say lower profile tires will ride harsher than higher profile tires. Because the sidewall hight is shorter and less "cushion" between the wheel itself and the road surface, a lower profile tire will have a stiffer sidewall. This stiff sidewalls are designed for stability under more aggressive driving and offer less flex upon changing of directions in a corner. They also offer less protection for the wheel upon sharp bumps and generally require higher air pressures to offer the best traction.

Ride stiffness and lower profile tires: The profile hight will be a factor in ride comfort and likelyhood of road noise in the cabin. Because this tire has less distance between the road surface and the wheel itself, it is going to have a stiffer sidewall. This is a relative term and not an exact science, but suffice to say the following. A 40 series tire will generally be "harder" than a 50 series and offer a bit harsher ride. If you go way down to a 20 series, for example, you will have a vast difference between this and what was probably nearer a 45 or 50 series OEM tire profile. There are some benifits to a lower profile tire and cornering response is one to consider. Be sure to address this ride comfort when purchasing new tires/wheels as this can make you HATE your new tires!

Fitting tires to wheels: All too common is the desire for a wider tire, but no budget to purchase the proper size wheels in which to run them. For a vast majority, this never comes into play as they just go down to the tire shoppe and ask for new tires. The person there may ask a few questions, but will ONLY install an approved tire for the application of the vehicle in question. If you are reading this thread, you desire greater input as to which tires you want based on your needs as a driver, so this will not do. When looking to purchase new tires, be sure to read the specification info so that you purchase tires designed to fit the wheels you have on your car.

Oversized: A wider tire will not always offer improved handling/traction and depends on proper fit to wheel itself. A tire usually performs best when the sidwalls are straight down from the wheel. This keeps the surface area of the tire, AKA contact patch, flat on the road surface and as the tire was designed. If one were to install a 275/40, for example, on an 8" wheel, you would have a bit of bulge on the sidewalls. This bulge creates some sidewall flex when cornering as the wheel is undersized and will "float", for lack of a better term, side to side within the tire under lateral loads. So when you go around a turn, the tire will stay on the road surface and the sidewall will flex more than designed. This flex will allow the wheel to somewhat "roll over" the sidewall and the tire will under perform when driven hard around a turn. The surface area in which the tire bead meets the wheel to retain air pressure and seal, can be reduced and leaks are more likely as well. The taller the tire in profile measurement, the somewhat greater variations of wheel widths in which it can be installed. The above has lots of variables and is a general statement. Not everything mentioned will happen in every case, but more likely than not.

Undersized: An undersized tire on a wheel will also under perform, but only when in the more extreme. This issue is very rare, as the look is not a popular one. Let's take a 235/40 tire on a 10" wheel, for example. The contact patch will now be farther from the side and less than the width of wheel. Under lateral loads of a corner, this can push the car over onto the edge of tire and there is no tread there to achieve grip. This will accelerate tire wear and not perform as well as the proper size. In addition, if the sidewalls are pulled too tight, they get strained and can cause tire failure as the structure of the tire can be compromised. Running a thinner tire will tend to make your car a bit more "darty" and especially if you were running a proper fitted or wider tire previously. The tighter sidewall will offer less flex and react more quickly to turning of the wheels. Something to think about as you may appear to have had a couple glasses of wine for dinner to the local law enforcement heading home after a late dinner on a Saturday night. This can also make your wheels a bit more vulnerable to damage via curbs from the side and bumps from below. The surface area in which the tire bead meets the wheel to retain air pressure and seal, can be reduced and leaks are more likely, just like an oversized tire. The above has lots of variables and is a general statement. Not everything mentioned will happen in every case, but more likely than not.

Jazzer The Cat 08-24-2009 09:55 AM

Is wider always better?: As Luke of Tire Rack explains, there are some fundamental changes in grip between different tire sizes mounted on the same size wheel. See ***HERE*** for a nice comparison.

Running staggered wheels: This is where one runs differing widths on front/rear of your car, say 9"s up front and 10"s out back. This can change the characteristics of how your car handles and usually create additional understeer to the Mustang. This is when the steering wheel is turned, but the front tires do not offer enough traction to maintain grip of the road surface. Remember your physics class and a body in motion wants to remain in motion?. The car wants to continue in a straight line as the rear of car has additional grip to that of the front. This can also be referred to as pushing or plowing, but don't be too worried, as this is only experienced under quite aggressive driving conditions and is prefferable to oversteer 90% of the time. A very popular way to go, but offers some things in which to be aware.Tramlining: This refers to the tendency for a tire to follow ruts and grooves in the road surface and can appear to be weaving by the driver. This is just something to think about when going wider wheels/tires up front. For more info, see Tramlining at Tire Rack.

Speedometer accuracy: The speedometer is connected to the drive train on the Mustang. If you install larger/smaller diameter tires, the speedometer accuracy will be effected. For the '79~'98 Mustang, one needs to replace the speedometer gear for the cable and is located inside the transmission. It is an inexpensive and easy fix, but should do some research as to how much effect you will experience with a given tire/wheel change. I installed a 1" smaller tire on my '85 GT and registered 5 MPH fast at 70 MPH. The first is a speed calibrator ('99~'04 models only) and requires some wiring and installation. To determine the differential you may have, head over to TRS Speedometer check and see the expected change. The second is via hand-held tuner and any performance or repair shoppe should be able to accomplish this for you. I am sure a Ford dealer could do this as well, but don't know cost for this service. You can also have a slight change in your overall gearing with a change in tire diameter. If you go larger overall diameter tires, you will "technically" have a bit less giddy-up off of the line due to slightly higher gearing. It is VERY unlikely you will even notice, but would not be honest if I did not include this info.

Tire rotation: If you should decide to go with wider wheels out back (staggered) you will not be able to address tire rotation effectively. In some cases you can go from one side to the other, but unable to rotate in the traditional sense. Personally, I have found that my tires wear pretty evenly and never found a great need to rotate my tires. Should you desire the need to rotate, you will must consider the tread pattern or design in which you choose. Head over to Tire Racks Tire Tech Information page for details.

TPMS: Tire Pressure Monitoring System and is OEM on '05+ (S197) Mustangs. This invoves a sensor mounted around the inside of the wheels (underneath the tires) that sends a signal to the driver to notify of a low tire pressure condition. Pepe the Tire Pressure Monitoring Systems thread in this section for more info and detail.

Please support the vendors that bring you this forum:
Tire Rack tires
Tire Rack wheels
Tread Depot

Those that support this site:
Cruizin Concepts
American Muscle wheels

RS Racing Tire Technical Information
RS Racing Wheel Technical Information

I think that will do it. Feel free to post on this page or a new thread in the Tires & Wheels section for some info on your prospective purchase.

Jazzer The Cat :)

WannaBeGearHead 08-25-2009 11:09 PM

Very good write up. I, myself don't know about tires and which to choose so this def. helped.

Can't believe with all the tires questions out there no one has responded to your write up. This deserves a sticky!

Thanks Jazz!

B-lake22 08-27-2009 12:51 PM

WOW!! I never new that so much went into choosing new wheels. I don't know if I am smarter having read all this or just more confused. LOL thanks for all the great info! This will help me a ton!

WannaBeGearHead 09-06-2009 12:54 AM

Bump for people who haven't discovered this thread yet ;)

Jazzer The Cat 09-06-2009 08:58 AM

(pssst.... thanks, but no need to bump, this thread is a "sticky" ;))

I do kinda wish the title was a bit more interesting so pepes would check it before posting a new thread. I am thinking about changing it to one of the following...

"Oprah Winfrey sex tape"

"Free Viagra for first 100 callers"

.... or my personal fave

"College girls pillow fight video with exclusive whipped cream segment"

I will speak to the Mods and see if I can get some help on better advertising.

Jazzer :)

Badgolferguy 09-07-2009 06:06 PM

Tire Size Help
I'm staying with the factory 19" wheels (on 2010 GT) which I've been told are 8.5" wide. I want to put 275x40x19's all around. You think there will be any problems front or back?

Can I put a 1" spacer or adaptor to move the rear wheels out a bit?

Thanks for your input.


disposablehero27 09-07-2009 06:21 PM

nice resource, look forward to using it next year when i get some bigger rims and tires

Jazzer The Cat 09-07-2009 06:39 PM


Originally Posted by Badgolferguy (Post 6477467)
I'm staying with the factory 19" wheels (on 2010 GT) which I've been told are 8.5" wide. I want to put 275x40x19's all around. You think there will be any problems front or back?

Can I put a 1" spacer or adaptor to move the rear wheels out a bit?

Thanks for your input.


A 275/40 is a bit large for an 8.5" wheel, but wouldn't have any rubbing issues and no need to install a spacer. I would recommend a 255 instead, as this is a better fit for said wheel, IMO.

Jazzer :)

Badgolferguy 09-29-2009 11:41 AM

Thanks for the input. I went with the 275x40's all around......they look perfect!

Thanks again.

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