MPGs on my Virginia to Florida trip
10-30-2010, 01:31 PM
#22
6th Gear Member
Join Date: Feb 2007
Location: state of confusion
Posts: 7,635
O.k. I'm going to the mountains tomorrow and this is my question. When I got my car she had this rubber on her 255/55/17s' and now she has 255/45/18s' with a 10x9 rims. What air pressure should I put in her? My mustang shop saids 50 lbs of air where as my place of work saids the same as the door jam saids even if she is a different tire.
So I'll start from what I do know - that the 235/50-18 is door-stickered for 32 psi front and rear.
Your new 255/45-18's are two load ranges bigger, meaning that they don't need quite as much air in them as the OE 235/50-18 in order to be at the same rated load.
Near as I can figure, there's about 3 psi difference between the 235/50-18 and the 255/45-18 for developing the same rated load. In theory, that means you could run the 255/45-18's at 29/29 psi (front/rear) for normal everyday driving that doesn't include much unusually hard cornering. That's based on assuming you'd be generally happy with Ford's specs for their 235/50 tires.
But . . . on this trip you're evidently going to be doing some cornering that's likely to be more than "moderately hard" at times. Generally that means slightly more air in order to stiffen the tire against cornering loads. It also means that you might want to stagger the pressures front to rear a little.
For the enthusiastic-driving part of this trip I'd strongly consider running 32 or 33 psi up front and about 3 psi less in the rear. Less pressure in the rears will tend to dial out some of the understeer, and while doing so give your rear tires more mechanical grip (translation: less sensitive to having road roughness get the back end loose).
Be kind of careful about adding throttle as you exit the corners - you probably won't know where the apex for every corner is, and it's entirely possible to add too much throttle too soon. Ease into the throttle and leave yourself plenty of margin. Corners and blind turns may keep turning further than you expected.
Norm
10-30-2010, 01:45 PM
#23
2nd Gear Member
Join Date: Oct 2010
Location: USA
Posts: 182
Mustangmaster, thats great mpg! My mpgs are taking a dump lately. I have a Bamachips 87 octane tune and at first I was getting 24 mpg as compared to 21 without it. I was thrilled! Then now I'm getting 17mpg driving the same highway route. So you are getting way better gas milage than me for sure lol
10-30-2010, 04:11 PM
#24
3rd Gear Member
Thread Starter
Join Date: Sep 2006
Location: Lewisetta, Virginia
Posts: 788
07 Showstopper, I seem to get the best MPGs when I run the Brenspeed 91 tune running 93 octane ethanol free gas.
Daniel60, when I bought my new Goodyear tires that hold a max PSI of 51, I emailed Goodyear and they said stick with what the door sticker says....32 PSI.
Daniel60, when I bought my new Goodyear tires that hold a max PSI of 51, I emailed Goodyear and they said stick with what the door sticker says....32 PSI.
10-31-2010, 08:57 AM
#26
5th Gear Member
Join Date: Dec 2004
Location: Small walk way by big walk way
Posts: 4,717
Are you sure that the tire size was 255/55-17? Reason I ask is because 235/55-17 is the stock 17" size and 255/50-17 is the usual 17" "upgrade".
So I'll start from what I do know - that the 235/50-18 is door-stickered for 32 psi front and rear.
Your new 255/45-18's are two load ranges bigger, meaning that they don't need quite as much air in them as the OE 235/50-18 in order to be at the same rated load.
Near as I can figure, there's about 3 psi difference between the 235/50-18 and the 255/45-18 for developing the same rated load. In theory, that means you could run the 255/45-18's at 29/29 psi (front/rear) for normal everyday driving that doesn't include much unusually hard cornering. That's based on assuming you'd be generally happy with Ford's specs for their 235/50 tires.
But . . . on this trip you're evidently going to be doing some cornering that's likely to be more than "moderately hard" at times. Generally that means slightly more air in order to stiffen the tire against cornering loads. It also means that you might want to stagger the pressures front to rear a little.
For the enthusiastic-driving part of this trip I'd strongly consider running 32 or 33 psi up front and about 3 psi less in the rear. Less pressure in the rears will tend to dial out some of the understeer, and while doing so give your rear tires more mechanical grip (translation: less sensitive to having road roughness get the back end loose).
Be kind of careful about adding throttle as you exit the corners - you probably won't know where the apex for every corner is, and it's entirely possible to add too much throttle too soon. Ease into the throttle and leave yourself plenty of margin. Corners and blind turns may keep turning further than you expected.
Norm
So I'll start from what I do know - that the 235/50-18 is door-stickered for 32 psi front and rear.
Your new 255/45-18's are two load ranges bigger, meaning that they don't need quite as much air in them as the OE 235/50-18 in order to be at the same rated load.
Near as I can figure, there's about 3 psi difference between the 235/50-18 and the 255/45-18 for developing the same rated load. In theory, that means you could run the 255/45-18's at 29/29 psi (front/rear) for normal everyday driving that doesn't include much unusually hard cornering. That's based on assuming you'd be generally happy with Ford's specs for their 235/50 tires.
But . . . on this trip you're evidently going to be doing some cornering that's likely to be more than "moderately hard" at times. Generally that means slightly more air in order to stiffen the tire against cornering loads. It also means that you might want to stagger the pressures front to rear a little.
For the enthusiastic-driving part of this trip I'd strongly consider running 32 or 33 psi up front and about 3 psi less in the rear. Less pressure in the rears will tend to dial out some of the understeer, and while doing so give your rear tires more mechanical grip (translation: less sensitive to having road roughness get the back end loose).
Be kind of careful about adding throttle as you exit the corners - you probably won't know where the apex for every corner is, and it's entirely possible to add too much throttle too soon. Ease into the throttle and leave yourself plenty of margin. Corners and blind turns may keep turning further than you expected.
Norm
Your right it was 235/55/17s' I forgot.
10-31-2010, 12:05 PM
#28
Not trying to be sarcastic, just honestly am curious how a tranny would be cheaper.
10-31-2010, 12:25 PM
#29
I think your car 'puter lied to you. The only way you can get over 30MPG from a 4.6 with stock gears and overdrive is if you actually took the car out of gear and coasted downhill at over 80 MPH
11-04-2010, 03:31 PM
#30
5th Gear Member
Join Date: Dec 2004
Location: Small walk way by big walk way
Posts: 4,717
I drove to Asheville, N.C from Charleston, South Carolina and couldn't acquire any non-ethanol fuel on the way and back but got decent fuel mileage considering I have 4.10 gears and few bolt-ons.' First Tank I drove 189.3 miles and averaged 21.6 miles per gallon and this includes three or four fly byes at 120 mph on idiots on pissed me off on the interstate. Second Tank I drove 164.8 miles and averaged 21.3 miles per gallon. Third Tank I drove 141.5 miles and average 19.2 in the city and on the Blue Ridge Parkway and the final tankful I got cheap and filled up with 87 octane and drove back on the interstate from Asheville, North Carolina to Charleston, South Carolina and average 22.4 with a few fly byes. Speeds average mostly 75 on the interstate and 30-55 in the city and parkway. Car drove like a race car with strut tower brace and again if you haven't had any suspension parts added to your car this would be the most beneficial in my humble opinion.