Fireguy28

Does anyone know how much actual HP you lose when you add 20x8.5 - 20x10 wheels and tires to the 05? I dont want to look good and not have guts.

MikeVistaBlue06

It isn't the HP you lose. It is torque multiplication that you change. Adding bigger tires is like adding a taller gear to the backend (making the gear number go down), and making it go faster on the very top end, but slower on takeoff.

You can figure this easily by simply taking the old tire diameter and dividing by the new tire diameter.

So if you have 300 ft-lb torque and a 3.55 gear then your RWTQ = 300 * 3.55 = 1065.

So, assuming the tire height is the same for both wheels and the wheel sizes are 18" stock and 20" new, then multiply that 1065 by 18/20 and you have your new torque figure.

1065 * (18/20) = 958

In essence your old 3.55 gear ratio is now 3.55 * 18/20 = 3.195

Now these figures are just for showing an example, your multiplication number may be way different.

Hope this helps.

Mike

CrazyAl

Mike, that is an incorrect analysis.

The diameter of the RIM (aka Wheel) doesn't matter. It's the outer diameter of the TIRE that matters. It is possible to switch to 20" wheels but maintain the same tire diameter as stock. Most people that switch to 19s or 20s choose tires that have the same (or very close) diameter to stock, in which case there is no "torque loss".


In that situation your only drawbacks to 20" wheels will be the thin sidewall (makes the ride rougher) and the slightly heavier wheels.

Fireguy28

OK so with heavier wheels what do i lose?? I read that with 20"s you reduce your power to less then stock......

nbk13nw

Hmm... Rotational weight / unsprung weight is the catch phrase here. Before (Running stockers) I pulled 285 / 310 on a DynoJet. Same mods with the Foose Nitrous 20's front and rear. 20x10 rear (15lbs heavier than stock) I pulled 275 / 290 on the same DynoJet. So, I added 30lbs of rotational mass and lost 10rwhp / 20rwtq.

Just so you know, using my scale the stock wheel / tire combo came in at 45lbs each, the Foose Nitrous 20x10 came in at 60lbs each.

Hopefully the new pullies, CMCV deletes and the aluminum driveshaft will put me close or above the 300rwhp mark.

Fireguy28

Ok im a noob what is that??

MikeVistaBlue06

I know that! that is why I said this "So, assuming the tire height is the same" so it would come down to the delta between the actual wheel diameters.

Thanks!

Mike

05gtdriver

You must take in consideration too that wider wheels/tires cause aerodynamic drag, resulting in more hp lose and changing the coefficient drag on any given car, same goes when adding body kits, chin spoilers/rear spoilers.....these may aid in stabilizing a car at well in excess of triple digit speeds, but more hp must be added because of added drag, of course some kits are wind tunnel tested to help a car with bad Cd #'s achieve better figures, but usually not the case, most body aides are for "look" purposes. Anytime you make something heavier and less wind cheating(meaning wider) you're gonna lose power.

CrazyAl

I think you're on the right track, but your post is still confusing.

Your above post "assuming the tire height is the same so it would come down to the delta between the actual wheel diameters" is true, but I still think it's confusing for two reasons:

1) You really ought to say "series" or "profile" rather than Height. Generally speaking, the Height of a tire is the Outside diameter. And with two tires of identical outside diameter, the rim diamter doesn't matter as far as effective drive ratio is concerned"

2) If you do make the above assumtion your statement is correct, but it describes a rare circumstance that hardly ever occurs.

Empirically, 99% of people that go with "plus sized" wheels, like 20's, also go with low-profile tires to match. For example, whereas the stock tire might be a 55-series on a 17" wheel, they switch to a 35-series on a 20" wheel. The result is that the OD of the tire changed very little, if any at all. Nobody installs 20" wheels with 55-series tires.


The common "problem" that people discuss when switching to 20's regarding HP is weight. Most of the time, a 20" wheel-tire combo will be heavier than the stock tires. Extra rotating weight is bad for acceleration becasue it takes extra HP to spin. One common "rule of thumb" is that adding rotating weight is roughly equvalent to having four times that much weight sitting in your car. So if you switch to 20's and each wheel/tire combo now weighs 20 lbs more, that'd be like having an extra 320 lbs in your car.

moosestang

A 20 inch rim will only decrease performance, so if performance is your goal stick with 17's or 18's and get some lighter rims. More important than the small performance lose is the tooth chipping ride with a 30-35 series tire. People with 20's will say different, but I know better. If I could i'd mount 15's on my car to give me more sidewall, it will help at the track or street when you lower the air pressure. No reason to lower the air pressure on your low profile tires because they aren't going to flatten out much if any and the sidewalls ain't going to flex even with no air in them.