How much rwkw is lost going up in wheel size?

[AMG-010]

When I had my car first tuned I had already put on my 20" wheels and it ran 260rwkw standard this figure rose to 297rwkw. Almost a 40 rwkw gain. Any idea what the figures would be with the factory 18's. I understand the gains would be the same but would be interested to know what loss there is with larger wheel size. Would it be around 15 - 20 rwkw?

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[looney100]

I assume by rwkw, you mean power. Wheel size doesn't impact rw power at all. Your wheel & tire combo could affect accelleration through its effects on gearing and the weight of the wheel & tire combo, but the power doesn't change.

[C63newdude]

The wheel size doesn't impact, what it matters is the overall weight (rim/tire together).

[propain]

Originally Posted by C63newdude

The wheel size doesn't impact, what it matters is the overall weight (rim/tire together).

Weight aside....


Bigger the rim the smaller the sidewall.

Smaller rim, bigger sidewall = better launch.
Smaller rim, better acceleration.
Bigger rim, better top speed.

Marginal differences but every bit helps.


Bigger sidewall better quality ride also. Also the smaller the rim the cheaper the tire. Big rims cost way more to put rubber on.


Thats about it. If your not worried about every 10th of a second and don't hit the track 10 times a year go with what you think looks best because the performance difference wont be that noticeable on the street.

[_AMG_]

Wheel diameter has nothing to do with power.
Propain described the aspects perfectly.

[AMS Performance]

Both wheel diameter and weight affect acceleration. Whether or not it shows up on a dyno depends on what kind of dyno you use.

Intertia = mass x radius^2 so the radius of the weight is actually exponentially more important than the mass itself (literally).

It is a delecate balance of the two that will determine the exact difference. On larger wheels with lower side walls more of the mass is concentrated at a larger radius hence the reduction in performance even if overall tire diameter is the same.

Hope that helps

[AMG-010]

Thanks for the replies. What I actually meant was if I put the factory 18's on the back of my car and did another run on the Dyno the number would be higher as the overall size of the wheel is slightly smaller. My last car came with 20" wheels and when I put on 22" wheels the Dyno reading was lower but I can't remember by how much.
My car did a run with standard tune of 348.5 rwhp on the 20" wheels and after tune with the 20's it did 398 rwhp. I haven't measured the difference of overall diameter between the factory 18's and my HRE 20"s

[DuaneC63]

Originally Posted by propain

Weight aside....


Bigger the rim the smaller the sidewall.

Smaller rim, bigger sidewall = better launch.
Smaller rim, better acceleration.
Bigger rim, better top speed.

Marginal differences but every bit helps.


Bigger sidewall better quality ride also. Also the smaller the rim the cheaper the tire. Big rims cost way more to put rubber on.


Thats about it. If your not worried about every 10th of a second and don't hit the track 10 times a year go with what you think looks best because the performance difference wont be that noticeable on the street.

This sums it up right on the mark.

[MB_Forever]

Originally Posted by propain

Weight aside....


Bigger the rim the smaller the sidewall.

Smaller rim, bigger sidewall = better launch.
Smaller rim, better acceleration.
Bigger rim, better top speed.

Marginal differences but every bit helps.


Bigger sidewall better quality ride also. Also the smaller the rim the cheaper the tire. Big rims cost way more to put rubber on.


Thats about it. If your not worried about every 10th of a second and don't hit the track 10 times a year go with what you think looks best because the performance difference wont be that noticeable on the street.

Good post

Going back to the weight issue, it is not neccessarily always the case that the bigger rim is the heavier one. For example, many people switch from 18" AMG rims to 19" HREs or BBS yet still save on rotational weight due to the wheel being made out of much lighter material. However, if the new wheel/tire combo does indeed weight more than the old wheel/tire combo, then your drivetrain loss percentage would become "slightly" higher since the car would now have to spend more energy accelerating the heavier wheel/tires, and similarly, the car would have to spend more energy stopping the heavier wheel/tire combo. And even though one may see a loss (or gain) of power on the dyno due to heavier rotational parts, it is important to note that the power loss (or gain) is not changing the crank horsepower output.

[PistolWhipC63]

I just put on 265/35/18 on stock 18's and i feel like my top speed may have decreased slightly. Maybe the 35 sidewall is too much?

[AMG-010]

Would it be fair to say 2 cars side by side with the same power just different wheel sizes would be fairly equal in a race.

[DuaneC63]

Originally Posted by AMG-010

Would it be fair to say 2 cars side by side with the same power just different wheel sizes would be fairly equal in a race.

A taller aspect ratio tire is going to change the differential ratio making it "taller" (ie less rear end gear ratio...a smaller number). So you're going to a wider tire with the same aspect ratio that tire is taller thereby reducing your rear end gear ratio. Technically this will cause the engine to gain revs slower but will also increase top speed. Off course the taller sidewall with more flex could make it hook up a bit better from the dig. Changing one tire size wouldn't make much a difference for example going from 255x35 to 265x35 will only add 3.5mm to the side wall or a total of 7mm to the overall tire diameter.

[AMG-010]

Thanks Duane. I have gone from the factory 18's with factory tires which I think were 255/35/18 on back to HRE 20's with 285/25/20, so there would be a slight loss of power showing on dyno due to a overall larger wheel