Those of you that have 19" and 20" wheels (Moderator's please don't move to wheels!!)
 

I ran a new thread through the wheels forum about the differene in straight-line acceleration with 2 different wheel/tire combos. One was a 17" wheel/tire that we estimated to be 21" in diameter and a 20" wheel/tire that we estimated to be 25" in diameter.

With those figures a couple of really smart forum members put together the physics formulas and the results are STAGGERING!!!

Even if our diameter estimates were wrong, if you increase your wheel size even FOUR INCHES....you lose a whopping 1.1 SECONDS off straight line acceleration.

There are many, many factors involved in calculating this, but we assumed that the car and driver were driving in identical weather and road conditions and with consistent shifting times, etc.

This is a huge issue and I wanted to shed some light on this forum and see what everybody thought about this, considering it could equate to roughly .5 to .7 seconds off the 1/4 times at the track or on the street.

YOUR THOUGHTS PLEASE!!!!

OH, I forgot to mention we also assumed that the two wheel/tire combos were roughly the same weight...

5" difference is a bit much.

most cases, the 20" rim just has 0 sidewall and thus the same overall diameter.

what this does show is if you reduced your tire size youd go faster, and that is true. I bet if you change the gearing so you hit redline in 6th at 110mph, youd get mid to low 13's with stock power. even if you changed it so 5th got you there and leave 6th for highway driving. but its alot tougher to change individual gears than just the final drive or tire diameter..

I'm thinking that a 3 inch diameter difference is reasonable to consider. Even so thats still 0.825 off the 0-60 figure...

We have 19" Axis Se7en's (chrome). The rears are 19x9. We run 275/35s. Our Z is completely stock. I have been fooled by a certain person many times about our "heavy" wheels. When it came down to it, he didn't pull at ALL from me thru third and fourth and then into fifth gear. And he wasn't stock either.

With the 275/35 19s, the overall diameter is very close to stock height.

Hope this helps :)

-Cory

I'll go ahead and post the two replies I gave in the wheel/tire section.

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POST 1
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a few notes..

1. There will be a difference. A larger wheel will induce a higher moment of inertia, even if it has the same overall weight as the smaller wheel.

2. The overall diameter of any proper wheel and tire combination will be equal. The things that change are wheel diameter and the size of the tire's sidewall.

3. The rotational inertia is proportional to the integral of mass across the radius of the wheel/tire.

4. The only way to actually calculate the rotational inertia would be to know what the mass of the wheel is at every point across the radius, and to know the same information about the tire. The results will vary from wheel to wheel depending on the wheel design.

5. Doing the math is nice, but in the real world, other factors come into play, like friction, wind, etc, etc. The only real answer is to do real-world testing on a track.

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POST 2
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one HUGE fact you guys are forgetting.. i mentioned this in my post above...

It doesn't matter if you have a 15, 16, 17, 18, 19, or 20 inch wheel. The overall diameter of a proper wheel and tire combination is THE SAME. ALWAYS.

When you buy a larger wheel, you choose a tire with a smaller sidewall to compensate.

Bottom's calculations above assume that there is a different rolling diameter, which isn't the case. The difference in acceleration comes from the rotational inertia component of the unsprung wheel/tire mass, which increases with a larger wheel due to weight distribution closer to the edge of the wheel and tire.

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I have no math to back this up, but my feeling is, upgrading to 19s or 20s could give maybe a 0.1 to 0.2 second difference on your ET, depending on wheel weight. Huge 20" chrome wheels would probably affect the ET even more.. Regardless, the difference is there, but probably not huge. People who truly care about those few tenths of a second will spend the extra money to pick up a set of Weld draglites for the strip and not worry about what they run on the street.

Let's not forget than when you change wheels and tires the difference is where the rubber meets the road. If you go with a wider tire (not neccesarily heavier) you may improve your traction and ability to accelerate.

'Nuff said.

realistic would be 2" IMO. thats assuming you maintained the exact same sidewall height and thus the larger diameter comes from the rim alone. I dont think anyone would do this, but if they did, then yes.

mind you Im not disagreeing, there the whole inertia arguement. but in most cases your gearing isnt effected.

and as someone said, mathematic forumlas regaurding unsprung weight are next to impossible to definitively measure.