rpietro

Can anybody give us the real scoop on the "larger wheels and/or tires will slow you down" myth or syndrome. Obviously there will more centrifugal resistance (I guess that's what we could call it) due to the additional weight but how much will it slow you down? Will you even notice? God forbid if it would restrict my ability to do burnouts. I looking to change my wheels on my 08 GT/CS (18" X 8.5" stock) to 20" with larger tire size, but I'm afraid to render the car lethargic. Has anybody gone through this mod already?

Riptide

Yes it will slow you down. The easiest way to tell how much it will slow you down is to try a smaller wheel and tire then run the 1/4 mile. See what happens. That will give you real world results/example.

rpietro

Makes sense. I guess a smaller wheel vs the stock would give me somekind of a ratio that would also apply going to bigger wheels. Thanks, good advise.

howarmat

it depends, larger doesnt mean heavier

heavier wheels will slow you down

just because you gets 20s does mean you will be slower, they can be the same weight as the 18s and same diameter and you will not see any difference. You can also get lighter 20s and actually feel faster

And you can easily tell the difference when you switch to lighter or heavier wheels as that is rotational weight and not dead weight.

rpietro

Wow Howarmat!!! I just took a look at your pics. Great mods. How was the fit on the TRUFIBER hood and ROUSH front facia? Sorry for switching subject, but I'm also looking at the same TRUFIBER hood.

howarmat

fit is great. the hood requires some work but looks good when done for sure

jz

In a sense a taller wheel will slow down your car because it basically reduces your gear ratio. It's a larger circle and therefore takes longer to rotate. However, like Howamat was saying, weight also plays a role since you'll be increasing or decreasing rotational mass.

I do not have, or honestly know an equation that would allow you to figure out what the gear ratio reduction would be with a larger wheel. What you need is one of the Mechanical Engineers to jump in and answer in layman's terms.

09blkgtcal

you'll definetly feel that jump in wheel size

ejanderson05gt

Here's a fairly simply explanation, though it's maybe not as accurate as you're looking for. Torque, a rotational force, is given by Force*Distance, so, obviously, the longer the distance, the torque required to rotate the mass with the same force will be proportionally higher. It doesn't deal so much with the bigger circumference, rather the bigger rotational mass. Just what I picked up as a ME student, maybe someone can offer a more precise explanation.

Eric

Here's something I read a while back from some truck guys.

"here's a useful formula. Your RPM at 60 mph equals REVS PER MILE (drive tires) times REAR AXLE GEAR ratio times TOP GEAR ratio. The lower your RPM is the better your truck will be on fuel.

Example (my own truck). I have a 13 speed tranny with 0.73 top gear ratio, and my rear axle is a 3.73. When I go to my tire manufacturer's web site I learn that my 295 low profile YOKOHAMA tire rotates 512 times per mile. So, at 60 miles per hour, my RPM should be

512 x 3.73 x 0.73 = 1394

For some reason it's never quite that high in my case because I'll probably get 512 revs/mile in a year when the tires that I put on in January 2008 will be more worn out.

But the idea is solid: there's a concrete tie between your tire size, axle ratio and top gear ratio. To compute your RPM at another speed, just divide the result by 60 and then multiply again by the new speed.

Example, if I want to know my RPM at 65 mph, I take 1394 from the above, divide it by 60 and multiply by 65. The result is 1510 RPM (too high). I have a CAT and they say your best RPM is below or around 1350 rpm. That's why I mostly cruise at 58 mph (1300 rpm)."

ChiDiddy

heres an answer to your question with real proof. Larger rotational mass= lower gear ratio, which in the end will make you slower in the 1/4 mile. Weight has the most influence though.

Way back, when i just got my car, it ran a 14.1 stock.... Yes, its sad. Those are with 20" Saleen wheels. Then i borrow my friends stock GT 17" wheels and ran a 13.5. Driving skill consistency is spot on because i have an automatic .

What howarmat said is true. Lighter 20's may even make you feel faster. But thats gonna cost you alot.