Can someone take a minute and explain tire size vs. acceleration (and more)

[Hotwatermusic]

I have 225/45's on 16" Fuchs on my 74 911 and when researching new tires I was told the proper size (for proper rolling diameter) on my car was 225/50's on a 16" rim. That's about an inch bigger than what I have on, and frankly I don't think there's room in my wheel arches for that size tire (car has been lowered). My question is, if it happens, what will increasing my rolling diameter do to the car's ride? It has to change accleration, but is it noticeable? Obviously an extra inch of rubber will soften the ride some, but what about cornering--will the taller side wall have a large effect on cornering feel? I'd keep the size I have on there--the 225/45--but on Tirerack.com alone there are countless mores tires available at 50 than 45. (70 vs. 4) And I am not liking any of the 4 that are in my size. Thanks to anyone who can give me any advice on this long winded question.

[DanielDudley]

That is a 3.6% difference in rolling diameter. When the speedo reads 60, you will be going a little over 57 MPH, and it will feel like a six HP difference, which might be enough to notice.

Cars with less HP tend to be really sensitive to tire size and weight. A performance tire in either size is going to handle better than a non performance tire. Get a really good set of tires, and I think you will be happier than if you make a compromise in quality.

[DanielDudley]

Of course, when I talk about cars with less HP, I am thinking early Miata. You don't want to put a really big set of tires on one of those bad boys...

[ruby911]

I recently had a similar discussion about my car. I am currently running 17 inch wheels with 45 series tires on the back of my '84 Carrera. It handles like a dream, but I noticed at a recent DE at Road Atlanta, that I was not getting the bite coming out of turn 5 that my instructer was getting in his SC with Fuchs. I had my foot flat on the floor and the car just seemed to lag a touch until I peaked the short hill coming out of 5. On the other hand, my instuctor's SC with fuchs (and stock motor/intake) would dig and go and seem to get the RPMs up quicker. We did not do any scientific tests, but both agreed that I was losing some because of the taller tire/wheel combo.

The difference between my car and his was quite noticable.

[Bill Verburg]

The taller tire gives less torque for acceleration and moves the curves to the right

I never did the analysis you asked for but I did some similar ones

245/45 vs 255/50


265/35 vs 275/40

[euro911sc]

So physics wise Torque is force * length. So when you increase your rolling radius you are reducing the force propelling your car forward, all other things being equal. See Bill's graph above. Remember, F = ma, so unless you lighten your car up you will not get back your lost a (acceleration)

Of course... with the bigger radius you will be going faster for the same RPM...

There are a lot of arguments to be had on wheel and tire size choice that should include: handling, acceleration, top speed, looks, tire availability, ride/comfort needs, intended use, etc..

Personally, I like the lower profile tires. A little harsher ride, but better handling and a little kick in the pants torque wise. But, you are correct in that the tire choice is getting slim these days... Try discount Tire Direct as well as they have different brands than Tire Rack.

-Michael

[mxessin2]

I changed to the larger tires and I noticed the gap in RPMs was greater when shifting betweens gears.

[Bill Verburg]

Originally Posted by mxessin2

I changed to the larger tires and I noticed the gap in RPMs was greater when shifting betweens gears.

You are misinterpreting what you observe. the rpm drop on shifts is not affected in any way by cwp or tire changes. The available force for acceleration at any given speed is changed and is what you are observing additionally a large change in wheel/tire weight(~9#>) can also be felt as less acceleration or longer time to go through the gears.

[mxessin2]

It seems that after a shift it takes a little longer to get the motor back into the upper RPMs where it pulls harder. I will go back to the smaller tires when its time to get new ones.