Sizing Tires

scot w.

GNSperformance.com
Joined
Feb 19, 2005
Something rarely mentioned or even noticed is "Diameter Difference"

On my set of Toyo Proxy T1R's I had someone recommend the sizes I have on now and even though they fit fine there was one thing that I found WAY off. That is the difference in Diameter! The diameter difference is recommended by Most manufactures to be +or- 2.5% as the MAXIMUM difference in diameters. Well with that said and according to the tire sizes on my car the diameter difference is a whopping 7.7% !! And that is where the "Rake" look comes into play with my car...

After speaking to Richard Winchester @ BFG they also recommend the +/- 2.5% MAX.

So with lowering a car like we did to my white T and to avoid rubbing issues the front tire needs to stay under or just at 25" tall so the front tire will stay 255/35-18 resulting in a 25.03" tall tire.

The rear will now be a 295/30-18 resulting in a 24.97" tall tire giving me a Much better diameter difference of only -o.31 which will also get rid of the 80's "Rake" look!

Once I have my new tires with the new sizes I will post a few pictures....

Here is what I use to size tires properly. Use the TOP portion as your "Front" size and the bottom one as your "Rear" size and you will see the diameter difference... http://www.boosttown.com/wheels_tyres/tyre_size_calculator.php

Here is a better visible comparison of my old tire sizes vs the new.

OLD:
255/35-18 = 25.03" Tall
285/40-18 = 26.98" Tall resulting is a + 7.7% Dia. difference = 1.95" height difference

NEW:
255/35-18 = 25.03'' Tall
295/30-18 = 24.97" Tall Resulting in a - 0.31% Dia. difference = 0.06" Height difference


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hello; I was always told to keep the tires as close to the stock height especially for a street driven car. Some cars are designed with differant height tires from front to back. Have you ever seen a stock Delorean? It's looks weird if you have. Then if the height is too far off your speedo is all wrong. I see drag racers with tall rear tires but that's them and I know nothing of that.
IBBY
 
hello; I was always told to keep the tires as close to the stock height especially for a street driven car. Some cars are designed with differant height tires from front to back. Have you ever seen a stock Delorean? It's looks weird if you have. Then if the height is too far off your speedo is all wrong. I see drag racers with tall rear tires but that's them and I know nothing of that.
IBBY

What is there that is "stock" about a handbuilt car?:p
 
Scot make sure you post up some pictures of the car after the new tires if you don't mind. I'm curious to see how yours will sit comoared to mine now. I always wondering about the rear of your car being higher since my car sits level.
 
Scot make sure you post up some pictures of the car after the new tires if you don't mind. I'm curious to see how yours will sit comoared to mine now. I always wondering about the rear of your car being higher since my car sits level.
Mike,

What are your tire sizes?
 
Scot, normally I run 235/60/15 Radial TA tires on all four for the street. They are 26.1" if i remember correctly. The Nitto drag radials are 275/50/15s and are 25.83 or something like that. When those are on people think my car is sagging in the rear. The car sits dead level with the same tires on. I guess the difference in the wheel well shape front to rear is what makes people think my springs are shot lol.
 
I have a 275/40/18 in the rear and running 245/40/18 in the fronts. I already have my DSE lowered springs, but was also thinking how to avoid that "Rake" as well. And in the rear with with 275's on 18x10's I only have a quarter of an inch before I touch the frame. Interesting what the 295's will do? Heres a pic of my car in motion, I don't want cut my rear springs but would be nice to drop another inch....;) No rub issues also..
Thanks again Scott for your research!
IMG_5203.JPG
 
I have a 275/40/18 in the rear and running 245/40/18 in the fronts. I already have my DSE lowered springs, but was also thinking how to avoid that "Rake" as well. And in the rear with with 275's on 18x10's I only have a quarter of an inch before I touch the frame. Interesting what the 295's will do? Heres a pic of my car in motion, I don't want cut my rear springs but would be nice to drop another inch....;) No rub issues also..
Thanks again Scott for your research!
View attachment 171226
Cars stance looks "Perfect".... Very nice, I wouldn't change a thing!

BTW: How are you liking your DSE springs?

Here is the low down on your tire sizes:

245/40-18 = 25.72" tall tire
275/40-18 = 26.66" tall tire - The rear is +0.94" taller than the front so there is pretty much the 1'' difference.

Now if you were to go to a 275/35-18 = 25.58" tall tire now your diameter difference would go down to -0.46 = -.3 mm

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The taller your tire is the larger the contact patch, 1" taller adds more contact with the road than 1" wider.
 
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