THE Tire Thread

[otis]

Nope the circumference and diameter are exactly the same amount percentage-wise. Circumference equal d*pi so it doesn't matter if you increase the diameter the ratio is always the same. I hate to believe it, but I think this math is off.

Circumference of the Goodyear rears is 90.81 and for the michelins 85.25. That is exactly a 6.12% reduction. Same as the diameter. Correct me if I'm wrong.

An 18.91% reduction in the front tires between the goodyears and michelins makes them from 26.33" to 21.35" not truly 25.7" which is the actual diameter.

Otis

 

[jcthorne]

A tire's circumference and its effective rolling distance per revolution are not the same. The tire manufacture often will publish a revs per mile spec for the tire properly inflated on the spec rim. Has to do with the tire flatening out in its contact patch causing the effective tire radius to be smaller. Its different depending on the construction of the tire.

 

[otis]

Rear:
Rev/mile Goodyear 722
Rev/mile Michelin 765
Difference 5.62%

Front:
Rev/mile Goodyear 793
Rev/mile Michelin 809 245/40/18
Difference 1.98%
Rev/mile Michelin 832 245/35/18
Difference 4.69%

No real difference.

Otis

 

[Superfly]

Otis, I think you're trying to stir the pot! :lol

The difference shown on the original chart is the difference in the SIDEWALL. Consider also that the sidewall needs to be doubled for the entire height of the tire. BUT, there is NO difference in the majority of that height, (which is the overall diameter of the tire) since the 18 or 19 inch wheel portion stays the same, regardless. So yes, the sidewall is 20% (or whatever) smaller for the michelin, but the overall height might only be 6% since the WHEEL diameter (which makes up the majority of the height) is 4-5X the size of the sidewall diameter. The 20% (or whatever) is limited to the SIDEWALL!!!

So, Fubar is right with his calculations of the SIDEWALL. He does say it's "SIDE HEIGHT". I take that to mean the height of the sidewall from the ground to the wheel, NOT the overall wheel height.
You're right with your calculations of overall DIAMETER.

So great, you're both right. Isn't this confusing? LOL :facepalm: Now, instead of confusing us all with math (well, me anyway, I'm a lawyer not an accountant :willy) get some miles on your car let us know your thoughts of the michelins, and if you've got any additional clearance issues or aesthetic issues with tires that are that much smaller vs the performance increases you gain from the michelin compounding.

BTW, based on all the math, the Michelin is nearly 2 inches smaller in overall diameter for the rear tires. That's gonna leave you with a LOT of fender well clearance on a car that's about an inch lower than stock at the back, or a REALLY LOW car if you're adjusting the suspension to remove that fenderwell gap. I'd make my decision partly on the roads around my house and how/where you're driving it. If I had better roads, I'd have probably gone to 19/20" or 20/20" wheels rather than the Michelins in those sizes. Just my $0.02

 

[otis]

Otis, I think you're trying to stir the pot! :lol

The difference shown on the original chart is the difference in the SIDEWALL. Consider also that the sidewall needs to be doubled for the entire height of the tire. BUT, there is NO difference in the majority of that height, (which is the overall diameter of the tire) since the 18 or 19 inch wheel portion stays the same, regardless. So yes, the sidewall is 20% (or whatever) smaller for the michelin, but the overall height might only be 6% since the WHEEL diameter (which makes up the majority of the height) is 4-5X the size of the sidewall diameter. The 20% (or whatever) is limited to the SIDEWALL!!!

So, Fubar is right with his calculations of the SIDEWALL. He does say it's "SIDE HEIGHT". I take that to mean the height of the sidewall from the ground to the wheel, NOT the overall wheel height.
You're right with your calculations of overall DIAMETER.

So great, you're both right. Isn't this confusing? LOL :facepalm: Now, instead of confusing us all with math (well, me anyway, I'm a lawyer not an accountant :willy) get some miles on your car let us know your thoughts of the michelins, and if you've got any additional clearance issues or aesthetic issues with tires that are that much smaller vs the performance increases you gain from the michelin compounding.

BTW, based on all the math, the Michelin is nearly 2 inches smaller in overall diameter for the rear tires. That's gonna leave you with a LOT of fender well clearance on a car that's about an inch lower than stock at the back, or a REALLY LOW car if you're adjusting the suspension to remove that fenderwell gap. I'd make my decision partly on the roads around my house and how/where you're driving it. If I had better roads, I'd have probably gone to 19/20" or 20/20" wheels rather than the Michelins in those sizes. Just my $0.02

Sorry I am stirring hard here. Really want michelins instead and I want to be sure my math is correct before experimenting. Hate to blow 2k on bad math. I love the advantage of better grip and acceleration. I truly believe this is a superior compound too.

I agree with the lowering. The car will be about 0.8" lower from the tires and if I add another 1" of lowering with the coil-overs it's going to be near the original GT40's 40" mark. At least I have a beater front diffuser on the car and a new one in the box.

Ordered them and will let you all know. Only problem is I can't give you feed back until our road are free from snow and salt.

Otis

 

[Fubar]

I will admit to being susceptible to error from time to time. I remember going thru a lot of numbers when I was doing this. I’ll check my math later today, when I have a little more time. Feel free to critique… it won’t hurt my feelings.

 

[otis]

Fubar,
If your numbers are sidewall height then I can't comment and would bet your math is perfect. I interpreted your post to mean total side height or diameter of the tire. The small numbers you posted must have been sidewall height in millimeters? I posted the actual difference in total wheel diameters. For me rev/mile and total wheel diameter are the most important to balance. Therefore I chose to pick the largest rear they made in the PSS for a 19" rim 345/30/19 which was a 6.1% reduction from the stock and a front 245/35/18 equally smaller at 5.8% reduction. For me sidewall height is important for comfort and handling. Comfort should be a tad worse with the shorter sidewall and handling should be sharper with the reduction.

Just hope the car isn't too low with the smaller tire and lowering. Hate to scrape on things.

No disrespect meant. Just looking for help to make a sound decision.

Otis

 

[HHGT]

I have the Bridgestones on and the car is lowered. No issues, just have to be careful getting into driveways or approaching speed-bumps.

 

[fjpikul]

Otis, Kirby Viera ran the Michelins on stock rims with no complaints. Look for some of his old posts.

 

[2112]

I thought Ralphie runs them too?

 

[GTED]

Does anyone know where I can get a pair of the Bridgestone RE050A Scuderia 345/35-19 with a date-code newer than Oct 2010? I'm told by Big O tires that the newest one they can put their hands on for me, in California, is in 2010, but manufactured before Oct. So, the tires are already or almost 2 years old.... thx

GTED

 

[2112]

The Tire Rack?

 

[Fubar]

No disrespect meant. Just looking for help to make a sound decision.

Otis

Not at all Otis. It was my intention to make this thread helpful anyone who wanted a quick idea of what options where available and the affect a change might have on the car. Having good information in critical to making a decision you can be happy with. This was my math in the original post:

315/40-19 meaning the manufactures estimated traction width is 315mm. The sidewall height is calculated as a percentage (40% in this case) of the width (so 315 x 0.40 = 126mm of sidewall at any given point along the tire, not a combined function of the overall tire from top to bottom). The rim diameter is 19” and irrelevant for the purpose of this comparison.

Subsequent tires were measured in comparison to the OEM tires sidewall. The difference was issued as +/- X% from OEM. Doing so would give the buyer the ability to see what the change would do to the balance of the GT and the change in wheel spin from front to back.

In the first example, the Michelin PS2 345/30-19: These tires have a est. traction width of 345mm, giving them a sidewall height of 103mm (345 x 0.30 = 103.00mm). Then I took the stock height minus the new tire, difference being 23mm and calculated the percentage 23mm would be of the stock height (126 – 103 =23, then 23/126=0.1825 and thus an overall change of -18.25%

As you can see, it was simple math but a lot of number crunching and certainly open to little errors. Embarrassingly enough, the front PS2 width should have been 98mm not the 85.75 that I originally posted. So the change from stock would have been 7.75mm , a change of 7.33%. I have corrected the original post and thank you for detecting the error. In the end, it helps everyone to have good information.

Cheers.

 

[otis]

Well I wanted to post a quick update on the new tires. The fronts are Michelin PSS 245/35/18 which are 25.7" diameter versus the stock 26.3" diameter. The rears are PSSs 345/30/19 which are 27.3" diameter versus the stock 28.9". My rears were pretty worn about 60-70% and the difference was only about 1.3" for the rears from the stock tires to the new ones and the fronts about 0.5".

The fronts look smaller than the measurements would indicate and the rears look better than the measurements would indicate. Funny what the eye tells you versus what the tape measure shows. They definitely need the lowering T&A shocks to improve the stance. As far as ride goes they are much quieter than the worn stock tires. I cannot tell if the grip is all that much more since they are pretty new. The first gear rolling 10 MPH experience is about the same with the back end letting loose easily about the same maybe a touch better than the old tires. The second gear hold is better with the new tires as before the 1-2 shift could break the stock tires pretty easy and the new ones hold the 1-2 shift without any tire spin.

Here are a few pics to appreciate the new stance prior to dropping the car an inch or so.

Otis

 

[Fubar]

Glad to see somebody got a set of PSS on there. I have heard nothing but good things about these tires.

 

[RPM217]

I'd be curious as to a side by side comparison with the Bridgestones. I've been very happy with them, but it'd be nice to have an alternative.

 

[roketman]

Hoosier time!

 

[tmcphail]

Hoosier time!

Bingo

 

[GTMikey]

I put 6000 miles on my Scudera's, changed the rears fir to new ones for 2 track days at 31psi, loved them... Pumped them up to 40psi for the 350 miles back to San Diego, love them. However, I'm considering getting new wheels and tires for track days, sounds like Hoosiers, what wheels?? B

 

[GTMikey]

I put 6000 miles on my Scudera's, changed the rears fir to new ones for 2 track days at 31psi, loved them... Pumped them up to 40psi for the 350 miles back to San Diego, love them. However, I'm considering getting new wheels and tires for track days, sounds like Hoosiers, what wheels?? B