This will go a 996TT

Looking to match some front tires to my new 315/30-18 tires 25.7" OD

I can do 235/40-18s at 25.4" OD or 265/35-18s at 25.3" OD

Same Brand / Make

Are these fronts CLOSE ENOUGH too avoid hurting the viscous coupling?

 

You should be within 4% on 996. On 997 the rears should be .5" taller.

As close to each of these you can get.

 

Thanks Damon. Updated the detail to 996TT

How do you claculate?
delta is .4/25.7= .0155 or 1.55%

Looks close enough?

 

is that how its calculated? I thought you do 25.4/25.7 = .988 so about a 1.2% difference

 

I was actually calculating for the 265 front tire

25.3/25.7 = .984 = .0156 (close enough)

 

I go to tirerack.com and select "tires by size".

Then I put in the size I like and get a "proceed to results".

In the results page, you will see the tire choices and each one will have a grey bar at the bottom which has a link that says "specs".

Click that link and you get detailed information on exact dimensions of the tire and, if you have chosen a tire made in many sizes, you can probably get information on a bunch of different sizes and find out what will work for you.

I just did the search while typing this and here is a link for an example....hope it works.

http://www.tirerack.com/tires/tires....=yes&tab=Specs

 

4%, are you sure? Is this from Porsche?


Many of us have been living by a 1% rule and if I'm not mistaken some people that have been just 2-3% over have blown their front diffs in short order.

4% is a lot.


To answer the OP, you should be ok with those diameters. I've never gone over 1% though.

 

BINGO.

What damages a diff is actually compensating for a different revs per mile between front and rear.

The factory spec tires are 832 and 829 revs/mile for 295/30 and 225/40 a 'delta' of 3...or 3 per 800 or pretty small

A 315/30 is 817 and a 235/40 is 819 (GT2 sizes)...again very small difference

A 265/35is 822. a bit more difference, 5/822 or 5/8 of one percent.

(Edit: These numbers all for Mich PS2s from Tire Rack)

Ive run both the 235/40 and the 265/35 with the 315/30 rears

The rule of thumb I've seen is 3% and up you risk diff. Not sure I've seen issues below that.

 

This is what I'm running for a track setup. This shows 1.5% change from front to back. Any thoughts?

Save this setup in your Favorites Add comments (optional): -chart 1 chart 2-
Wheels:Wheel 1Wheel 2Wheel size18x11 ET 4518x8 ET 50BackSpace197 mm (7.7'')164 mm (6.4'')
33 mm (1.3'') shorter.Rim Lip43 mm (1.7'') widerTires:Tire 1Tire 2Tire size315/30-18225/40-18Section width315 mm (12.4'')225 mm (8.9'')Sidewall95 mm (3.7'')90 mm (3.5'')Tire diameter647 mm (25.5'')637 mm (25.1'')
10 mm (1.5%) shorter.Rim sizes18x10 to 18x1218x6.5 to 18x8.5Circumference2032 mm (80'')2000 mm (78.7'')Revs per mile792.2804.6SpeedometerAssuming that this is OEM tire and speedometer readings are correct When speedometer reads 60mph (96.6km/h) actual speed will be 59.1mph (95.2km/h): 1.5% slower.Clearance:Package 1Package 2SuspensionAssuming that this is OEM wheel/tire and there is no any problems with clearance on both sides 100% clear (leaves even more room than OEM) Fenders100% clear (leaves even more room than OEM)Wheelwells100% clear (leaves even more room than OEM)

 

How about 245/40/18 with 315/30/18? What percentage difference is that combination?

 

Those should be close +/- 1% depending on the tire manufacturer.

 

809 revs per mile versus 817 revs per mile. So every mile the diff is spinning 8 revs. Right at 1%

Versus 2 or 3 for stock sizes, or ~0.25%

Like I said I've run a 265/35-18 which is a difference of 5.

TimAZ- you are running a GT2 rear tire size with a regular TT front tire. This is 'unmatched' IMO: 817 to 829 revs or a delta of 12 revs. 6 times the abuse the factory sizes cause. I wouldn't... in fact tossed out a pair of 225/40-18 ps2s with plenty of tread when I went to GT2 sizes for this very reason.

A

 

Hi A,

From everthing I read here 1.5% should be fine. That is what this difference works out to be.

Tim

 

Like I explained, it isn't just a matter of 1 or 2%, but as the rotational mismatch builds, the amount of heat/force that is input into the diff increases dramatically.

1.5% is 4-5 TIMES more than the stock spec, so a 400-500% increase in the amount of energy into the diff.

Least this is the way I see it...

 

In the brand I'm looking into (got some new Hankook rs2 for a great deal) a 245/40 this is actually the perfect match for a 315. Generally speaking a 235/40 is the best match.

I'm buying some rims and want to go as wide as possible (partially to justify buying the 18" rims when i own the stock hollow spoke 18s).

I've running a bit more front camber and setup the rear H&R bar for less under-steer. I figure fat fronts will help a bit as well. That's why I want to go with the 265/35s..

ARD: what size rims were you running the 245 on? Do you happen to know the offsets? I'm contemplating some 9" x 43 offset