I did an extensive research on the issue before I bought my set of tires. I don't know why most people here go for 235/35/19 when 255/30/19 clearly a better match for 315/25/19 size rear tires. Here are the specs on all:

STOCK TIRE - 225/40R18

Section Width: 8.85 in 225 mm

Rim Diameter: 18 in 457.2 mm

Rim Width Range: 7.5 - 9 in

Overall Diameter: 25.08 in 637.03 mm

Sidewall Height: 3.54 in 89.916 mm

Radius: 12.54 in 318.51 mm

Circumference: 78.79 in 2001.2 mm

Revs per Mile: 829.3

Actual Speed: 60 mph 100 km/h



TIRE SIZE 235/35/19

Section Width: 9.25 in 235 mm

Rim Diameter: 19 in 482.6 mm

Rim Width Range: 8 - 9.5 in

Overall Diameter: 25.47 in 646.93 mm

Sidewall Height: 3.23 in 82.042 mm

Radius: 12.73 in 323.34 mm

Circumference: 80.01 in 2032.2 mm

Revs per Mile: 816.6

Speedometer1: 59.0 mph 98.4 km/h

Speedometer Difference: 1.555% too slow

Diameter Difference: 1.54%



TIRE SIZE 255/30/19

Section Width: 10.03 in 255 mm

Rim Diameter: 19 in 482.6 mm

Rim Width Range: 8.5 - 10 in

Overall Diameter: 25.02 in 635.50 mm

Sidewall Height: 3.01 in 76.454 mm

Radius: 12.51 in 317.75 mm

Circumference: 78.60 in 1996.4 mm

Revs per Mile: 831.3

Speedometer1: 60.1 mph 100. km/h

Speedometer Difference: 0.241% too fast
Diameter Difference: 0.24%



TIRE SIZE 315/25/19

Section Width: 12.40 in 315 mm

Rim Diameter: 19 in 482.6 mm

Rim Width Range: 11 - 12 in

Overall Diameter: 25.20 in 640.08 mm

Sidewall Height: 3.10 in 78.74 mm

Radius: 12.60 in 320.04 mm

Circumference: 79.16 in 2010.6 mm

Revs per Mile: 825.3

Speedometer1: 59.7 mph 99.5 km/h

Speedometer Difference: 0.484% too slow
Diameter Difference: 0.48%

 

Agree on the wider fronts, but you can't run that on a stock 8" rim.

 

996TTSCAB....just to be clear...this data is published theoretical?....you can't go with theoretical sizes....you need to actually measure the diameters (BTW no problems running 235/35-19 and 315/25-19)

 

Guys tvm, this topic has obviously got the experts out it has been extremely helpful and I will be buying some new tires as well as a new diff.....

 

Still would have been good to know what sizes you had that led to the problem...

GL- sounds like you can pick up a diff from someone doing a 2WD conversion, but I suspect the shipping will be a bear!

A

 

So how much is it okay to be off on the diameter and the revs per mile?

 

richardch, I thought you are a long time 996tt owner? I can't believe you let this happen? this is a well known issue.

 

STOCK FRONT: Overall Diameter: 25.08 in 637.03 mm
STOCK REAR : Overall Diameter: 24.96 in 633.98 mm

They are within 3.05 mm difference

 

Saw this happen to a Gallardo recently as well. Such a shame...

Why people screw with rolling diameters I will never understand.

 

it is the ratio of front to rear diameter that matters, not the absolute diameter or revs per mile. In the 997tt the 235/35/19 : 305/30/19 combo has a height/diameter of 25.6 and 26.3 inches. That ratio is 0.973. I run 245/35/18 : 315/30/18 track combo with a height of 24.8 and 25.5 inches. That ratio is 0.972. The difference between the 2 setups is 0.1% and perfectly safe despite nearly and inch difference in heights between the 18's and the 19's.

 

Eclou, by my calculations your differential is nearly 3%.

 

differential between the different (19" vs 18") setup ratios is what I was calculating : (.973-.972)/.973 = .001. I am not concerned with the front to rear differences.

 

eclou, are you calculating your ratios from the actual tire spec sheet or from the
theoretical interpretation of what 235/35/19 means (235 mm, 35% etc)?

Here's a repeat of an old post I made:

Hi folks. danger danger. ... Never, never use any online generic
'tire size calculator' if you are dealing with close tolerances, such
as trying to prevent rubbing or if you need identical front and rear
diameters like we do. They all mean well, and are simply designed
according to what the tire size nomenclature is *supposed* to
mean, but in the real world there is a wide range of variance
between what a tire is designated as and what it's real dimensions
are. For the real facts you have to go to the tire manufacturers
spec sheet. I have seen two different 245/45-16 tires differ in
diameter by 1/2 inch *from the same manufacturer*! They were
just different model tires. Recently one tire manufacturer stopped
making it's 305/30-18. Now they list a 315/30-18. *It's the same
tire*! They just decided that the actual tire dimensions were closer
to the 315 meaning, so after 6 years they just changed what was
printed on the sidewall.

To let you know how poor a ballpark answer they provide. Go to
tirerack.com and look at the spec sheets for 225-40-18 tires, and
you will find diameters varying between 24.8" and 25.5". The latter
value is taller than the average 235-40-18!

225-40-18 tires:

Hoosier A6 - 24.8"
Bridgestone expedia S-01 - 24.9"
...
Michelin Pilot Sport - 25.3"
Dunlop Sp Sport 01 - 25.5"

Now look at the supposedly larger 235-40-18 tires, eg:

Dunlop Direzza Z1 - 25.1"

I'm saying that however helpful these generic formulas are trying
to be, they are quite easily capable of being way wrong, precisely
when it matters the most to be right.
Joe

 

http://www.carbibles.com/tyre_bible.html

Page two has the formulas for it all

I had with my recommended winter tyres for my Cargraphics 235/35/19 & 295/30/19 (recommended by their official Swiss distributor) and you will see it is +1.93% difference, with the REARS being bigger than the FRONTS.

In fact if you plug in the stock summer tyre recommendations 235/35/19 & 315/25/19 there is still a -1.08% difference but is seems this is within tolerences and maybe crucially the FRONTS are BIGGER than the REARS maybe this is the key

My problem is that simply there are no Wider Winter Tyres made more than 295

 

Hi Richard, you just missed my post.