LVDell

Since I'm new to the world of AWD with the recent purchase of the Turbo, I am trying to find in the manual, etc, (with no luck) what the requirement are for tire size tolerances.

For example, is the importance of front to back variance more important than staying within a specific window of sizing (diameter) on the single axle?

What happens to the AWD system if the tire is too big or small?

I just can't find any PCNA/PAG documentation on tire size variance tolerances.

My car is delivered from the factory with the following diameters:
F: 25.1"
R: 25.0"

My car as delivered from the dealer a few weeks ago with new rubber from the PO:
F: 25.3"
R: 26.0"

I don't feel anything adverse but you never know.

redridge

according to the dude in the pca website (I forgot his name, Joe something)..... no more than 4% diff f/r for the abs to start acting funny. Thats all i got...

LVDell

It's not the ABS I'm worried about as much as the AWD system

redridge

Its a mechanical AWD system via viscous coupling.... as long as the diff has good fluids you should be fine., very few has reported failure and the 4% rule appies to the awd setup as well.

I have heard the 600hp+ TT has burned their coupling.... having abs issues. I think the abs is more of an issue than the front diff is concerned.....


So when are you gonna convert to rear drive setup.

LVDell

Never going to convert to RWD.

Just hoping I am fine with the current tire diameter setup. It's the rear tire I am concerned about.

MechanicalEng

Dell
The variance is 3%, I always check the Revs Per Mile of the tire to determine this. The front differential is the one to suffer the most and gets damaged easily if the radial speeds of the front and rear tires are unequal or way out of spec.

LVDell

So how do I know if what I have is acceptable or needs to be addressed?

MechanicalEng

what size/brand of tires do you have now?
According to your data (25.3 and 26) and my shady math you are at about 2.69% variance front to rear

LVDell

The 26" tire is 2.77% larger than the 25.3

The tires are Toyo Proxes T1R in 235/40 and 295/35. The published diameter by Toyo is what I listed.

ROK

You should be just fine here

SH || NC

Time for another diff fluid change! (link)

pongobaz

Dell, use this tire calculator to see if the rolling diameter difference between the front and rear is greater than 3%.

http://www.1010tires.com/tiresizecalculator.asp

redridge

its not so much of the difference between the stock rear tire vs the new rear tire, but the error between the front and rear tire..... optimally you want as close to 25" since thats what the designers had intended for the 996. But the error of diameter between the front and rear is also very important.

plug you tire front and rear specs in this calculator.... on the bottom it will tell you the % error.

http://www.miata.net/garage/tirecalc.html


edit: 2.9% difference, between the front and rear.

Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
235/40-18 3.7in 12.7in 25.4in 79.8in 794 0.0%
295/35-18 4.1in 13.1in 26.1in 82.1in 772 2.9%

LVDell

That's a really good "generic" calculator. It just doesn't know the exact specs of different tires which is why I used the manufacturer specs instead and did my own computation.

I'm still looking for PCNA/PAG documentation regarding this.

LVDell

Oh you can count on it! I already have this planned. Just need to order some Delvac.