Smaller Tires bad for IRS?

[SgdriskillGT]

I just upgraded my 275/40/19 rear tires to 305/30/19 which are 1.5" smaller in diameter (actually replaced the front 255/40/19 with the 305's as well to make a square setup). A friend suggested going to a shorter tire like this is bad for the differential. He also suggested the car is setup for staggered so I should keep that to decrease front wheel rolling resistance/increase cooling to brakes.

The PP2 S550 has 305/30/19 square setup, but I don't know if they tune the car differently. I've also read a square setup is better for the track, even though my car came staggered. I feel like gaining front grip at the expense of rolling resistance would be favorable.

I guess my main question is will the 305/30/19 setup create issues with the differential? What do others think?

Sponsored

 

[M151A2]

I don't think you differential would care as long as both rear tires are the same size. Slight speedo change but nothing else.

 

[SgdriskillGT]

I don't think you differential would care as long as both rear tires are the same size. Slight speedo change but nothing else.

Good, that’s exactly what I thought. I guess this also means flat out acceleration will decrease b/c the same engine RPM will equal lower mph.

Is this right?

In general, is it better to give this up in favor of more overall grip with the wider tires?

 

[Norm Peterson]

Good, that’s exactly what I thought. I guess this also means flat out acceleration will decrease b/c the same engine RPM will equal lower mph.

Is this right?

No. Acceleration generally increases because the same amount of axle torque produces more forward thrust at the contact patches. More thrust = stronger acceleration.

There are brief periods of time when shorter tires cost you some acceleration, those specifically being in the few mph between where you'd had to have upshifted with the shorter tires but before you'd have to with the taller ones. I am assuming that traction is not a problem here.

As long as the shorter tires don't cause a ground clearance problem, it's an easy way to drop your car (mostly meaning its CG height) without sacrificing suspension geometry. The tires on the car in my sig are about 1.3" shorter than the OE tires it came with, my other set is about 0.9" shorter than the OE tires, and I have no intention of ever going back to OE diameter tires.


Norm

 

[qtrracer]

Your T/C may take exception with the different sizes. I once ran a 255/40/17 on the rear with and 245/45/17 on the front and the T/C engaged under certain conditions. This was an 01 Cobra. That may have been too much difference for that older design.

 

[SgdriskillGT]

No. Acceleration generally increases because the same amount of axle torque produces more forward thrust at the contact patches. More thrust = stronger acceleration.

There are brief periods of time when shorter tires cost you some acceleration, those specifically being in the few mph between where you'd had to have upshifted with the shorter tires but before you'd have to with the taller ones. I am assuming that traction is not a problem here.

As long as the shorter tires don't cause a ground clearance problem, it's an easy way to drop your car (mostly meaning its CG height) without sacrificing suspension geometry. The tires on the car in my sig are about 1.3" shorter than the OE tires it came with, my other set is about 0.9" shorter than the OE tires, and I have no intention of ever going back to OE diameter tires.


Norm

Got it, thanks. I did find a warning in the '18 owners manual that is probably the source of this confusion. Not sure why this warning is in there:

"Extended use of tire sizes other than the manufacturer’s specified tire size on these axles could result in a permanent reduction in effectiveness. This loss does not affect your normal driving and should not be noticeable."

Interesting...

 

[West TX GT]

I ran a 305/30/19 for over a year on a PP and never had any issues. I think they look a little funny being that short and you probably give up some straight line traction vs an equally wide tire with more sidewall but not much. Just enjoy the increased grip of the wider rubber, it'll be fine. It will tramline more with that wide of a front tire but it is hard to escape that.

 

[Norm Peterson]

"Extended use of tire sizes other than the manufacturer’s specified tire size on these axles could result in a permanent reduction in effectiveness. This loss does not affect your normal driving and should not be noticeable."

Not nearly enough explanation in that to know for sure what the concern might be. Keep in mind that even if the computers are comparing the speedometer sensor signals against the ABS wheel sensor signals they're still going to remain consistent with one another as long as you don't make significantly unequal changes to front vs rear tire diameter.

Going 'square' from the PP1's smallish amount of 'stagger' shouldn't cause any problems; the PP1 tire stagger is less than 3% on diameter, and tires can wear that much from new to wear bars.

My guess is that they're trying to tell people that extremes in "big-n-little" tire choices could be problematic without saying so directly. But it's only a guess at this point.


Norm

 

[SgdriskillGT]

Thanks everyone, I feel confident this won’t cause any issues. Really appreciate the insight!

Sponsored