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Is changing to shorter tires bad for rear differential?

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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. This setup is going to be for road course/track use.

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?
 
1,246
1,243
In the V6L
Differential will perfectly fine so long as the tires on both sides of the car are the same size. The only effect will be on your speedometer, which will be off by about 5.5% - when it reads 100MPH, you're only actually going 95.
 
Differential will perfectly fine so long as the tires on both sides of the car are the same size. The only effect will be on your speedometer, which will be off by about 5.5% - when it reads 100MPH, you're only actually going 95.

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?
 

ArizonaBOSS

Because racecar.
Moderator
8,730
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Ignoring the weight increase of any wider wheel and tire setup, you will actually accelerate FASTER with the smaller tire, assuming you don't spin. But you will reach redline in each gear faster as well, so you will do a bit more shifting.
 
Ignoring the weight increase of any wider wheel and tire setup, you will actually accelerate FASTER with the smaller tire, assuming you don't spin. But you will reach redline in each gear faster as well, so you will do a bit more shifting.

Ok good. FYI here's the warning in the '18 manual about using different size tires than OE on the rear axle w/ torsen diff. Doesn't sound like y'all think this will matter, but the warning is 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."
 

steveespo

Lord knows I'm a Voodoo Child
Moderator
4,008
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Cookeville TN
As long as the tires are the same size on both rear wheels there is no effect on the differential or gears. The abs and advancetrack look for wheel speed difference so the actual values do not matter just the delta. As for engine tune there are speed tables that reference the wheel speed/gear ratio equation but there is leeway in that to the point where a 5% change won't matter, there are many other variables considered to keep the engine performing optimally. You should be just fine, enjoy the better cornering and braking performance.
 
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8,183
As long as the diameter is the same. don't worry about it. most guys run 18 inch wheels because there are plenty of take off race tires in that diameter.
In one respect, that your friend sort of alluded to, you will need to increase front brake cooling, regardless of the tire diameter if you are really pushing the car hard. With regards to "stagger" that is actually called "rake" and is most apparent when you scale the car., or try to pass IMSA tech (long story).
 

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