Are there any benefits in running larger/smaller sidewalls? I have two sets of tires, 245/40 and 245/45, and wondering which to keep. I know the difference isnt much, and if anything, barely noticeable as the 45s fill up the wheel well more.

 

My bad. Did not notice this is the autocross forum.

 

Comparing different sidewall "height" on the same brand of tire, the less sidewall, usually the less deflection in the tire in cornering...basically a stiffer sidewall feel for the 40 series vs. the 45 series. This can make for a rougher ride but better cornering.

Also, a lower profile tire has a smaller diameter and thus, when talking about drive wheel tires, it changes the gearing.

I would go with the lower profile tire

 

Excellent, this is what I was looking for. I was thinking the lower profile would also help a bit with the reduced weight (as small as it may be).

Both tires are identical brand/model. Its the RE01s and they seem to have a fairly stiff sidewall, so I think I'll switch to the 40s.

 

I'm assuming you have 18" wheels?Keep in mind the correct diameter for your car.
The 45 series is the OEM size,the 40 series is still within the tolerance level and a bit lower profile.I sure would go with the 40 for the handling and the looks.Depending on the make of the tire,the ride difference should be minimal.

 

Lower sidewall height does not necessarily mean better handling. We got better lap times on high horsepower BMWs, Vipers, and 240Zs by running a taller sidewall height in back. That allowed the cars to put power down better and it was a bit easier for the driver to consistently turn faster laps. For the Solo2 FP national championship BMW going with a slick with a 2.5% taller sidewall pulled .1 seconds off course times.

 

So, for a amateur who doesn't have a lot of money to buy multiple sets of tires and do lots of back to back testing, is there a standard that can be applied...for example, lower sidewall in front and taller sidewall in the rear on a Z? Or, is it basically try and see which works best?

 

For most racers the 40 to 45 series difference we are talking about here would be unnoticeable. It comes into play at the upper levels of competition in your class or series. If you're a mid-pack competitor, work on your driving skills first.

 

Ya I figured 40 vs 45 would be negligible. Thanks for all the good info!

 

Run your 40's up front and the 45 in the back.

 

if its time for new tires just buy RS3s