I do atleast 4 roadcourse and 7 auto cross events every season. Just started last summer. I bought a set of 2005 GT500 wheel's (18x9.5) with the thought of putting 275 35/18 tires on them. The though was to have two sets of tires, DD and track. I figured for the track I wanted at least 275's and tires that size are cheaper in 18's vs. 19's so I went with the GT500 wheels. The 275 35/18 will have a AOD of 25.6" vs. the stock 27".

What effect will this have other then throwing my speedo off and lowering the speed at which I hit redline in every gear. Does this change change my current 3.73 rear to an effective 3.55's?

Am I better off with 18's or 19's?

I have a 2012 5.0 Brembo lowerd with Koni yellows and Steeda Sports.

 

I think you made the right choice regarding 18's vs. 19's if cost was your objective. Seems like you are already aware of the effects to going to a smaller AOD set up. Does it change your effective gear ratio...I cant say for sure. My gut says no.

 

A shorter tire will effectively make your 3.73's act like 3.90's. Your speedometer will read slightly high while on track as well.

 

A Shorter tire will definitely make your speedometer read faster than your going, giving you an overall shorter gearing.

Effective gear ratio will be 3.95.

To make your shorter tire equal to the factory w/3.73's, you would need a 3.55 rear gear.

Speedometer error with a 1.5" shorter tire (approx based on tread depth):

Reads 20 = actual 18.90 (-1.11 mph)
Reads 40 = actual 37.75 (-2.25 mph)
Reads 80 = actual 75.50 (-4.50 mph)
Reads 120 = actual 113.38 (-6.67 mph)

Based on the tracks you run at, max speeds, corner speeds, etc... you many find the 3.73's are too much gear. You want to be able to accelerate out of a corner without already being at near top of a gear, or lugging too much in the next.

 

Thanks for the info