Diff vs Tire size

Daryl Smith · Post by **Daryl Smith** » Wed Sep 28, 2005 2:15 am

With Joaquin going with a 3.7 rear diff and 185/60/14 tires, and the discussion going there, I began to think interms of performance:

Which is better, assuming same relative speed, for performance and milage:

1. A higher ratio rear end with smaller wheels/tires?
OR
2. A lower ratio rear end with bigger wheels/tires?

hport · Post by **hport** » Sun Oct 02, 2005 8:04 pm

Its the same final ratio +/- a few points.

JC

TR · Post by TR » Sun Oct 02, 2005 9:56 pm

The smaller wheel setup has less inertia, therefore it will perform better in terms of mileage. All around performance leads to many preferences, but if you are talking acceleration, the smaller wheels will outperform the larger.

If you dyno'd a car setup each way, the one with the smaller wheels would actually show more WHP. Even though the final ratio is the same, the diff gear has less inertia than the wheels, therefore just like a lightened flywheel, the smaller wheeled setup will rob less HP from the car.

And I think you meant to say a smaller ratio with bigger tires compared to a larger ratio with smaller tires? TR

Post by **dbrick** » Tue Oct 04, 2005 7:04 pm

Other variable would be if you had 2 tires with the same width, the taller tire would have a larger contact patch. Also, all else being equal, the smaller diameter tire would improve brake performance by having less "leverage" , improve aerodynamics by lowering the car and having less frontal area, but increase wear on bearings due to higher rotational speed.

I think it would need to be a big change in diameter to notice.

I used to use a set of 22" height sports car racing slicks on a 340 Dart, big difference in accelleration from the 28-29 inch high or so street tires.

Used to get them for free from this guy who raced SCCA and had a tire sponsor. They didn't last long, but really stuck.