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markeb01 · 11-06-2010, 02:50 AM

A bit more food for thought on the rear axle ratio – the street tires on the back of my truck are 29.1” tall. The truck normally accelerates nicely with a launch ratio of 10.8 (2.64 1st gear x 4.09 rear axle).

I recently swapped over to my snow tires which are 27.4” tall and the truck accelerates like a bullet. I wanted to determine how much of a difference the tire diameter made in final drive. Using a standard gear ratio calculator here’s what I discovered:

To achieve 2600 RPM with the 27.4” tires (actually 2608 rpm) and 4.09 axle the truck is traveling 52 mph.

To achieve the same 2600 RPM with the 29.1” tires requires a 4.33 ratio, and the relative launch ratio changes to 11.4. I actually enjoy the harder acceleration with the shorter tires, but the engine runs out really fast.

With a 2.20 low gear, a 4.88 rear axle would provide nearly the same launch ratio I have at 10.7. And a 4.88 with a 33” tire will also turn 2584 rpm at 52 mph. It won’t make a very efficient freeway cruiser, but would certainly be fun around town.

11-06-2010, 02:50 AM	#19
markeb01 Senior Enthusiast Join Date: Feb 2007 Location: Spokane Valley, WA Posts: 8,356	Re: 3.73 gear swap for ???? A bit more food for thought on the rear axle ratio – the street tires on the back of my truck are 29.1” tall. The truck normally accelerates nicely with a launch ratio of 10.8 (2.64 1st gear x 4.09 rear axle). I recently swapped over to my snow tires which are 27.4” tall and the truck accelerates like a bullet. I wanted to determine how much of a difference the tire diameter made in final drive. Using a standard gear ratio calculator here’s what I discovered: To achieve 2600 RPM with the 27.4” tires (actually 2608 rpm) and 4.09 axle the truck is traveling 52 mph. To achieve the same 2600 RPM with the 29.1” tires requires a 4.33 ratio, and the relative launch ratio changes to 11.4. I actually enjoy the harder acceleration with the shorter tires, but the engine runs out really fast. With a 2.20 low gear, a 4.88 rear axle would provide nearly the same launch ratio I have at 10.7. And a 4.88 with a 33” tire will also turn 2584 rpm at 52 mph. It won’t make a very efficient freeway cruiser, but would certainly be fun around town.