Article by: RobRed

Applicable Models: 1998 – 2007 / 100 Series Land Cruiser or Lx 470 (automatic transmission A343 and A750)

Last updated: April 24, 2013 at 22:40 pm

One question that comes up inevitably when modifying a Land Cruiser, especially if you are changing to locking differentials, is “Do I regear?”  Well there are a number of personal factors involved as well as technical. For example popular opinion is to change out to 4.88 differential (ring and pinion) gear ratios to improve acceleration when running 35″ tires. That’s probably good advice for the trail but may not be the best for your needs. I’m not going to answer that question here since there are a thousand variables. What I am going to do is give you a tool so you can calculate changes in performance for yourself and see what might make sense.

The math itself is pretty simple:

We need to understand final drive ratio to calculate our speed at RPM.

Differential Ratio x Transmission Ratio (ie; 4.3*2.804)

Rollout (The distance traveled by one revolution of the engine, in inches)

(Tire Diameter x Pi)/ Final Drive Ratio
(Pi = 3.14159 in my chart)
The higher the rollout number (distance) the slower acceleration

Speed at a given RPM

(Tire Diameter x RPM) / (Final Drive*336.13552)

Tire revs per mile

Tire Circumference / 63,360
(63,360 in the number of inches in a mile)

RPM at 60 MPH

Final Drive x Revs Per Mile

A note regarding revs per mile of tires:

Tire revolutions per mile cannot be calculated by simple math because the tire tread and sidewall bend and stretch (deflect) when the load of the vehicle presses the tire against the road. The only way to know what it really is to measure your tires at load and pressure. Though contradictory to the above we are in fact using simple math to calculate our table – this is academic since the real world will be different. Tire Revs Per Mile in the calculator is entered manually should you want to change that value. The calculated value is shown next to the field for reference.

Final thoughts: In order to make a 4 speed (A343) accelerate as quickly or quicker than a 5 speed (A750) you would have to install 5.29 ratio differential gears. Even then the advantage on the 4 speed is in the first 3 gears only. Probably the most popular is the 4.88 change which provides some restoration in acceleration when going to larger tires but keeps the engine revs down to make it livable day-to-day. As they say, Your Mileage May Vary (YMMV).