An accurate measurement of your rear wheel circumference is the most difficult part of calculating your rear axle ratio. You can measure the diameter and multiply it by pie (3.14159). However, this also multiplies the error by 3.14159! Or you can mark your tire and the ground with chalk. Roll the car until the chalk mark comes back EXACTLY to the same angular position and measure the distance covered on the ground.

To calculate your rear axle ratio you need to drive your car in fourth gear (1 to 1) ratio ("Drive" with automatic trans.) and note the RPM and speed, say at 60 mph. If you're in another gear then divide the RPM by the gear ratio of that gear before using the following formula:

Circ. In inches. Unfortunately, this method doesn't prove to be that accurate. Inaccuracies in the tachometer and speedometer are so great that it throws the calculation off.

Here are some ID numbers to identify the gears in your car. (from hypertech)

Lower gears (higher number) will increase the torque at the rear wheels leading to a higher force at the tires.

F=MA.

If the force is higher, same car mass, then your car accelerates faster. The horsepower throughout the powertrain will not change.

HP = Torque X RPM/5252

If torque goes up then RPM goes down equally resulting in the same horsepower.

Gears work wonders with small block V8s. I've installed 4.09 rear end gears. This really helped autocrossing and has my car finishing the quarter rapping out in 4^th gear (perfect). With the stock 3.45 gears I shifted into 4^th about 200 feet before the finish. 4.09s has the engine at the peak horsepower rpm range one more time in the quarter mile and will improve launches. Fortunately, for everyday driving, I still have two more gears. Get this, if you had enough horsepower 4.09 gears would still go 225 mph in sixth gear at 6000 rpm, 5^th gear tops at145 mph, and 4^th gear tops at 112 mph. Heck, six speed Corvettes and Camaros should come with 4.09s in them.

Here is a table comparing the gear ratios at 6000 rpm.

Gear ----3.45s ---- 4.09s

1^st------ 50mph------38mph

2^nd------75mph------60mph

3^rd------102mph-----80mph

4^th------130mph-----112mph

5^th------180mph-----148mph

6^th------270+--------228mph

HOW MUCH DO GEARS HELP?

You should gain about a tenth of a second in the quarter for every two tenths change in the gear ratio. For example going from 3.45s to 4.10s is about 7 tenths change in the gear ratio. So the gain would be 0.35 seconds in the quarter.

Thanks to the abusive way Justin Vandersall punishes his car some very valuable lessons were learned. The main lesson is that the Dana 44 rear carrier assembly will break in major horsepower applications, 450 HP and above. The carrier has a tendency to break anywhere in the support beam from the support points near the wheel to the edge of the differential housing. The carrier beam resembles an I-beam with the differential housing in the center. To stiffen the carrier beam up I had 1/4" structural grade aluminum plates welded on the front and back of the carrier I-beams all the way across up to the edge of the differential housing. This basically changed the I-beam to a large square tube, which is much more rigid in torsion.