GM-GMC: gear ratios, rear axle ratio, gm diesels

Question
Can you please explain in detail how the ratios apply to the diffrent trucks? Example3.73 on a 1500 extended cab, versus a 4.10

Answer
Al,

I'll be glad to help.

The axle ratio is the number of times the driveshaft turns in relation to one full rotation of the rear wheels.

3.73:1 (read as "3.73 to 1") means that the driveshaft turns just under three and three quarters turns to make the rear wheels turn one complete rotation.

I have two GM diesels.  My van, with 3.08:1, has an engine that turns more slowly on the highway than my pickup, which has a 3.73:1 ratio.  My van gets better fuel economy, too.  But my pickup can tow more weight and does not slow down when going up hills and towing a car (or carrying a heavy load) behind it.

Unless you are going to tow heavy loads frequently, stay with the manufacturer recommended rear axle ratio.  4.10:1 will result in an engine that runs very fast at highway speeds.  But, on a farm, it might be ideal.

So, axle ratios are a trade-off.  What is good for power and towing is not as good for economy and highway speeds.  Engines have a "sweet spot" where they make the best power and torque with good economy.  A rear axle ratio that is selected for obtaining this engine speed during highway travel is usually the best selection.  Allowing the engine to turn too slow will result in low power and lower fuel economy, so choose wisely.