# Differential

What is the meaning of a 4.1 or 4.3 rear end. How does this affect towing, engine RPMs, torque, gas mileage?

It is the gear ratio of the differential. Basically, a 4.1:1 ratio means the drive shaft and the engine (unless you have overdrive) rotates 4.1 times for every rotation of the wheels.

A higher number is a “lower” gear ratio; generally resulting in higher engine rpm’s, lower gas mileage, and increased towing capability.

Thanks. I suppose this explains why a Toyota Tundra (4.3:1) tows more than a Chevy HD half ton 4.1:1 which tows more that a Chevy 3/4 ton HD (3.7:1).

That’s part of the reason, it also has to do with engine power, suspension, etc.

This reflects the ratio between the pinion gear (on the input shaft at the end of the driveshaft) and the ring gear around the carrier assembly. The higher the ratio is, the greater number of times the driveshaft will spin for each turn of the ring gear.

Basically this means that you can put more energy from the engine into each rotation (because the engine is “firing” a lot more for each wheel rotation…there’s angular momentum from parts involved too, but I’ll ignore that to keep it simple). Or, with the same amount of energy you can spin the tires faster.

What this means in effect is that the higher the ration, the better towing power you have but teh higher the engine will spin at a given speed. Mileage is an interesting question, because a higher ratio might actually provide better mileage in the city.

Click on this Wikipedia web article for a good read on differentials: http://en.wikipedia.org/wiki/Differential_(mechanical_device)

I would be interested in hearing about the “angular momentum”.

Another question: how would a higher ratio provide higher mileage in the city?

It might and it might not provide higher mileage in the city. A lower ratio at the differential shifts into a higher gear sooner so at your usual speed on your usual route it might (or might not) have you in a higher gear than a higher ratio. For someone else on a different route it might be different. It just depends. On the interstate, OTOH, a higher ratio, within reason, will produce better fuel economy.

When they say high gear, they mean low numbers and low means high numbers.

The tire size, transmission gear ratios, engine size, intended use also play a role in the rear-end gear ratio.

Your gear ratio alone does not determine the towing capacity. The 3/4 Chevy HD with 3.73 actually has the same size rear end (10.5in) as the Toyota. The ratio is something that the buyer can choose when buying the truck from a dealer to suit his/her needs. You can get 4.56 gears in the Chevy HD if you want, or even 5.13 if you want some crazy low gearing.

Yes Mb, I’d like to hear/read your take on ‘angular momentum’ too.

Angular momentum is basically just rotational inertia. As with the inertia in a mass moving in a straight line that requires some force to change its velocity (slow it down or speed it up) so too does a rotating mass. This does have some effect on the power output off the flywheel. The power output at a constant speed is the angular momentum plus the power from the cylinders less the resistive forces.

My thought on a higher gear ratio possibly getting better mileage on a city driven vehicle is that it might be able to make better use of the engine’s torque curve for the speed range between zero and 35 at which a city vehicle is always going. Cars’ gearing is designed to take advantage of the speeds included in the EPA testing, which includes lots of highway mileage, and balance that with the “city mileage” testing protocols. My thought is strictly a theory. I have totally no data to back it.