[VAC] Re: Rear End Ratios

[Jim Dunmyer <jdunmyer@xxxxxxxxxxx>]

Greg,
 You're right in your basic thinking. A higher ratio (4.10 vs. 3.08, for
instance) means that the engine turns faster for a given speed. Be aware
that some folks call that a "lower" gear, as it simulates shifting the
transmission to a lower gear; you must be careful in your terms.

General rules of thumb: rear end ratios in the low 3's, such as 3.54 or
3.08, or even in the 2's (2.73) are considered "economy" gears. A ratio
of 3.73 would be considered "midrange" or "compromise", while a 4.10 or
higher would be considered a "towing" or "performance" number. Lower
ratios are called "taller", higher are "shorter" to some folks.

It's not as simple as all of the above, however, because transmission
ratios also come into play, along with tire size. Larger tires give you
the effect of a lower axle ratio because of their larger circumferance.
The particular engine (gas or diesel) also comes into the equation, as
most Diesels are much happier at a lower RPM than most gas engines. IE:
my Cummins likes to be between 1800 -> 2000 RPM, where a 360 V8 would
really be pulling hard between 2000 -> 3000 RPM.

Usually, the manufacturer will install gearing that's "correct" for the
intended application. For instance, my Dodge uses a 3.54 rear axle,
considered "tall" for towing. However, the transmission is matched to
the axle and engine, so the engine is generally running in its best
power band, or can be made to do so by proper shifting.

As Dr. J points out, a higher ratio rear axle makes it much easier to
start a load, whether you have a stick shift or automatic. It'll also
give you better compression braking on downhill runs. The downside is
(maybe) slightly lower mileage, especially when running solo, and
increased engine noise due to the higher revving. The mileage really
won't change much when running solo, and might well be better when
towing.

Other than the Dodge P/Us, you would probably be unhappy with a vehicle
with a less-than 3.73 axle ratio. You'd be VERY unhappy with your
Gutless Wonder if it had a 3.08. If you're otherwise satisfied with the
vehicle, you can get the axle gears changed out for around $500.00 in
most cases. If you're much into DIY, you can sometimes buy a complete
rear axle from a salvage yard, rebuild the brakes, and slide it under
your vehicle for very reasonable $$$. Bear in mind that changing the
axle ratio will throw your speedometer way off, but it's simple and
inexpensive to correct that. Some vehicles require changing a little
plastic gear in the rear of the transmission (a 10-minute job), others
require a different procedure.

If you don't know the axle ratio, it can be sometimes be found on a
sticker in the glove compartment, in the Owner's or Shop Manual, or by
asking the dealer. He can put the VIN into his computer and tell you a
lot about your vehicle. There is sometimes a tag on the rear axle
itself, usually under one of the cover bolts. At the very worst, you can
jack up the rear wheels, chalk mark them and the driveshaft, and count
the turns.

That make it a bit clearer?

                                             <<Jim>>