- Torque multiplication
By Dave Lum
Gearing - generally a confusing and
misunderstood aspect of motoring. The correct gearing - both final drive (axle) ratio, and
transmission gearing - will maximize the power you have and keep you accelerating at the
maximum rate your engine can provide. The wrong gearing can make an engine/car combination
sluggish and inefficient.
Getting your gearing right requires taking
into account the use of the car, engine power characteristics, weight of the vehicle, and
even tire sizing. It is important to remember that tire sizing is also part of your
overall gearing. In the following, we'll start from the "ground up", covering tires, axle, and
transmission, in that order.
If you have 4x4 truck with 44" tall tires, you
will want different gearing than an economy car with 24" tall tires, as the taller
tires reduce your torque multiplication just like a "taller" (lower numerically)
final drive (top gear * axle ratio) would.
Here's an example. the 44" tire will rotate about 458 times in a mile, whereas the
24" will turn 840 or so. With the same axle ratio and transmission ratios, (we'll
assume a 3.5:1 axle ratio and 1:1 (let's call it forth) gear, the truck (we'll call it a
Nissan Hardbody) will turn 1600rpm at 60mph, and the car (we'll call it an Altima) will
turn about 3000.
In our above example, we'll put in Nissan's
2.4L motor and give it 150hp @ 4600rpm and 135ft/lbs of torque @ 2800rpm. Right here we
can already see the Altima will have an advantage, as it's right near the torque peak,
while the truck is WAY below it. Same gearing, same engine, and we haven't messed with
weight yet, but we can see if it's ALSO the same between the two (it isn't) the car will
smoke the truck JUST BECAUSE OF THE TIRE SIZE!! Also keep in mind the tire size affects
every gear and every speed.
Your axle (final drive) ratio, like your tire size, affects
performance in every gear. Most people choose an axle ratio for the intended use of the
car, which is OK as long as you are also looking at the rest of your equipment. Let's say
you have two Z cars, and you want one for highway cruising, and the other for racing. No
problem, just put a 5.33:1 differential in one, and a 2.08:1 in the other, right? Sure, it
will point them in the general direction, but the 2.08 car will be a slug, and the 5.33:1
may be short on top speed. The key here is to match it to the tire sizes, transmission
ratios and engine characteristics. The 5.33 will be fine in the race car if A)
It's a shorter track with no long stretches or B) The engine can rev to
8-9000rpm. The 2.08 car will be OK if it makes a LOT of torque at LOW rpm (more like a big
displacement V-8 than a V-6). With this in mind, you can "do the math" and see
what axle/tire/transmission combination will give you what rpm's at what speeds.
These days you can get Nissan 5-speeds with .75:1 overdrive (anything lower than 1:1 is
overdrive, by the way), and the GM 6-speeds are 0.50:1(!!), so you don't have to sacrifice
a snappy launch of shorter axle gearing for relaxed highway cruising like you used to
before overdrive transmissions. Also, the more gears in between 1st and top gear (5th or
6th), the better (to a point - I don't think Kenworth's with 13+ forward gears come into
play here). The extra gears help keep your engine "on the boil" for a longer
period of time, aiding acceleration - ESPECIALLY if your engine has a narrow powerband
(ever ride a two-stroke motorcycle??). If your engine has a nice wide powerband (see Torque&HP) this is less of an issue, but it still applies.
You want a short enough first gear to get you a great launch, but not so short that you're
shifting at 15mph. the next several gears should keep your engine between the torque and
horsepower peaks, and shifting up shouldn't cause the engine to "bog".
An example of less than ideal (from a
performance standpoint) gearing is the '94 Civic DX 5-speed mated to the 1500cc 4 cyl. The
1500 has a peaky (small) powerband (fun, but small) and winding it out in first, shifting
to second produced a VERY noticeable loss in acceleration. The 1st-2nd shift was around
30, and the motor wouldn't get back into its powerband until 40 or so. Gearing for
performance cars should have this result, and thus acceleration would be much quicker and