Although you should take the time to do the math and find out which
ratio is best for your truck and tire combination, to make things easier
we've put together the following chart. This information will give you
a general idea what ratios to shop for, but always discuss your gearing
needs with a professional at SOUTHWEST FOUR WHEEL DRIVE (local 602-468-0022 out of area 1-800-653-5918) before buying.
The chart is divided into three categories.
The rpm highlighted in black are
optimum for the tire size and ratio combination. These will most
closely give you the best overall drivability.
If you're looking for a ratio that will give better fuel economy for
highway driving at the expense of some acceleration and overall
performance, choose a ratio in the yellow
shaded area.
If you're looking for a ratio that will improve acceleration and towing
power at the expense of some fuel economy, choose a ratio in the blue shaded area.
The Formula
2242 = 65 x 3.08 x 336 / 30
To use an example, let's say we want to know what gears should be
installed when putting 36" tires on a transmission, 3.08 gears, and 30"
tires in stock form. Since the truck doesn't have a tach, we first need
to find out what RPM the engine is spinning at a given speed, say 65
mph. With this information, the formula looks like this:
rpm = mph x gear ratio x 336 / tire diameter
At 65 mph, the truck's engine is spinning at 2242 rpm, which is right
in the optimum powerband for most V-8 engines. Now all that's left to
do is plug in the new tire size and a couple different ratios available
for the truck to see which one closely matches the RPM.
2263 = 65 x 3.73 x 336 / 36
As you can see from the above formula, 3.73 gears with 36" tires is
optimum for restoring stock performance with this truck. Keep in mind
that not all ratios are available for every axle, so do some research to
make sure there's a ratio close to what you need available for your
truck's axles.
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Whenever larger-than-stock tires are installed on a truck, it will have a
direct effect on the truck's performance. Why? It has to do with the
effective gear ratio. Your truck comes from the factory with the
optimum axle gear ratio to work with the truck's engine, transmission,
and stock tire combination as well as provide a good balance between
acceleration and fuel economy. When taller tires are installed on a
truck but the axle ratios stay the same; the effective gear ratio is
reduced. This means the engine is forced to operate below its power
band, and performance and fuel economy suffer as a result. In order to
restore the effective gear ratio (and the truck's performance), you'll
need to have the axle gears swapped to lower (numerically higher)
ratios.
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Fortunately, finding the correct gear ratio to run with the tires that
you plan to install is simple using a mathematical formula. There are
actually several different formulas available to determine the correct
ratio with varying degrees of difficulty. The simplest and most
reliable method we've found is to determine what engine RPM the truck
runs with the stock gears and tires at a given speed, and then closely
match this rpm with the larger tires. In order to use the following
formula, you'll need to know the stock axle gear ratio as well as the
stock tire size. It also helps to know what RPM the truck runs at a
given speed with the stock setup, but it's possible to find this
information with the formula. Keep in mind all RPM readings should be
taken with the transmission in a 1:1 ratio (Fourth with most manuals or
Third with an automatic).
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One
last note regarding trucks equipped with overdrive. The Overdrive gear
in a transmission allows the drivetrain to run below a 1:1 ratio, which
brings down engine RPM for good fuel economy when running down the
highway. Most Overdrive gears run somewhere around 0.73:1. As we said
before, the best option for people who use the truck primarily on the
street is to match the formula given above. However, Overdrive offers a
unique opportunity for dedicated off-roaders. Since overdrive
decreases the reduction at highway speeds, it's possible to "cheat" and
select gears that are a little lower (numerically higher) for better
slow gearing off-road without sacrificing highway drivability. This
advantage becomes apparent when you do the math by multiplying a
prospective gear ratio by the Overdrive ratio of your transmission. To
use an example, 5.13 gears in a truck without an overdrive and
moderate-sized tires would offer awesome slow-speed crawling ability for
off-road situations but would leave the
engine screaming at highway speeds. However, if the same 5.13s are
installed in a truck equipped with an overdrive of 0.73:1 and we do the
math (5.13 x 0.73 = 3.744), we find that the Overdrive gear gives the
effect of a very streetable 3.73 ratio for highway cruising. In other
words, the truck has the same good crawling speed of 5.13 gears (because
Overdrive is never used off-road) combined with a very street-friendly
effective ratio of 3.73 gears. Of course the engine will still rev
higher on the highway compared to having the proper ratios found by
following the steps we laid out earlier. Overdrive does offer dedicated
four-wheelers the option of installing low, off-road friendly gears and
still be able to drive around on the street with some fuel economy left
intact. To plug overdrive into the gearing formula above, simply
multiply the Overdrive ratio by the axle gear ratio.
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Note: This chart is based on engine rpm at 65
mph with the transmission in a 1:1 gear ratio (Third with a three-speed
manual, Fourth with a four-speed manual, or Third with an Automatic).
Remember, the actual rpm indicated in the chart will be slightly higher
(between 100 and 300 rpm) on vehicles equipped with an automatic
transmission. This is due to the slip present in an auto transmission's
torque converter.