What's rear
end gear ratio?
How does
rear en ratio affect your vehicle's performance?
How to
calculate your rear end, gear, rpm ratio?
Rear end
Ratio is the ratio of how many times the drive shaft turns to how many times the
axle turns. This is set
up by how many teeth are on rear end's ring and pinion gears.
A higher ration
number will give you more torque (power) at the rear wheels, better
acceleration ( more low end pulling power), but lower top speed and less fuel
mileage.
A Lower
ration number will give you lower acceleration but higher RPM's, more high
speed (RPM's) and also more fuel consumption, the higher RPM's the poorer your
gas mileage is.
A bit
confusing but... Higher ratio is referred to as lower gearing (geared down) and lower
ratio is referred to as higher gearing. Power is
transmitted from the transmission to rear end 1:1 in high gear.
For example,
If your vehicle is equipped with 3.55 gear, the ratio is 3.55 to 1. Meaning
that for every
3.55 times that the driveshaft turns, the rear axle turns one time and you will have a torque advantage of 3.55x. So for every 10 turns of the (rear) wheels, your drive shaft will turn 35.5 times and the engine will only turn 35.5 times in high gear.
3.55 times that the driveshaft turns, the rear axle turns one time and you will have a torque advantage of 3.55x. So for every 10 turns of the (rear) wheels, your drive shaft will turn 35.5 times and the engine will only turn 35.5 times in high gear.
If your
vehicle is running a 3.73 rear end gear set up... the ratio is 3.75 to 1. This means
that for every 3.73 turns of the driveshaft, the rear axle will be turned one
time and you will have a torque advantage of 3.73x. In this
case every 10 turns of the (rear) wheels,
your drive shaft will turn 37.5 times and the engine has to turn 37.5 times in
high gear.
Another
example to make it clear... when your vehicle is running a 4.10 rear end gear
set up... the ratio is 4.10 to 1. This
means that for every 4.1 turns of the driveshaft, the rear axle will be turned
one time and you will have a torque advantage of 4.1x. In this
case every 10 turns of the (rear) wheels,
your drive shaft will turn 41 times and the engine has to turn 41 times in high
gear.
Keep in
mind... that the gearing is relative to the size tire your are using. As
mentioned above... first two examples, the 3.55 will give better gas mileage
over the 3.73 for a given tire size range. Get above this range, say for
3.55 gears, and the performance will suffer, as well as putting additional
strain on the tranny, gears, etc.
Didn't mean
to complicate this article, but you can't talk gearing without mentioning tire
size, and effecting the RPM's.
The "higher"
gearing number (ex. 4.10 vs. 3.75) will allow for larger tires that will come
close to offsetting the RPM diff.
RPM - Rear
End Ratio Chart.
Following chart
can help you decide which rear end ratio is the best for you / your vehicle.
Listed are
the expected RPM's at 60 MPH with a given rear end ratio and tire height.
The Chart
applies to all Ford, both Automatic & Manual transmissions.
NON
OVERDRIVE... such as: C-4 three speed automatic, C-6 three speed automatic, FMX three speed
automatic, Top-loader three speed manual and Top-loader four speed, manual
Running an overdrive transmission, the RPM's will be about 32% less.
The Formula to calculate RPMs is...
RPM = MPH X Trans Ratio (in high gear) X Rear End Ratio X 336 : Tire Height
Below are three useful ratio / gearing / rpm calculator Links,
Clear as mud
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