Gear Ratios. Last month one of our club
members challenged me to explain what gear ratios meant. I considered this a pretty tall
order, and I thought it could take me a while to explain. However, I decided that others
might also like to know so I have recruited assistance from a mechanical expert - John
Parkinson, who has been able to put it very simply for us.
The term "Gear Ratios" means the difference in speeds of 2
meshed gears expressed in numerical terms. In other words, a gear ratio of 3 to 1 (usually
written as 3:1) means that a drive gear rotates 3 times for every 1 turn of the driven
gear that it is meshed with. Altering the number of teeth on a gear will obviously change
the ratio.
To understand ratios in a 4WD you must understand the mechanical layout
of the vehicle, and it goes like this:- The engine drives (via the clutch) the gearbox,
which then drives the transfer case, which then drives the rear axle (or both front and
rear axles when in 4WD). All these driven units have gear sets with many different ratios.
Let’s do the gearbox first - The ratio of first gear (on a Pajero
NJ) is 3.95:1. This means the input shaft (driven by the engine) turns 3.95 turns to every
1 turn of the gearbox output shaft. This rotating output shaft then drives the input shaft
of the transfer case. in high range, the input and out put shafts of the transfer case
rotate at the same speed (ratio 1:1). This then drives the rear axle, and its ratio (often
referred to as the diff. ratio) is 4.9:1 (4.9 turns of the tail shaft to 1 turn of the
rear axle).
You can work out the OVERALL RATIO by multiplying them all together,
like this: 3.95 X 1 X 4.9 = 19.35. So in this gear (first gear, high range) the engine
rotates 19.35 turns for every single turn of the axle.
Let’s select low range in the transfer case, which has a ratio of
1.925:1. The overall ratio calculation is now: 3.95 X 1.925 X 4.9 - 37.26. So in first
gear / low range, the engine turns 37.26 times for every single turn of the rear axle
shaft.
Speedo Error. Correcting an incorrect speedo is easy - simply replace the speedo
drive gear (on the transfer case end of the speedo cable) with one of a different size.
(Refer to the relevant workshop manual in the club library for fitting instructions).
These gears come in many sizes, and they all interchange. Simply measure the percentage of
error, count the teeth of your existing gear, decide on the size of the new one using the
table below and fit it.
There are three ways to find the percentage of error. Run on a dyno and
compare the speedo readings, drive over a fair distance and monitor your trip meter
against the kilometre posts on the highway, or go the RACQ for a check for only a few
$’s.
