a diff to better suit larger wheels/tires is a good idea and in some cases is a
If fitting larger tires to a 4x4 which are 3-4 sizes bigger (stock was 30 inch,
new tires 33-34 inches) or even larger than that, it will affect the way the
vehicle will drive.
The differential gear ratio on a stock standard 4x4, are made to suit the
tire diameter which the vehicle is released with.
Using bigger diameter wheels/tires with stock ratios will cause more wear and
tear on clutch, driveline and engine.
Low range gear will also suffer as 1st
gear low will now roll too fast as the tires are bigger, this becomes a problem
when descending steep hills and climbing rocks and logs.
to fix the problem:
a stock 2005+ Toyota Hilux has 29 inch wheels/tires and fitting 33 inch rubber
is quite bigger (4 inches in diameter bigger) this
will cause the engine and
entire driveline more stress and more power is then required due to the larger
The final gear ratio is suited for 29 inch, this ratio in
this particular case is 3:5, referring to a chart it will state the same (29'
recommended ratio is 3:5) and looking at 33 inch diameter the chart will
Going by the chart any higher will make
the 4x4 rev too much at highway speeds causing poor fuel economy.
Changing the crown wheel and pinion is how the ratio is modified. Basically
installing a new or reconditioned crown wheel and pinion with the recommended
ratio will bring the 4wd driveline back to stock specification rpm and fuel
Once differential gear ratio mod is done the vehicle will regain its 1st gear low
range and take off in high range will be improved or even back to stock.
Differential Gear Ratio
This chart is an approximant tyre size to differential gear ratio; it’s based on an RPM at 105 km/h (65 mph) and 4th gear on a manual transmission or 3rd gear on an automatic gear box (1:1 gear ratio).
This is only a guide to get a general idea of which ratios would suit the
vehicle in question.
To get an accurate RPM the following steps
and formula will give an accurate answer:
Take a drive and record the rpm
at 105 km/h (65 mph), the next step is to work out what the current ratios are
in the vehicle. This can be revealed by calling the manufacturer or even
calling a differential repairer giving him/her the Vehicle Identification
Number, also known as VIN.
Once all this information is known use
the following formula to get the accurate differential gear ratio for the
The rpm is 2806 at 105 km/h (65 mph), the current tires are 29 inch, the factory ratio is 3.73 and the new size upgraded tires
are 33 inch.
Stock tires: 29
Stock ratio: 3.727
New Tyre size: 33
2806 = 65 x 3.727 x 336 / 29 (rpm = mph
(not km/h) x diff ratio x 336 / tire size)
Now the correct rpm is known for the
vehicle (2806), the next step is working out what ratios are available for the
vehicle, some makes and models won’t have as many choices as others.
Say the following 3 ratios are
available 3.909, 4.100 & 4.88.
The new tire size wanting to be fitted
is 33 inch and the rpm needs to be a close to the 2806 as possible in order to
keep it to stock specs. Just a quick scan at the differential gear ratio chart it’s obvious that a ratio of
3.909 is too low at 2587 rpm, ratio of
4.88 is too high at 3230
rpm. But looking at 4.10 with the rpm at 2713 is the
closest to 2809 stock rpm.
New tires: 33
New ratio: 4.10
Fuel Economy after
the tire & diff mod:
Even with the help from the differential
gear ratio chart and the right diff ratio installed, the vehicle will
never regain its fuel economy it had before simply due to the fact that larger
tires will have a bigger rolling resistance.
reason for not changing diff ratio:
is not always required as keeping to stock differential gear ratio with only a slightly bigger tyre size can help
with fuel economy at higher speeds. This happens as a larger tyre will reduce
the amount of rpm at speed.
Example: 29 inch diameter at 100kph = 2674 rpm in 4th gear manual or 3rd gear automatic (gear box ratio of 1:1) with 31 inch diameter at 100kph = 2501
rpm in 4th gear (gear box ratio of 1:1). At cruising speeds on a highway 100kph
this will save fuel.
When & why you should regear your Wrangler, technical guide & approximately cost click here.