# Technical forum ( Diff ratios)

```There has been some discussion about back axle ratios and top speed.
If the final differential ratio is too high the car will have good
acceleration but will have to pull ridiculous revs to give a high top
speed. To calculate what the top speed will be for a car you need to
find how many MPH per 1000 engine revs the car will do in top gear
then multiply by the max revs the engine will pull in top. To follow
the formula you will need to determine:

Tgr : 	Top gear ratio for the gearbox to be used. Look in a manual.
It is often 1 or 1.1 for Ford gear boxes.

Rr : Rolling radius of tyres and wheel. Measure from center of wheel to the
ground in mm or calculate as follows: Wheel diameter in inches * 12.7 +
(Tyre size * aspect ratio/100)
For my car with 13 inch rims fitted with 175/70s
13 * 12.7 + (175 *70/100)  = 287.6

Dr : Differential ratio. For Vauxhall Viva/Magnum axle the ratio is indicated
by a letter stamped on the pinion housing - on the top for 1800&2300cc
OHC engines,  bottom for 1256cc:
A - 3.88, B - 4.125, D - 3.45, K - 3.727, none on later 1256 cars - 4.125

First calculate revs of back wheels per hour :
(60000 * Tgr ) / Dr		=  Rph
For my car :	(60000 * 1)/ 3.45	  	= 17391
Secondly calculate distance per rev of back wheels in miles:
(Rr  * 6.263)  /1000 * 0.000621  	= Dpt
For my car :  	(287.6 *6.263)/1000 * 0.000621  	= 0.00011185
Finally : 	(Rph * Dpt ) 	     	= miles per 1000 rpm
17391 * 0.00011185	     	= 19.45 mph per 1000 rpm

This means at 4000 RPM (comfortable cruising) the car will do (19.45 * 4) = 77.8 mph.
At max revs (19.45 * 5.5) = 107 mph. You can use the same formula for the other
gears by using that gear ratio in Fdr to get the top speed in any gear.
You can also see the affect of changing a wheel and tyre size by altering
the rolling radius and reworking the figures. Note: Max engine revs may be higher
than best cruising revs.```