Differential sizing

[milliemarlin]

I hope this might be useful to compare ratios. My car, Millie, is based on a 2litre Ford Sierra with a Pinto engine, type 9 gearbox and 3.62 differential. The engine has been replaced with a 2L Zetec.

Millie as she is 3.62 diff

			[B]Engine rpm
Gear          	Ratio	1000	2000	3000	4000	5000	6000	6500	7000
Gear 1st[/B]	[COLOR="#808080"]3.656[/COLOR]	5.3	10.5	15.8	21	26.3	31.6	34.2	36.8
[B]Gear 2nd[/B]	[COLOR="#808080"]1.97[/COLOR]	9.8	19.5	29.3	39.1	48.8	58.6	63.5	68.3
[B]Gear 3rd[/B]	[COLOR="#808080"]1.37[/COLOR]	14	28.1	42.1	56.2	70.2	84.2	91.3	98.3
[B]Gear 4th[/B]	[COLOR="#808080"]1[/COLOR]	19.2	38.5	57.7	76.9	96.2	115.4	125	134.6
[B]Gear 5th[/B]	[COLOR="#808080"]0.82[/COLOR]	23.5	46.9	70.4	93.8	117.3	140.7	152.5	164.2

This is the standard 2L Sierra Pinto engine with type 9 gearbox with 195/ 65 R15 wheels. The original Sierra donor had 185/70 R13 Wheels and tyres (195/60 R14 are virtually the same). The larger wheels on Mille mean slightly less acceleration. At 70mph in 5th is about 3,000 rpm in the Sierra it would require 3,200 rpm. The difference is more than compensated by a lighter car.

Millie with 3.9:1 Diff

						
[B]Gear 1st[/B]	[COLOR="#808080"]3.656[/COLOR]	4.9	9.8	14.7	19.5	24.4	29.3	31.7	34.2
[B]Gear 2nd[/B]	[COLOR="#808080"]1.97[/COLOR]	9.1	18.1	27.2	36.3	45.3	54.4	58.9	63.4
[B]Gear 3rd[/B]	[COLOR="#808080"]1.37[/COLOR]	13	26.1	39.1	52.1	65.2	78.2	84.7	91.2
[B]Gear 4th[/B]	[COLOR="#808080"]1[/COLOR]	17.9	35.7	53.6	71.4	89.3	107.1	116.1	125
[B]Gear 5th[/B]	[COLOR="#808080"]0.82[/COLOR]	21.8	43.5	65.3	87.1	108.9	130.6	141.5	152.4

This diff compensates for the larger wheels 70mph is again at about 3,200 rpm.

A useful way to see the data is what rpm is needed for a given speed (3.62 diff).

[B]	RPM	1000's[/B]							
[B]Speed mph	20	30	40	50	60	70	80	90	[/B]
[B]Gear 1st[/B]	3.8								
[B]Gear 2nd[/B]	2.0	3.1	4.1						
[B]Gear 3rd[/B]	1.4	2.1	2.8	3.6	4.3				
[B]Gear 4th[/B]		[COLOR="#FF0000"][B]1.6[/B][/COLOR]	2.1	2.6	3.1	3.6	4.2	4.7	
[B]Gear 5th[/B]		1.3	1.7	2.1	2.6	[COLOR="#FF0000"][B]3.0[/B][/COLOR]	3.4	3.8

With the 3.62 diff 30mph is a comfortable 1,600 rpm in fourth and 70mph 3,000 rpm in fifth.

I would be interested to see which diff ratios and gearboxes others have used. I would particularly like to know how happy they are with the result.
Paul