There was some discussion at Stoneleigh about differential ratios. There has also been some discussion on these forums. I am a somewhat confused as to what the best diff ratio would be for Millie. The Sierra donor had 13 inch diameter wheels and Millie has the same gearbox & diff but 15 inch diameter wheels. Millie is lighter than the Sierra but has a greater wind resistance. The car is ok but at 30mph the slightest incline requires changing down. This does result in more gear changes round town that is really necessary. Or more likely, I leave her in 3rd gear more than I should. She copes ok in top gear at 70mph (or thereabouts).
My calculations show that by changing from a 3.62 to a 3.9 diff the engine revs to speed would be returned to within 1mph of the Sierra. I imagine that this is the optimum ratio for her. But others seem to think going lower (3.14?) is better for their cars.
So what would be the best ratio for a 2 Litre Zetec?
What would be the best ratio for other engines?
Engine speed to vehicle speed charts.
Millie as she is 3.62:1 Differential
Engine rpm -> 1000 . 2000 . 3000 . 4000 . 5000 . 6000 . 7000
Gear Ratio Speed in mph
1st .. 3.656 .... 5.3 .. 10.5 .. 15.8 .. 21.0 .. 26.3 .. 31.6 .. 36.8
2nd . 1.970 .... 9.8 .. 19.5 .. 29.3 .. 39.1 .. 48.8 .. 58.6 .. 68.3
3rd .. 1.370 .. 14.0 .. 28.1 .. 42.1 .. 56.2 .. 70.2 .. 84.2 .. 98.3
4th .. 1.000 .. 19.2 .. 38.5 .. 57.7 .. 76.9 .. 96.2 . 115.4 . 134.6
5th .. 0.820 .. 23.5 .. 46.9 .. 70.4 .. 93.8 . 117.3 . 140.7 . 164.2
Engine revs at 30mph in 4th gear 1560 rpm. Engine revs at 70mph in 5th gear 2984 rpm
3.9:1 Diff within 1 mph of the Sierra
1st .. 3.656 .... 4.9 ... 9.8 .. 14.7 .. 19.5 .. 24.4 .. 29.3 .. 34.2
2nd . 1.970 .... 9.1 .. 18.1 .. 27.2 .. 36.3 .. 45.3 .. 54.4 .. 63.4
3rd .. 1.370 .. 13.0 .. 26.1 .. 39.1 .. 52.1 .. 65.2 .. 78.2 .. 91.2
4th .. 1.000 .. 17.9 .. 35.7 .. 53.6 .. 71.4 .. 89.3 . 107.1 . 125.0
5th .. 0.820 .. 21.8 .. 43.5 .. 65.3 .. 87.1 . 108.9 . 130.6 . 152.4
Engine revs at 30mph in 4th gear 1680 rpm. Engine revs at 70mph in 5th gear 3215 rpm
Apparently, according to the Lowcost web site, the Ford diff is available in the following ratios (some must be very rare):
3.12, 3.14, 3.26, 3.31, 3.33, 3.36, 3.56, 3.58, 3.59, 3.62, 3.77, 3.82, 3.842, 3.92, 4.056, 4.27, 4.29
My calculations show that by changing from a 3.62 to a 3.9 diff the engine revs to speed would be returned to within 1mph of the Sierra. I imagine that this is the optimum ratio for her. But others seem to think going lower (3.14?) is better for their cars.
So what would be the best ratio for a 2 Litre Zetec?
What would be the best ratio for other engines?
Engine speed to vehicle speed charts.
Millie as she is 3.62:1 Differential
Engine rpm -> 1000 . 2000 . 3000 . 4000 . 5000 . 6000 . 7000
Gear Ratio Speed in mph
1st .. 3.656 .... 5.3 .. 10.5 .. 15.8 .. 21.0 .. 26.3 .. 31.6 .. 36.8
2nd . 1.970 .... 9.8 .. 19.5 .. 29.3 .. 39.1 .. 48.8 .. 58.6 .. 68.3
3rd .. 1.370 .. 14.0 .. 28.1 .. 42.1 .. 56.2 .. 70.2 .. 84.2 .. 98.3
4th .. 1.000 .. 19.2 .. 38.5 .. 57.7 .. 76.9 .. 96.2 . 115.4 . 134.6
5th .. 0.820 .. 23.5 .. 46.9 .. 70.4 .. 93.8 . 117.3 . 140.7 . 164.2
Engine revs at 30mph in 4th gear 1560 rpm. Engine revs at 70mph in 5th gear 2984 rpm
3.9:1 Diff within 1 mph of the Sierra
1st .. 3.656 .... 4.9 ... 9.8 .. 14.7 .. 19.5 .. 24.4 .. 29.3 .. 34.2
2nd . 1.970 .... 9.1 .. 18.1 .. 27.2 .. 36.3 .. 45.3 .. 54.4 .. 63.4
3rd .. 1.370 .. 13.0 .. 26.1 .. 39.1 .. 52.1 .. 65.2 .. 78.2 .. 91.2
4th .. 1.000 .. 17.9 .. 35.7 .. 53.6 .. 71.4 .. 89.3 . 107.1 . 125.0
5th .. 0.820 .. 21.8 .. 43.5 .. 65.3 .. 87.1 . 108.9 . 130.6 . 152.4
Engine revs at 30mph in 4th gear 1680 rpm. Engine revs at 70mph in 5th gear 3215 rpm
Apparently, according to the Lowcost web site, the Ford diff is available in the following ratios (some must be very rare):
3.12, 3.14, 3.26, 3.31, 3.33, 3.36, 3.56, 3.58, 3.59, 3.62, 3.77, 3.82, 3.842, 3.92, 4.056, 4.27, 4.29
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