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Thread: Differential ratios

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  1. #1 Differential ratios 
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    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
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  2. #2 Re: Differential ratios 
    Club Member dcunn's Avatar
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    My Hunter has the 2300 dohc and five speed box. The fitted 3.92 was noisy and I "happened" upon a serviceable 3.62.

    I don't understand all the techy bits, but I still have reasonable acceleration, and tend to drop to 4th whenever in
    a 30 mph area (mental reminder ref speed limit)

    To be honest, the change has not really been that noticeable

    Rgds DC
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  3. #3 Re: Differential ratios 
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    Changed mine from 3.62 to 3.38 on Bmw 6 cylinder/5 speed. Mike Garner has I think 3.14 which he thinks is probably to tall.
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  4. #4 Re: Differential ratios 
    Club Member Mike's Avatar
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    Quote Originally Posted by dogoncrazy View Post
    Changed mine from 3.62 to 3.38 on Bmw 6 cylinder/5 speed. Mike Garner has I think 3.14 which he thinks is probably to tall.
    Yes, I have a Sierra 3.14, which matches the BMW 3.15 that went with my engine in the E36 325i. (My tyres are a match for the rolling radius of the BMW, so my gearing matches the donor perfectly).

    I had expected with a 2.5 straight 6 with 200HP and 200ft.lbs of torque that it would pull the same gearing in my Cabrio quite easily.

    As Alan says, I am contemplating whether it may be too tall, requiring me to change down from 5th when travelling slowly (below 50mph) up a mild hill, though having driven to Stoneleigh and back I am beginning to wonder if the Vanos (Variable Valve Timing) seals are shot in my engine which all the forums suggest will account for a slight rattle at 3000rpm (which mine has) and sluggish performance upto 3000rpm, but very good acceleration thereafter to redline (which again mine has).

    So, before I change my diff back to a 3.38, I am considering fitting a Vanos seal repair kit (not too expensive if you do it yourself) to see how it then performs.

    And if all that does not give me the low down torque I crave, then I have another lump of metal in my garage that should solve the problem...........???? watch this space, as they say!!

    To answer your question regarding your Zetec, it comes back to personal preference. How much acceleration do you want, and what cruising rpm do you want?
    Personally I do not want a screamer of an engine, which is why I chose a straight 6 hoping it would have bags of low down torque, which would allow me to gear my Cabrio pretty tall to keep the cruising revs right down - it is currently pulling 70mph at 3000rpm, so identical to yours in your overdriven 5th gear. With the Zetec producing less torque though you will need to change down more frequently.
    Since you have gone for bigger wheel/tyres, a 3.9 diff may give you an easier drive, but at the expense of a few more revs at 70mph.

    It comes back to what you prefer.
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  5. #5 Re: Differential ratios 
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    FMB has a 3.38:1 diff coupled with 205x60 15 inch wheels mainly cos the gearing was correct for the SD1 Speedo.
    The V8 has huge amounts of torque. When I first ran it the gearbox remote failed and I drove back from Derby to Cheshire in 4th. Traffic lights and roadworks, nothing seemed the phase it.
    The gearing has a maximum of 135mph but I doubt it would pull that sort of speed. However 3rd is good for 85mph and is a great overtaking gear.
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  6. #6 Re: Differential ratios 
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    Hi Millie, We hope we have the right car and driver . We were chatting at Stoneleigh and I think you are from the Isle of Wight. We are over there next week after the London to Brighton and would like to meet up if it fits in with you. Apologies if it is not you ! Email ian.papworth@yahoo.co.uk
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  7. #7 Re: Differential ratios 
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    Hi - There is lots of discussion on ratios in older threads if you want to spend the time going through it. My car has a tuned 3.5 V8 good for about 200bhp, so its about as powerful as they come. Wheels+tyres are 15" + 195/65. The diff is a 3.92, ex Sierra 1.8, and in 5th at 70mph the engine is doing 2800rpm, so not excessive to my mind. Should be good for about 150mph flat out, but I doubt any Cabrio would do more than 120mph as it would be like driving a block of flats! 1st gear is pretty redundant, but I don't seem to suffer unduly with wheelspin even in that, but I know Derek has some trouble with it (are you sure it is in standard tune Derek?). On the Scottish trip I averaged 24mpg over 800miles, but if economy is a primary concern, you don't fit a V8! The only caveat on all this is, are the gauges, etc to be believed? Hope that helps someone, but in my opinion, the vast majority of Cabrios will be fine using 3.92 or 3.62 diffs which are cheap and plentiful, and pretty much bullet-proof. Dave.
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  8. #8 Re: Differential ratios 
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    When I built my Berly the first time it had a 3.38 diff and at 70 in 5th it was just on 2000 rpm. This was too tall so this time with the restoration I am trying a 3.92.
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  9. #9 Re: Differential ratios 
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    Quote Originally Posted by dave.kitson View Post
    My car has a tuned 3.5 V8 good for about 200bhp, so its about as powerful as they come. Wheels+tyres are 15" + 195/65. The diff is a 3.92, ex Sierra 1.8, and in 5th at 70mph the engine is doing 2800rpm, so not excessive to my mind.

    The only caveat on all this is, are the gauges, etc to be believed?
    Dave.
    Hi Dave
    Are you running a gearbox with a 0.8 over driven final gear?

    Using one of the web site tyre calculators your tyre's rolling radius (maximum when new) is 1993mm. If you then calculate using a 0.8 over driven gearbox and a 3.92 diff your actually speed should be 66mph at 2,800rpm.
    70mph should come up at 2,950rpm.

    This suggests your speedo is over stating your true speed by around 7% ? - not a bad thing if you want to avoid speeding tickets!
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  10. #10 Re: Differential ratios 
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    Hi Mike
    As far as I know, the gearbox is standard agricultural SD1, and Haynes says that 5th is 0.833:1.

    The rolling radius you quote is for the wheel I think, clearly the tyre is about 50% more than this, so I'm not sure where that leaves your calculation. Even if I'm doing 3000rpm at 70mph, I don't think this is that high.

    The speedo was pretty much spot on at SVA 8 years ago, so if an error exists I'd suspect the tacho. More likely there is a discrepancy due to tyre wear, as I've now done 20k on one set of Toyos. Pretty good compared with some other folks experience! Having said that, as the circumference is directly proportional to the diameter, I can't see worn tyres making more than 3% difference to the readings. So probably some tacho error too!
    Cheers - Dave
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