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Thread: Differential ratios

  1. #11 Re: Differential ratios 
    club member Club Member Mike's Avatar
    Join Date
    Nov 2011
    Gosberton, South Lincs
    Hi Dave

    Sorry, should have read "Rolling circumference is 1,993mm".............. but there was still a fault in my maths somewhwere!

    Calculation based on:
    Rolling circumference = 1993mm = 78.4" (source: )
    63,360" per mile

    If your gearbox is an early one using 0.833, then
    Engine rpm = 3,000 rpm
    Gearbox output rpm = 3000/0.833 = 3601rpm
    Diff output rpm = 3601/3.92 = 918.7rpm
    Tyre revs/hr = 918.7 x 60 = 55,124 r/hr
    Speed = 55,124/hr x 78.4"/63,360 = 68.2mph - speedo reading of 70mph is perfect!
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  2. #12 Re: Differential ratios 
    regular forum contributor
    Join Date
    Jan 2012
    Hi, not sure if people know about this gear ratio calculator which can be found on the following website
    ( It is an excel spreadsheet that lets you play around with different ratios and tyre sizes and gives a chart and graph of the results. The spreadsheet link is towards the end of the page and can be downloaded. The site also gives some excellent information about Ford Sierra/Granada differentials.
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  3. #13 Re: Differential ratios 
    club member Club Member
    Join Date
    Nov 2011
    Thanks to all that have responded giving some very useful information.

    The Scottish run was my first chance to experience Millie’s engine after the rolling road tune up. The engine is running much better than it was and has more power than the old Pinto. So I find the current diff ratio is perfect. I also notice that the engine is running cooler than it was too.
    As for noise, when cruising at 70 on the motorway at 3,000 rpm I can hardly hear the engine. I still can’t listen to the radio on long journeys, because the wind noise with the top up or down is still deafening.

    For reference this is what I used to set the dip switches on Millie’s Speedo which the SVA inspector said was spot on.
    Tyre width: 195 mm, Profile: 65 %, Tyre Height: 126.75 mm, Rim: 15 inches = 381 mm, Flat at the bottom of the tyre allowance: 20 mm, Rolling radius: 297.25 mm, Circumference: 1867.68, mile = 1609.344 m, Wheel rotation: 861 revs/mile, Diff ratio: 3.62, Propshaft/engine: 3119 Revs/mile, Magnets: 4, Pulses: 12477 per mile.

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