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Gearing Formula

Somewhere along the line, I acquired "the formula" to calculate what sort of speed in mph I might expect, from running whatever 2CV gearbox, together with one of the final drive ratios that are available.

Most people use the standard 2CV6 gearbox (disc-brake version) which comes with : First gear 5.20, Second Gear 2.65, Third gear 1.78 and Fourth gear 1.31 and a final drive ratio of 4.125.

The first variable in calculating eventual road speed is tyre selection. 2CVs and Dyanes came with 125 X 15 or 135 X 15 radials. Using alternative wheels (like the MWS wire wheels Phil Gregory supplies) it is possible to fit tyres of up to 165/70 X 15 section (crossply Camacs of 5.60 X 15 size) which have a much increased rolling radius. Using these larger section tyres, an increase in road speed over standard gearing and tyres of around 10% is likely.

60,000/(gear ratio x Diff ratio) = Revolutions per hour per wheel

Revs per hour per wheel X rolling radius/36 = yards per hour

Divide result by 1,760 to get mph per 1,000 rpm in any chosen gear.

So, standard 4th gear (1.31) with 165 X 15 tyres (rolling radius 2049mm) and 4.125:1 final drive is 14.1 mph per 1,000 rpm

Standard 4th gear (1.31) with 165 X 15 tyres (rolling radius 2049mm) and 3.875:1 final drive is 15.03mph per 1,000 rpm

Standard 4th gear (1.31) with 165 X 15 tyres (rolling radius 2049mm) ans 3.625:1 final drive is 16.07mph per 1,000 rpm

The 3.875:1 and 3.635:1 final drives are both Dyane-based. The longer 3.625:1 version may be difficult to source.

Mike Meakin

Find out more about gearboxes and final drive calculations for Citroen CVs on Jeroen Cats' website. There is a huge amount of information on Citroën cars available on this site.

You should be able to see the effects of different gearbox/crown wheel and pinion and tyre sizes on the overall gearing of your car and predict its performance for various engines. The one unknown for a Pembleton is the "A" value (the drag of the car) which should be smaller than that of a Lomax. It should be possible to estimate "A" from the known performance of a Pembelton. It would be useful if this was done for a number of vehicles

There was an item in ePAG#10 page 3 on gearing by Crackleport. There's a lot of good stuff in the archives - it's well worth using the search facility on the website Home page.

The Longstone Tyres website has the rolling diameters for a large range of tyres

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