Speedo change and Diff Ratios
 

Hi all,

I need a new speedo for my 3.46 diff S124 230TE (UK model) and it's proving very hard to find one with the ASD warning triange as they weren't often specced on the 4-cylinder cars, and the original part isn't available anymore. I've found one new that is for a 230CE but the diff ratio for that is 3.27 ... does anyone know how bad the speedo error would be if I fitted the one for the coupe in the wagon?

Cheers.

Looks pretty close. 2 or 3 mph? I'd use it.

346/327= 1.058, so your speedometer will read 5.8 percent low at all speeds assuming the original setup was pretty close to right on.

Duke

Actually, a speed head calibrated for a 3.27 final drive will read high when driven by a 3.46 final.

You are correct!

Say tire revs/mile are 850, then the revs at 60 MPH in a direct drive gear = 850(3.46)=2941

With a 3.27 axle revs at 60 = 850 (3.27) = 2780, and if the speedo gearing is set up for a 3.27 axle the speedo will read 60 at 2780 regardless of the installed axle ratio.

So with an installed 3.46 axle and the speedo gearing set up to read 2780 at 60, actual speed at 2760 is 2780/2941(60) = 56.7, so the speedometer is reading 60/56.7 = 1.058, which is about six percent high, and this percent error is constant over the entire speed range.

Duke