View Full Version : Speedometer/diff.ratio/speed sender unit

Taxi Driver
07-09-2012, 02:16 PM
Hi, speedometers, are they adapted to what final drive the car was specified w i.e is it only the speedo k= #### value that decide the correct reading , are the sender units the same on 2.82 and 2.65 cars?

I know of one case where the G-box was changed on a 2.82 500E, to a G-box from one 2.65 500S, this car does not read
correct, missreading is quite optimistic. Obviously somthing has to be changed, my 2.82 500E speedo has# 1245429267
k=5585, my 420E 2.65 has# 1245422469 k=#### I didn't check, also this unit is w.o ASR window. Roger

ps that car w the 500S box has an AMG 300km unit and it may have the wrong k= value......

07-09-2012, 03:00 PM
You need the speedometer "K" number to match the rear axle ratio AND the tire size.

Your Euro-spec .034 has a 2.65 differential and with the 6.0L engine, I would leave that ratio, especially with the normal 400E-size tires. Just put a limited-slip diff in there. Moving to 2.82 will just make traction problems a bit worse and lower your top speed. If you leave it alone, no need to change the speedo...


Taxi Driver
07-09-2012, 03:28 PM
Hi, yes your right about the 2.65 I'm happy with that and together w say 225/55-16 it should make for a longlegged GT
That other Q about the 300km speedo 500S g-box and 2.82 (not my car) I take it, the change over to that G-box would not make any differance,or? This may indicate that speedo is suitable for the 2.65 rear best to check k=#### then,,Roger

07-09-2012, 03:45 PM
The 225-55-16 tire is not correct for the stock 400E speedometer. You would need a correction box for that. I'd use a 225-50-16 tire, which should keep the speedometer accurate.

Transmission doesn't matter, the sensor is the same on all the 722.3 with electronic inductive speed sensor. The difference is how the speedometer head is calibrated, i.e. the 'K=xxxx' number. If necessary you can use a YellowBox (2-wire inductive version) to correct the speed: