Need to Adjust Odometer Gears due to Change in Differential ratio

[Americo2054]

My car is an E21 320 1976 Euro model, originally fitted with the M10 engine, 4 speed manual transmission and 2.90:1 differential ratio.
I have changed the differential gears to 2.45:1, a great improvement. However, since the speedometer and the odometer read the driveshaft revolutions, both speed and distance show 2.45/2.90 less than before. Speedometer and odometer are in kilometers, not miles.
So, the solution is to change the odometer gears to those of a car that uses the 2.45 differential. Now the question:

What are the part numbers of the odometer gears that match the 2.45:1 differential ratio?

This is important because I use the car mostly in classic car regularity rallys, and acurate odometer (and speedometer) readings are crucial.
Any help here will be greatly appreciated.

[Jeroen]

2.45 is really long and not a stock ratio. Stock for the 76 316 is 4.10, sometimes 3.91. Any lower number would only make the final drive longer, like a 3.45 from a 323i. Shortest stock final drive known to me is that 4.10. The only model that uses the same diff is the 02 (same housing, different cover). But on these models the shortest final drive also is 4.10/4.11.

Of course there are ways to get to a shorter final drive by changing out internals, but if that's the case I do not know of any factory stock possibilities to adjust the odometer to that. However, there must be mechanical odometer specialists that should be able to solve this for you.

[Americo2054]

Hi Jeroen, thanks for your comments.
My E21 is 320, 2 liter engine. I fitted a Weber 40 twin body carburetor, IrishEngineering forged "high head" pistons to increase the compression ratio and dimensioned down pipes in stainless steel, all the way to the back. The car gained around 30 to 40 in power just with those modifications.
This engine now handles very well the 2.45:1 differential ratio. The car is much better now.
The issue I have, is to correct the odometer reading to reflect the difference between the 2.45 and the 2.9 ratios. That can be done by changing the small gears that drive the odometer cilinders. However, I need to identify those gears that will exactly match the 2.45 diff ratio.
thank you!

[BertjeConti]

Hello Americo,

Haha, thats a realy long diff ratio. With a 2.45 diff your car would do 145km/hour in second gear easily. Could you confirm that?

We do classic car rally's and regularity's also. We do not rely on the speed and odometer in the car, therefore we have fitted a period correct rally trip meter, which is verry accurate.

[Wilmo]

I have a E30 gearbox in my M10 E21, hence there is no mechanical drive for the odometer. I used a digital E30 odometer and put that behind the clock plate of the E21. At the driveshaft I pickup a signal from the bolts, put it in an Arduino to convert the signal and that controls the digital odometer.

Sort of a hybrid between Bertje's solution and the original.