What Differential Gear Ratio to what Speedo

[Orangener]

Some of the members of our E9-forum know that I have a E12 in restauration.
During this process the question arises, what differential should I use in combination with a overdrive 5-speed manual transmission (ratio of 0,87 in last gear).
Please, in advance, apologize bad translations of technical terms.

After contacting some fora, I found the calculation formula, to charge the correct tachometer, the assignment of the appropriate differential and the determination of terminal velocity at different rear axle gearboxes.
Maybe this sentence is very confusing or obscure, so please, Dang, change it into understandable english

According to the formula

W (stamped on rear of the speedo) = (rear axle ratio multiplied with 4) divided by (rolling circumference multiplied with 10)

the following W-results arises for the tire-dimension 225/50/16 (Alpina 16 inch) with 1.919-circumference on the rear axle:


Gearbox stamped with 10:41 ---> ratio 4,1 = coefficient W=0.854

Gearbox stamped with 11:43 ---> ratio 3,9 = coefficient W=0.8125

Gearbox stamped with 11:40 ---> ratio 3,64 = coefficient W=0.758

Gearbox stamped with 11:38 ---> ratio 3,45 = coefficient W=0.718

Gearbox stamped with 12:39 ---> ratio 3,25 = coefficient W=0.677

Gearbox stamped with 13:40 ---> ratio 3,07 = coefficient W=0.638

I have two speedos with W = 0.854 and W = 0.758 on stock.
One of those tachometers would need a diff with 11:40 teeth, it is the "shortest" diff, that has the lowest top speed.

With the diff ratio of 4,1 there is a possible maximum speed of 209 km/h at 6500 rpm or 150km/h at 4660 rpm
A decent value, as permitted in Austria anyway, only 130 km/h on motorways and the car is not built for racing at highspeed.
My son and I have to withhold..........:mrgreen:

With the "longest" differntial that I have, the 3.25, there would be theoretical 263 km/h highspeed and 150km/h with 3700 rmp, for example ......

Here is the thread with an EXEL-list that I have used to calculate the highspeed in last gear, unfortunately it is in german:

http://www.motor-talk.de/forum/bere...geschwindigkeit-ganguebersetzung-t745658.html

Please note : the circumference of the tire size entered in the program calculations is not correct, you may need 1.92 as result (tiresize 195/70/14 or 225/50/16).

 

[Tierfreund]

Hi Orangener

what you need to know is the "Wegdrehzahl" wich corresponds to the print on the speedos you have. Now, you can go through the trouble of trying to figure it out through theory and Excel

or

you can do a practical measurement after you´ve put the car together (btw, only the differential and the wheel cirumference matter with the E12 setup, not wether you have a 4 or 5 speed gearbox since the drive fot the speed is on the output shaft so directly linked to the diff)
for the practical measurement, here´s an explanation.

http://www.ka-ja-tacho.de/wegdreh.pdf

if you then find you don´t have a speedo that fits, you can have an existing one modified. I´ve had that done for my E3 since I run a longer geared 3,09 rear diff from an E12.

It´s not too expensive at about 50€ but the downside is that only the speed indicator is easily adjusted. The mileometer is difficult if not impossible to adjust. So I just add 10% to the indicated mileage in my head.

 

[phead82]

Some of the members of our E9-forum know that I have a E12 in restauration.
During this process the question arises, what differential should I use in combination with a overdrive 5-speed manual transmission (ratio of 0,87 in last gear).
Please, in advance, apologize bad translations of technical terms.

After contacting some fora, I found the calculation formula, to charge the correct tachometer, the assignment of the appropriate differential and the determination of terminal velocity at different rear axle gearboxes.
Maybe this sentence is very confusing or obscure, so please, Dang, change it into understandable english

According to the formula

W (stamped on rear of the speedo) = (rear axle ratio multiplied with 4) divided by (rolling circumference multiplied with 10)

the following W-results arises for the tire-dimension 225/50/16 (Alpina 16 inch) with 1.919-circumference on the rear axle:


Gearbox stamped with 10:41 ---> ratio 4,1 = coefficient W=0.854

Gearbox stamped with 11:43 ---> ratio 3,9 = coefficient W=0.8125

Gearbox stamped with 11:40 ---> ratio 3,64 = coefficient W=0.758

Gearbox stamped with 11:38 ---> ratio 3,45 = coefficient W=0.718

Gearbox stamped with 12:39 ---> ratio 3,25 = coefficient W=0.677

Gearbox stamped with 13:40 ---> ratio 3,07 = coefficient W=0.638

I have two speedos with W = 0.854 and W = 0.758 on stock.
One of those tachometers would need a diff with 11:40 teeth, it is the "shortest" diff, that has the lowest top speed.

With the diff ratio of 4,1 there is a possible maximum speed of 209 km/h at 6500 rpm or 150km/h at 4660 rpm
A decent value, as permitted in Austria anyway, only 130 km/h on motorways and the car is not built for racing at highspeed.
My son and I have to withhold..........:mrgreen:

With the "longest" differntial that I have, the 3.25, there would be theoretical 263 km/h highspeed and 150km/h with 3700 rmp, for example ......

Here is the thread with an EXEL-list that I have used to calculate the highspeed in last gear, unfortunately it is in german:

http://www.motor-talk.de/forum/bere...geschwindigkeit-ganguebersetzung-t745658.html

Please note : the circumference of the tire size entered in the program calculations is not correct, you may need 1.92 as result (tiresize 195/70/14 or 225/50/16).

Hallo Orangener,

what differential ratio did you eventually end up going with?
4.10 or 3.25?

I'm asking since I might be able to get a 4.10 LSD (I currently have a 3.64 installed) but am wondering whether it's too short for occasional Autobahn use (given I probably won't go much faster than 150km/h, even on the German Autobahn)

Grüße aus Essen!