Speedos and rear gear ratios

[Mike Goble]

When I swapped my E3 rear axle out for one from an E28 I was able to choose from various gear ratios from 3.73 to 3.25. One of the drawbacks is that the speedo reads incorrectly. I've been looking through realoem at rear axle ratios offered in the different E3 models, and discovered that some 2500 and 2800's used a 3:45 ratio, in contrast to the 3.64 from the Bavaria. I'm thinking that I could get a speedo from a 2500 or 2800 and use it in conjunction with the 3.45 rear gears from an E23, which fortunately bolt right in my E28 rear axle.
My questions:
Will this work? Are the speedos calibrated differently?
If I find a speedometer from a 2500 or 2800, will speed be displayed in km/hr or mph, or were both versions made?
Are there different markings on the speedo to indicate the calibration?

 

[teahead]

Going to be tough. I say get your speedo converted to GPS

 

[HB Chris]

Mike,

The 2800 and 2800CS both came with the 3.45 rear axle in the US and most if not all were limited slip, I know the 2800 all had LSD. You should be able to swap the clusters.

Chris

 

[Mike Goble]

Going to be tough. I say get your speedo converted to GPS

Already got GPS. I've got a 3.25 rear axle with a speedo calibrated for a 3.64 rear, and as you can see, there is a 12% error.

 

[m5bb]

Mike,
When I had my speedo rebuilt at one of the CA places they gave me instructions to help them calibrate to my car.
Once cluster was removed I put tape on end of speedo cable (like a flag) and then went a measured distance and counted how many times the flag went around.
They got it right according to my phone speedo and Waze. Maybe you don't want to have to have it calibrated. Just an idea.

 

[teahead]

Are you saying you already converted your analog speedo that's in your dash to GPS and it STILL doesn't work?

 

[Mike Goble]

Are you saying you already converted your analog speedo that's in your dash to GPS and it STILL doesn't work?

No. I have an aftermarket GPS h/u display sitting on my column that reflects off the center of my cluster, producing the 94 that's in the picture. The reason I got into this thread was that I discovered that 2800's have 3.45 rear gears and I have a set as well. It's not a big deal - I have driven the this car for 12 years the way it is, and I was just looking at a possibly inexpensive fix to a minor problem.