RPM's didn't change when i swapped diffs

[BlapBlap]

I bought a m50/ZF tranny swapped car with s50 cams installed running the tms s50 cam chip. The car didn't run when I bought it but I got it running and noticed that the RPM's were really high like i was running a 4.10LSD.

I looked under the car to find out that the diff had a 2.93LSD tag. I know you can't always trust the tags as they can always be switched around. I did some more research and found out that with a ZF tranny coupled to a 4.10LSD at 60mph in 5th gear I would be running around 4000-4200rpm's which I was at 4100rpm's at 60 mph.

I figured I was running a 4.10LSD but was getting annoyed of the high rpm's. I swapped in a 3.46 LSD a couple days ago but the rpm's are the exact same, they didn't change. At 4100 rpm's in fifth gear I'm going 60 mph.

I don't know where to start looking for the problem so any help would be greatly appreciated.

PS I think in the beginning I was running a 2.93 because when I put in the 3.46 my car felt faster and pulled harder.

[e30trooper]

do you have the correct coding plug for the cluster?

[devon.818]

why not turn the input flange of the diff and calculate the drive ratio?

[jlevie]

If I remember correctly, that transmission has a 1:1 fifth gear. Assuming standard sized tires, the engine speed at true speed of 60mph in fifth gear for some of the differential ratios would be about:

2.93 2500rpm
3.46 2900rpm
3.73 3200rpm
4.10 3475rpm

None of those are really anywhere near the engine speed you report. But, it would not be unusual to find the speedometer is reading high by 3-4mph at an indicated speed of 60mph. If I assume that and a 3.46 differential and assume that the coding plug in the cluster is one for a four cylinder engine the tach could read about 4100rpm at an indicated 60mph.

[BlapBlap]

If I remember correctly, that transmission has a 1:1 fifth gear. Assuming standard sized tires, the engine speed at true speed of 60mph in fifth gear for some of the differential ratios would be about:

2.93 2500rpm
3.46 2900rpm
3.73 3200rpm
4.10 3475rpm

None of those are really anywhere near the engine speed you report. But, it would not be unusual to find the speedometer is reading high by 3-4mph at an indicated speed of 60mph. If I assume that and a 3.46 differential and assume that the coding plug in the cluster is one for a four cylinder engine the tach could read about 4100rpm at an indicated 60mph.

Since I didn't build this swap and the car is a 91' 318is with I'm guessing the original cluster and coding plug that's what's causing me to have such high rpm's.

What coding plug would I need to correct the rpm's?

[Killacortes]

damn nucka! i didnt know your whip was still reading that high. A coding plug from a 6cyl car should read much lower.

[BlapBlap]

yeah I fixed it thanks for all the help guys. It was just a matter of switching the coding plug.