I recently adjusted the mileage up by some 40,000 on a replacement cluster for a 240. I haven't had to take my 740 cluster apart so I don't know how similar it is but I think it is pretty close.

On the 240 I took the speedo loose from the cluster - needed to replace the small gear that always breaks off a tooth or two causing the odometer to stop working. I removed the broken small gear and then used a bamboo shish-ke-bob stick - pointed like a toothpick on the end but long and stuck the pointed end into the large gear until it was tight. then I put the other end of the stick into a hand drill and started it up - it worked great and I was able to add over 40K miles in about a half hour. I would only run the drill for about 5 minutes then let it cool off. Once I had the correct mileage displayed I removed the stick, inserted the new small gear and screwed the cluster back together.

Don't know for sure if this will work on the 740 but good luck!

I managed to update the mileage on my odometer after replacing one of the gears that drive it. You need to take the speedo to bits to update your mileage. I can't remember how the 1990 speedo differs to the earlier ones so perhaps someone can advise on how to dismantle it. Anyway, you need to disconnect the stepper motor from the mileage counter by removing a couple of screws that go through the motor. Then you can manually wind the counter up using the least significant digit on the counter. It could take you half an hour to add 10,000 miles though!

If you know a bit about electronics and have a signal generator you can connect it to the speedo to simulate the car being driven. If you do this then make sure that you don't exceed the maximum clock frequency of the odometer circuit.
Ignore this last paragraph if you don't understand. It is much easier and faster to correct the mileage manually.

Cheers
Will

1986 740 GL 235k mi.

No easy way I know of to adjust the mileage reading. (to make it harder to fraudulently alter)

If your car is a 90, the replacement cluster must be from an 89 or 90, or the speedometer won't work correctly (voice of experience speaking)

To remove and install the two knobs you have to remove the face from the cluster. There are several screws on the back that hold it in place. Remove these and you can swap them over.

Take the old one apart first to avoid making a mistake on the good one.

See the FAQ for further information on disassembling the cluster (which screws to remove etc.)