quick, inaccurate speedo question.

[quikveedb2]

my speedo reads +/-5 mph under what the car is actually doing, based on readings from local speed traps and some testing with other cars. the tires are 195/65/14 all around currently. iirc later model (plastic bumper) cars got the 65 series, and earlier cars got 60 series. mine is early model. is this the cause of my inaccurate speedo, or is something else going on? i'm badly in need of new rear tires and if i can kill these 2 birds, well i'd love to...

tia.

[kronus]

I think this is a trend of most cars. My speedo under-reads reliably by 5mph anywhere above 40.

[quikveedb2]

thanks for the quick reply kronus. any fix anyone is aware of? mine reads under well into the low 30s. so at 30 i'm going 25, 25=20, etc. not mission critical, but very annoying.

[archemedes]

you can send it in to be certified, but still can be off a couple mph (dot requirement is 5 mph at 55 for new cars) I have seen police cars after certification still be off 2 mph at 55

[Ferdinand]

my speedo reads +/-5 mph under what the car is actually doing

You mean it reads over, right? When it says you're going 30, you're really only going 25.

You can see when you're stopped the needle doesn't point to where zero should be. It points to about where 5 mph would be on the dial, even though you're stopped.

What I did was pull the needle off the speedo and move it back one spline on the post. Now it's fixed and accurate.

[fretburnr]

Mine has always read over - 45 on my speedo is more like real 39

[bostonbmw]

There is a resistor on the back of the spedo that you can rotate to +/- what you need. I am not sure what the range is but it should help.

[kkamakasi]

^ any pictures of this said potentiometer?

[jlevie]

A BMW speedometer is supposed to read high. As long as the error is no more than 2.4mph + 10% it is within specification.

[FLG]

I tried offering a calibration service...no one seemed interested.

http://www.r3vlimited.com/board/showthread.php?t=267955&highlight=speedo+calibration

[FLG]

Squid read my post above, in my thread i link to a e30tech thread about adjusted potentiometer. You need a circuit to accurately adjust it, it would be near impossible to adjust while driving or just by guessing.

edit: yep, sorry.