Speedometer question.

[gsxrcop]

My speedometer reads 10mph to fast (via gps) is there a way to calibrate these? I recently switched out diifs from a 4.10 to a 2.93 and used the same speed sensor. The speed reading was the same with the other diff as well. I'm thinking the speed sensor has nothing to do with this correct? Any help would be welcomed.

[Cletonius]

I read that BMW allowed up to a +10% error factor on our cars. It's annoying, mine is consistently 7% high. I would love to know of a way to recalibrate also.

[nando]

the "error" is simply the needle on the dash itself. it's typically optimistic by 5mph.

a 10mph difference would probably be something else, like tires that are too small.

[jlevie]

The spec for the speedometer is that it should read fast by 2.4mph + as much as 10% of actual speed. Typically that is about an indicated 65mph at a real 60mph. But it could read 68.5 at 60 and still be in spec.

The speed signal is independent of the differential ratio. The sector wheel that drives the sensor is the same on all diffs. I'd agree that undersized tires could be a factor here.

[gsxrcop]

Well I do have a smaller than stock tire size with the 16 x 9's, but it has always been 10mph over even with the stocks and 18's. The only thing that changed is the rpm at different freeway speed as depicted by the ratio change.
I would just like it to be a little closer to the actual speed I'm going :( Nando can it be calibrated with a yellow box or something of that sort? Thanks. Kenny

[nando]

your tires are undersized. that's all there is to it.

tire size definitely affected your speedo, whether you realized it or not. the question is, how much undersized?

there is a speedo calibration actually, but you really need a way to bench test it as you need to adjust a potentiometer with a known-reference speed sensor.

[gsxrcop]

Do you have a link for this? I'm sure I'm not the only one who would like to see an accurate speedometer. Thanks again.

[nando]

only one I've seen:

http://www.e30tech.com/forum/showthread.php?t=73541

I was going to go through this but it turned out with 235/50-15 tires (slightly oversized) the dash speedo is dead on. you can also see what BMW built into the needle of the gauge with your OBC, which is supposed to read dead on - hold down the reset button while you're in the MPH mode to see your "real" MPH without the safety factor added in.

[gsxrcop]

My 318 has no OBC :(

[Curt Hayes]

I simply pulled the needle off of mine and dropped it down about 5mph. Makes me much happier.

[Kershaw]

^

this.

clock yourself via gps and figure out how much it has to go down.

[IS300E30]

My speedometer is 10mph fast......Say my speedometer says I am going 70, my gps says I am going 60. The faster I go the bigger the difference the slower I go, the smaller the difference.

But, as I said, for example, if my cars speedometer says I am going 70, then I am actually going 60.

I have stock rim size but with tire size of 205/60/R14 instead of the stock tire size of 195/65/r14.......

But, I really doubt that small difference in current tire size vs stock tire size would make much of a difference of the accuracy of the speedometer.

What could be the reason for the accuracy being off like it is?

[e34john]

^
You are less than 2% off with the new tire size.

[IS300E30]

^
You are less than 2% off with the new tire size.

So, what you are saying is the difference in tire size is making that much of a difference in the accuracy of the speedometer? Keep in mind the OBO is accurate, just not the speedometer.

So, what your saying is if I put the correct 195/65/r14 tire size on the car then the speedometer should be accurate?

[DREWHALL]

Typically yes. Tire size has a large impact on what the speedo reads. Even two different brands of the same labeled size could show a difference on your speedometer.