Speedometer accuracy

[Cybmx]

I'm curious, how is everyone's speedometer accuracy? Mine is off(faster) by about 5-7 mpg depends on how fast I'm going. I used a gps to measure the speed, at 80mph it shown 87 on the speedometer. I bought the car used, and the wheels and tires are stock spec. I suspect that I might have a shorter final drive...

[Taylorguy74]

Originally Posted by Cybmx

I'm curious, how is everyone's speedometer accuracy? Mine is off(faster) by about 5-7 mpg depends on how fast I'm going. I used a gps to measure the speed, at 80mph it shown 87 on the speedometer. I bought the car used, and the wheels and tires are stock spec. I suspect that I might have a shorter final drive...

On my E90 I have the digital Speedo coded to display - it shows a much lower speed than the needle. At around 70 on the digital, the needle shows between 74 to 76 mph. I don't know on the M, however the norm is between 3 to 5% difference in Speedo versus actual speed in a BMW.

[Rick F.]

My 2006 3.0i's speedometer "reliably" reads 2 mph faster than the car's true velocity at highway speeds.

Rick F.

[Rated M Roadster]

It's required by European law (likely other govts as well) that a speedometer never read lower than the actual speed; thus they set it higher by a percentage.

[vachss]

I calibrated my 3.0si's speedo with a GPS some years ago. It consistently reads about 4% high at all speeds. There was some discussion of this and it was suggested that this small positive error was a conscious choice on BMW's part (since erring the other way might have legal/liability implications).

[Cybmx]

Thanks guys! What that makes my rpm reading off too? Supposedly I would be at 69mph at 2750rpm, my rpm reading match that speedometer reading but I was only going about 63? I'm confused...

[Cybmx]

At speed of 100, my speedometer is reading almost 110.. That's about 10% off >_<

[mcoupe5/6]

I have a 2008 z4mc and my wife has a 2011 328i coupe, both read 5 mph higher than actual speed. Seems silly on cars with otherwise precise engineering.

On my z4 mc... I noticed the space Btwn 0-10 mph is only half that of the space btwn 10-20 mph. Could be nothing, but on what seems like an analog instrument. That tells me something.

There's also a gap discrepancy between 80-90 I believe and at that speed the car seems to be roughly 10 mph slower than the speedo indicates. Had anyone else noticed this?

[Rated M Roadster]

Originally Posted by Rated M Roadster

It's required by European law (likely other govts as well) that a speedometer never read lower than the actual speed; thus they set it higher by a percentage.

Originally Posted by mcoupe5/6

...both read 5 mph higher than actual speed. Seems silly on cars with otherwise precise engineering.

Did you read? There's a reason for it.

Why is this thread still going on?

[jdoublehcubed]

BMW speedometers read 4% higher than actual by design. If you have lower profile tires than stock (say 40 aspect ration instead of stock 45) thats about another 5%. there's your 10% error.

There's an option to fix the design inaccuracy in NCS expert, I believe its called True Speed or something similar (although probably in German).

[Cybmx]

Originally Posted by jdoublehcubed

BMW speedometers read 4% higher than actual by design. If you have lower profile tires than stock (say 40 aspect ration instead of stock 45) thats about another 5%. there's your 10% error.

There's an option to fix the design inaccuracy in NCS expert, I believe its called True Speed or something similar (although probably in German).

I have stock spec tires. The speedometer is not really my concern, is the RPM. If the RPM is accurate, then it means we are not going 69mph at 2750. If the RPM is inaccurate like the speedometer, then how would I know my true RPM?
Sorry if my concern sounds dumb to you, but it really bothers me…

[jdoublehcubed]

I think the RPM is accurate, because it has a lot more implications for safe operation of the car, and I don't think they would mess with it.

you could easily check it by using an OBDII scanner to read the RPM info from the CAN stream, then compare it to the dash. the data in the CAN stream has to be accurate, because other sensors and computers in the car are depending on it for various computations.

[psygnosis]

I know the speedometer reads high and I'm OK with it. I still treat it as though it was accurate.