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[Superfly_M3]

Hello,

Today I thought that I was going a little slower than was indicated and decided to check with GPS. Mind you its only an iPhone app, but it can't be off that much. I checked and my jaw dropped. Going indicated 80km/h GPS showed 73km/h. Going 140km/h GPS showed 129km/h. Thats 9%!!!

I just switched to 235/40/18 winter tires on OEM winter package rims - they have at least 60% thread left. My summers are 245/35/19F and 264/30/19R.

The diameter for the tires are:
235/40/18 - 25.40"
245/35/19 - 25.75"
265/35/19 - 26.30"

What could be wrong? Can it be fixed at the dealership through programing?
Where do our cars pick up the speedometer reading front or rear tires?

Thanks in advance.

[smmmurf]

you should be running 265/35R19 not 265/30R19 in the rear

i used gps to verify my speedometer. it's about 2% error for me on stock 18's (75 indicated is 73)

speedometer is picked up off rear but the car gets wheel speed data from fronts and rears

if you do a burnout they know you are going 0-5mph even though your rears are going 30mph... that is how they have tried to get out of warranty claims in the past

[Superfly_M3]

right ... whatever stock size is, its a typo.

[ben@tirerack]

This is not completely shocking when you factor in that BMW speedo is usually reading a few percent high. Stack the ~3% error on the speedo with 3.5% error due to tire size variation and maybe a couple percent error in the GPS and you're right there.

[M3PO]

The M3 guys at the Airstrip attack this past weekend were noticing a significant difference between the speedo and their Vboxs. I have been driving past one of those police speed things that warns you to slow down and at 60 mph indicated, the display on the police thing shows 54. I am running 255/35/19 and 275/35/19.

[m3an]

Originally Posted by ben@tirerack

This is not completely shocking when you factor in that BMW speedo is usually reading a few percent high. Stack the ~3% error on the speedo with 3.5% error due to tire size variation and maybe a couple percent error in the GPS and you're right there.

+1; I don;t think thats unreasonable at all

[Superfly_M3]

This is insane. Can this be corrected at the dealer or through a tune?

[MSteve5036]

I just thought thats how bimmers were. Most I or my friends have owned read high

Not trying to thread jack but anyone hit their limiter and at what speed?

[Z K]

You have 235/40/18 all around!? That is way smaller in diameter than the front or rear stock size of the car. You get less ground clearance, higher rpms for the same speed, and just plain looks weird. The speed sensor is attached to the transmission which reads rear wheel speed. You should be running 245/40R18 or 235/45R18 all around if you want something closer to stock sizing.

[ben@tirerack]

Originally Posted by Z K

You have 235/40/18 all around!? That is way smaller in diameter than the front or rear stock size of the car. You get less ground clearance, higher rpms for the same speed, and just plain looks weird. The speed sensor is attached to the transmission which reads rear wheel speed. You should be running 245/40R18 or 235/45R18 all around if you want something closer to stock sizing.

235/40R18 all around is shorter, but it is the size that BMW supplies with their winter package, so it's certainly not an unreasonable choice.

[Superfly_M3]

Originally Posted by MSteve5036

I just thought thats how bimmers were. Most I or my friends have owned read high

Not trying to thread jack but anyone hit their limiter and at what speed?

I don't get what you are trying to say.
What does speed and limiter has to do with this thread? ... not that you are trying to jack it or something like that.

[M3SQRD]

Every BMW I've owned has had an error between 5-8% and the error is worse at higher speeds (e.g., on the track; also, it appears the speedo "calibration" curve is non-linear). Below 70 mph my errors have all been < 5%.

[MSteve5036]

Originally Posted by Superfly_M3

I don't get what you are trying to say.
What does speed and limiter has to do with this thread? ... not that you are trying to jack it or something like that.

Well its limited to 155...so when you get to the limiter does your car say 155 or something else?

[MrD1198]

I hit the limiter at 269 km/h on the digital read out which is about 167 mph. This is in a stock ZCP.

[Sleeper519]

Mine has a 5% speedo error with stock wheels and tires. so, 2 mph at 40, 4 at 80, etc. I documented this by one of those radar "Your speed is ___" signs near my house. Now, my track tires are 18s, so that introduces another 2% error for a total of 7%, so when my speedo says 153 at Road Atlanta my gps tells me its really only 143.

[MSteve5036]

Originally Posted by MrD1198

I hit the limiter at 269 km/h on the digital read out which is about 167 mph. This is in a stock ZCP.

I was also around there with stock 19s. I got to 168...wooo! im faster! lol

[Franken///M]

I hit an indicated 169-170 in Germany. The car does read high compared to a GPS.