[kuaffa]

I have a 330i with the sports package. Comparing the speedometer reading with the speed displayed on a GPS device (which should be accurate to within 0.1mph), I see that the speedometer reading is about 2.5mph too high at 60mph. The tire pressure is correct. Is this common for BMWs? My old car was right on the mark. Are there adjustments for this that a dealer could make?

[stressdoc]

There have been other strings on this. But hey, if you can check your speed within .1 mph, do us all a favor and see what the top speed is when the limiter kicks in. There must be someplace safe, maybe in E. WA

[E90Fleet]

As your tires wear the speed reading will become more accurate

[egtlover74]

Originally Posted by E90Fleet

As your tires wear the speed reading will become more accurate

Are you serious?

[E90Fleet]

Originally Posted by egtlover74

Are you serious?

Yes

As your tread wears down for any given speed the tire will be rotating faster due to it having a slightly smaller diameter

So your speedo shows the speed more accurately

[JVFan]

Not sure 5 or 6 thirty seconds of an inch will make an observable difference on the Speedo E90....Like to see the math on that one!

[kuaffa]

Originally Posted by E90Fleet

As your tires wear the speed reading will become more accurate

No, it's the other way around. As the tires wear, the speedometer will become less accurate.

A smaller tire has to rotate more to cover the same distance. The more a tire rotates, the higher the speed is shown.

[E90Fleet]

Originally Posted by kuaffa

No, it's the other way around. As the tires wear, the speedometer will become less accurate.

A smaller tire has to rotate more to cover the same distance. The more a tire rotates, the higher the speed is shown.

You are correct

I read the top post wrong, i read it asif his speedo was underreading

[E90Fleet]

I wouldnt believe that a handheld GPS in a car could be accurate to 0.1mph

[mapezzul]

analog vs. digital.... analog has a +/- 3% mechanical error. This is pretty common, on an Audi and A VW there is a way throught the climate control/radio to get the actual digital readout of the MPH. On the E90 I believe it is the digital cruise control is the only way.... I have not picked mine up so I can not verify this. But I am sure it is the analog thing, the tires should have nothing to do with it, modern speedos are done via the tranny and the revolutions there.

[F32Fleet]

Originally Posted by E90Fleet

I wouldnt believe that a handheld GPS in a car could be accurate to 0.1mph

I've wondered that myself.

There was a thread that the odometer was reading incorrectly as well. Actual miles driven would be less than what the odometer was saying. Now that would be something.

[kuaffa]

Originally Posted by E90Fleet

I wouldnt believe that a handheld GPS in a car could be accurate to 0.1mph

Actually, even the cheapest handhelds are very accurate at measuring speed.

See this page for Garmin Geko 201:

http://www.walmart.com/catalog/produ...uct_id=2129164

Position accuracy is listed at 15m, and velocity accuracy is 0.05m/s. That's 0.11 mph.

By the way, it's not just the analog display that is 3mph wrong. When I set cruise control to 65mph, the car still only travels 62mph.

[UncleWede]

I went out and did some testing (minimal accuracy on my part) and I seem to be exact on the odometer, but 10% slow on the speedo. There is a marked 1 mile odometer test in town. I stopped at it, reset the trip, drove down. Clicked 1 mile right at the sign. Then I drove a bit to set the cruise at 60, got up to speed and measured the time to cover that 1 mile. At 60MPH, should have been exactly 60 seconds. Took me 66. So if my math is right, I was only traveling at 54 MPH. ASSUMING a constant linear misread of 10%, my top speed of buck-10 readout means I have not crossed into triple digits yet

[wac77]

Originally Posted by kuaffa

I have a 330i with the sports package. Comparing the speedometer reading with the speed displayed on a GPS device (which should be accurate to within 0.1mph), I see that the speedometer reading is about 2.5mph too high at 60mph. The tire pressure is correct. Is this common for BMWs? My old car was right on the mark. Are there adjustments for this that a dealer could make?

An errror of up to 5%, on the "optimistic" side, is normal for all cars.

[ex-poster]

While I don't have a citation to point to, I recall hearing about a German law or regulation that said no speedometer could ever read LESS than the actual, true speed, regardless of tire wear or wheel substitution.

So, BMW/Audi/Merc et al add a certain percentage.

That way, you can't ever say "I didn't think I was going that fast, my speedo said 75" as an excuse - usually you'll look down at the speedo and think you're going faster than you actually are.

[Tierfreund]

Originally Posted by smithrh

While I don't have a citation to point to, I recall hearing about a German law or regulation that said no speedometer could ever read LESS than the actual, true speed, regardless of tire wear or wheel substitution.

So, BMW/Audi/Merc et al add a certain percentage.

That way, you can't ever say "I didn't think I was going that fast, my speedo said 75" as an excuse - usually you'll look down at the speedo and think you're going faster than you actually are.

True. The german la states the the speedo may at no time indicate LESS than the true speed. The same law also stated the it´s fine for the speedo to indicate a higher speed as long as the deviation is no more than 7% of the END-VALUE of the speedo. So on a 330 with an end-value of 160mph that ould mean a deviation of 11mph at any speed. Not 7% of the driven speed.

To be sure to comply with the law, german automakers all have speedos that show slightly too much. In the old analogue days it used to be up to 10mph (less at lower speeds). Nowadays with digital signaling, the car usually knows pretty much the exact speed (up to 2-3mph, total accuracy is not possible due to the diameter chancges of different tire sizes, tolerances in tires, different diameters from different tire manufacturers and yes, tread wear), but to make sure to comply with the law, the analogue readouts are usually trimmed to show about 5km/h = 3mph too much.

And about that tread wear issue: I once did the math for an old VW Golf I had (tire sizw 185/50 R14). The difference between new tread (8mm) and old (2mm) would have been about 3km/h=2mp/h indicated

[DozyDriver]

I drove along a freeway where a large gantry contains a speed checker. This is to allow drivers to check the accuracy of their speedometers. My indicated speed was 102kmh. The speed checker said 97kmh. 5% out is too much in my books. But it meets the local design rules.

[E90Fleet]

These cars are no different than any other make

[UncleWede]

Originally Posted by E90Fleet

These cars are no different than any other make

I beg to differ. I took my GMC Sonoma pickup on the same route. I don't have cruise control on it, but I ran the 1 mile at 60MPH in almost exactly 60 seconds. Those front tires have about 35,000 miles on them, I am almost ready to get new ones. The GMC odo read exactly 1 mile like the BMW. I used that mile marker to calibrate the digital speedo/odo on my motorcycle.

[jhbodle]

Originally Posted by E90Fleet

These cars are no different than any other make

I agree

My dad's Ford Fiesta was taken to an indicated 106mph on the speedo whilst on the motorway. The car can only do 96mph according to the manual...

[David328M-Sport]

So the way I see it is that this error may help us to a degree in possibly avoiding a speeding ticket. Not a bad trade off.

I concur with DozyDriver about the speed gantries. There are a few dotted around the state where I live and you can conduct a speed check. Haven't done this with my e90 yet but my e46 was always indicating a faster speed than the actual speed - provided you can believe the accuracy of the speed gantries in the first place.

Oh, by the way, welcome to E90POST DozyDriver. Stick around. You will really enjoy yourself here. Trust you are enjoying your 320i

[segfault]

Originally Posted by UncleWede

I beg to differ. I took my GMC Sonoma pickup on the same route... Those front tires have about 35,000 miles on them, I am almost ready to get new ones. The GMC odo read exactly 1 mile like the BMW. I used that mile marker to calibrate the digital speedo/odo on my motorcycle.

I think the odometer/speedometer on the Sonoma is tied to the rear wheels, not the front.