Speedometer calibration

While on a recent trip to West Texas, traveling along with another car, we discussed how fast we were driving and my buddy following me said that I was driving 3-4 mph slower than my speedometer was indicating. In other words, at 80 mph indicated, i was actually traveling at 76 mph.

I thought maybe it was his speedometer that was off some, but after verifying with a very accurate GPS, on a long straight stretch of roadway, our 335d was in fact running about 4-5 miles per hour slower than indicated.

Being a stickler for accuracy, I want my speedometer to read exactly how fast I am traveling and don't want to have to constantly make a mental adjustment. Can anyone tell me what could be causing this, and more importantly, can it be easily corrected? I know it's not caused by the tire size because I am running the same size and type of tires that came originally on the car, which is a 2010 model. Thanks for any help!

I think I read some previous threads on this which indicated that BMW deliberately sets the car in this fashion. Try searching and you may find some info on a fix.

Originally Posted by Runnerman2

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Being a stickler for accuracy, I want my speedometer to read exactly how fast I am traveling and don't want to have to constantly make a mental adjustment. Can anyone tell me what could be causing this, and more importantly, can it be easily corrected? I know it's not caused by the tire size because I am running the same size and type of tires that came originally on the car, which is a 2010 model. Thanks for any help!

As Art mentioned, there is lots of discussion of this (mostly on the main E90 forum, probably on many others.) I think if you search for my handle and speedometer you should be able to easily find it.

It's the way it is because of an intersection of US/CA/EU laws & regulations: has to read fast, etc. There is no way to get it changed. If you have iDrive, you can somehow get the actual instantaneous readout in digital form, I believe.

Here is the one I was thinking of. http://www.bimmerfest.com/forums/sho...d.php?t=579643

All BMW's are 3mph fast.....when indicating 65mph, you're really doing 62. I've verified mine with GPS and radar.

http://forums.bimmerforums.com/forum...1266637&page=2

Has the service bulletin text.

My speedometer ran about 3-4 mph fast.I've read that it's because there's a German (or European) law which heavily fines a car maker which builds a car featuring a speedometer that's even a tiny bit slow so,to avoid the fines,they deliberately set them a bit fast.

Runnerman2: Same here for my 335d...seems to be off 3-4 mph, but in my testing it seems the odometer is pretty much dead on. If you change your tire diameter, then you will mostly likely throw the odometer off( in your favor, for once).

Ditto, well documented.

There is something you can do, install slightly larger diameter tires. I changed my tires recently and now I'm just a little bit closer to actial. Diameter (circumference) was slightly greater on the newer tires.

I usually go by my GPS any way.

Thanks guys for all the input! Sounds like I will just need to keep the difference in indicated vs. actual speed in mind in the future. We do have iDrive, but haven't looked at that to see if there's a way to show current speed all the time on the screen. May be a possibility as well.

Originally Posted by Flyingman

Ditto, well documented.

There is something you can do, install slightly larger diameter tires. I changed my tires recently and now I'm just a little bit closer to actial. Diameter (circumference) was slightly greater on the newer tires.

I usually go by my GPS any way.

What size did u move up to?

Sent from my MB525 using Bimmer App

Of course if you change tires to "fix" the speedo reading now your odometer readings will be off (not that I'd care but I do not care about the speedo either).

Originally Posted by Runnerman2

Being a stickler for accuracy, I want my speedometer to read exactly how fast I am traveling and don't want to have to constantly make a mental adjustment.

I'm just accustomed to doing the 'mental adjustment' now.

For example, if the speed limit says "55" I set my cruise control to "65", then bump it down one MPH. That is really close to the maximum legal speed. If you are feeling particularly feisty you could leave it at 10 MPH above the speed limit.

I just tend to always speed and if my radar detector goes off then I hit the brakes to whatever point on the speedo is the speed limit. I am not going to worry about if I am truly doing something like 77 in a 65 instead of what I think is 80 in a 65.

What really bugged me was on the West Texas run I mentioned, set it on 120 for about 40 miles (wide open spaces, you know). Just kills me to know now that I was "only" going 116. Will have to go back and do it again.

On a non sport 335d, go to 245/45 - 17 and your speedo will be very close to dead on. Your odometer will be about 3% under.

Originally Posted by BMWTurboDzl

What size did u move up to?

Sent from my MB525 using Bimmer App

I have the 18" Sport, changed from the OEM RFTs B/S to the Michelin Pilot Super Sport, but went slightly wider up front, from a 225 to a 235. Rears are still 255. Turns out the diameter of that size is just slightly larger.

Now comes the question: Are the front wheels or the rear wheels driving the speedometer/odometer? I suspect the rear wheels as Odometer is usually driven off the Transmission, but since I changed tires my indicated speed seems to be closer to my GPS.

Regarding swapping to a different size tire, thanks for the suggestions. But having just purchased a new set of "non-run-flats", that's not an option for at least another few thousand miles. I'll just live with knowing there's a difference in actual vs indicated speed. Thanks all!

Originally Posted by Flyingman

Now comes the question: Are the front wheels or the rear wheels driving the speedometer/odometer? I suspect the rear wheels as Odometer is usually driven off the Transmission, but since I changed tires my indicated speed seems to be closer to my GPS.

Neither. The ABS/DCT computer uses the wheelspeed sensors to derive it.

Originally Posted by floydarogers

Neither. The ABS/DCT computer uses the wheelspeed sensors to derive it.

So some sort of average of the indicated speed of all 4 wheels?

That would help explain Flyingman's observation.

Originally Posted by Runnerman2

Regarding swapping to a different size tire, thanks for the suggestions. But having just purchased a new set of "non-run-flats", that's not an option for at least another few thousand miles. I'll just live with knowing there's a difference in actual vs indicated speed. Thanks all!

Hey, compare the specs on your original tires vs the new ones, maybe they new ones have a larger diameter. It really can affect your speed/miles reading.

Originally Posted by TXPearl

So some sort of average of the indicated speed of all 4 wheels?

That would help explain Flyingman's observation.

This is just too high tech for me! But explains why I'm seeing a slight closing of the gap between indicated and actual.

So why would speedometer and odometer read different? i.e. if I go down the highway at 60mph and the odometer says I actually did 63 miles in an hour.

My OBC is notorious for saying my mpg is around 27-28mpg but when I calculate my actual fuel consumed vs miles driven (odometer), I always get the fuelly value below, which is some 1-1.5mpg better.

Regardless, after over two years and some 34k miles, my smiles/miles are still up there!

Originally Posted by Flyingman

if I go down the highway at 60mph and the odometer says I actually did 63 miles in an hour.

The easiest comparison which is accurate is to borrow someone's portable GPS unit and use it's speed measurement. GPS speeds are typically extremely accurate.

Originally Posted by Flyingman

This is just too high tech for me! But explains why I'm seeing a slight closing of the gap between indicated and actual.

So why would speedometer and odometer read different? i.e. if I go down the highway at 60mph and the odometer says I actually did 63 miles in an hour.

My OBC is notorious for saying my mpg is around 27-28mpg but when I calculate my actual fuel consumed vs miles driven (odometer), I always get the fuelly value below, which is some 1-1.5mpg better.

Regardless, after over two years and some 34k miles, my smiles/miles are still up there!

Different systems.

Originally Posted by Flyingman

My OBC is notorious for saying my mpg is around 27-28mpg but when I calculate my actual fuel consumed vs miles driven (odometer), I always get the fuelly value below, which is some 1-1.5mpg better.

Most MPG computers are not that accurate. While the fuel measurement is quite precise over small intervals for the purpose of controlling combustion, when the data is integrated over time to provide the volume of fuel used over a long period, a fair amount of error can creep in.

To a certain extend the ECM does not have to really know precisely the absolute volume of fuel it is injecting, as it simply uses feedback from the exhaust gases measurement to adjust the fuel trims to provide the right amount... a little bit like nudging the water faucet a little bit to increase or decrease the flow of water out of the garden hose without actually knowing the precise flow rate. If the "nudging" required to get the correct sensor readings gets to be too great, you get a MIL and an error code for fuel trim out of range or at the limit.