Speedometer Accuracy? SOLVED... sort of.

[StudioWizard]

My 2005 2.5i speedometer seems to be running fast. When I GPS a location, Google gives a readout of my speed. It's always about 5 mph SLOWER than my speedometer is reading.

I've read that this can sometimes occur with cars as they age.

Has anyone experienced this with an old Z4? If so, any suggestions on how to address this issue?

 

[MikeyH]

My Transit, wife’s Focus both under read. For some reason my zed doesn’t. It has 40 profile tyres on the back so maybe this helps.?

 

[TheDan]

All cars are designed (with stock everything) to read high. This is purely from a legal standpoint to make certain that their speedometers never read below the speed the vehicle is travelling, accounting for variations in tyre brand and pressures.

As far as the Z4 goes, when I had standard wheel and tyre sizes it read around 3-7mph above what Google Maps was telling me I was doing (depending on what speed I was going).

And yes Mikey, as far as I know most BMWs read the speed from the rear wheels (which is why the speedometer goes crazy when driving on snow), meaning that your larger profile tyre and therefore larger circumference is leading to a more "true to vehicle speed" reading. For example, if your speedometer was previously reading 60mph with a 35 profile rear tyre, it will read 57.7mph with a 40 profile rear tyre.

 

[smorris_12]

Legally speedos must never underread so you can't say "my speedo said I wasn't speeding" with a tolerance of something like -0/+5%. Since that is effectively impossible manufacturers work to +/-2.5% with an offset of 2.5% so most cars will read 3-5mph over.

They also have to allow for tyre wear and other variances.

Cars that get inaccurate as they get older will have mechanical speedos - and BMW stopped using those 35+ years ago.

 

[StudioWizard]

Ah OK! This is very interesting. I never would've guessed or thought about this. The silver lining is that the car probably has substantially less miles on it which is good for me because I bought for a pretty low price partially due to high mileage (160k miles), but bad news if/when I sell it. haha I may never sell it, though, because it's such a fun car and so far, seems VERY reliable.

 

[MikeyH]

All cars are designed (with stock everything) to read high. This is purely from a legal standpoint to make certain that their speedometers never read below the speed the vehicle is travelling, accounting for variations in tyre brand and pressures.

As far as the Z4 goes, when I had standard wheel and tyre sizes it read around 3-7mph above what Google Maps was telling me I was doing (depending on what speed I was going).

And yes Mikey, as far as I know most BMWs read the speed from the rear wheels (which is why the speedometer goes crazy when driving on snow), meaning that your larger profile tyre and therefore larger circumference is leading to a more "true to vehicle speed" reading. For example, if your speedometer was previously reading 60mph with a 35 profile rear tyre, it will read 57.7mph with a 40 profile rear tyre.

Yes, sorry I meant that the transit and Focus read faster than I was going.

 

[Mr Tidy]

I think every car I have owned had a speedo that read higher than the actual speed!

But my MC is more accurate since I fitted rear tyres one profile higher than the OE size.

 

[StudioWizard]

Does that also affect the odometer reading then? If so, that's not right since it would affect warranties, etc.

 

[Mr Tidy]

I've no idea, but if it did it would probably just make it more accurate! Warranties aren't an issue on a 16 year old car!

 

[inkey$]

I always worked to the 'speed limit +4mph' as a general rule of thumb in all my cars on motorways.

 

[smorris_12]

Knowing the innacuracy I happily set the cruise to an indicated 55 thru 50 average cameras and lightly stroll past everyone.

 

[StudioWizard]

I've no idea, but if it did it would probably just make it more accurate! Warranties aren't an issue on a 16 year old car!

True, but I'm just talking in general terms here. My Z4 is 18 years old!

 

[sars]

From regulation No 39, EU/UN, which is the type test for odometers

The speed indicated shall not be less than the true speed of the vehicle. At the test speeds specified in paragraph 5.2.5 above, there shall be the following relationship between the speed displayed (V1) and the true speed (V2).
0 ≤ (V1 – V2) ≤ 0,1 V2 + 4 km/h

This also means that your actual mileage will be much lower than your recorded mileage.

 

[axelleveau]

This also means that your actual mileage will be much lower than your recorded mileage.

That would be assuming displayed speed and "integrated speed" for mileage would be the same, which I am pretty sure they are not. I expect the ECU(s) use as close as possible to real speed for mileage calculation and a tweaked one for driver indication

 

[B21]

AFAIK the KOMBI module does the calculation based on wheel impulses from the ABS sensors..there is a table that is used to calculate 'true speed' based on the correct tyres/wheels etc.. BMW then applies a fudge factor to show the 'indicated speed' to ensure even with some minor errors it undereads.

I assume true speed is used for calculating total mileage..

Of course if you change the rolling radius it goes to ratshit!

 

[plenty]

As above, speedos over-reading by 5-10% is entirely normal for all cars. Manufacturers build in that tolerance as under-reading is illegal.

 

[StudioWizard]

LOL!! So many nuances about mileage I never even imagined. Somebody sat down and figured this all out. haha Hmmm... odometer vs speedometer accuracy...

 

[Z4Mariner]

My former 2008 BMW 128i had an analog speedo, but one could enable a digital speed in the Body Computer (BC) display. When coding for that, the 'fudge factor' can be coded "on" or "off" - so you can figure out the factor that's been applied.

 

[StudioWizard]

Interesting! The 2005 Z4 also has some secret gauge functions that I've toyed with... not sure if the speedometer accuracy can be turned on/off, though.

 

[Pondrew]

I had a Mazda6 a few years ago that had HUD and an analogue speedo. Both would read different speeds at any time. It was really annoying.
I would just pick whichever was showing lowest at the time.

Mazda have gone away from analogue speedos now, just a digital display with analogue rev counter. No idea why!