Speedometer Inaccuracy???

[53helofe]

I have a Garmin NUVi 765 that has its own spee display and it is always less than the speedometer. Below 40 mph the vehicle indicates about 2 mph less and up to about 100 mph it indicates about 3 mph less and at 120 indicated on the vehicle the gps indicated 115 mph.

I honestly believe the gps is accurate.

I do not know what wheel size the vehicle originally came with but since my spare is 1 inch smaller than the other wheels perhaps I should suspect that is my cuplrit.

I am not near the vehicle now so I can not post what is on the vehicle and what is in the "trunk".

Anyone else running an aftermarket gps that disagrees with the vehicle speedomoeter?

[motordavid]

Standard equipment, eg speedo error. These always read very optomistic.
Threads and threads of info about it, here on the Board.

Try Search Tab; it's your friend. 2 threads I pulled up in 5 seconds,
below.
GL,mD

http://www.xoutpost.com/x5-e53-forum/...edometer+error

http://www.xoutpost.com/x5-e53-forum/...ht=speedometer

[Jedi Mind Trick]

Actually that's happened to me before with Garmin as well.

I was doing 160kmph and it was telling me 150kmph. It was always less by 10. I figured it was a signal delay and I stand on that because the speed cameras agree with the car's speedometer and they're what I care about.

[x5skiddr]

I don't know about the gps's; but one day Iwas driving alongside my brother's;and he called me to tell how good I looked on new x5 lol. then I asked him how fast was he going,.. he was going 10 mph faster then me;and we were rite alongside ????? you be the judge!!1

[JCL]

MD has it right on, as usual.

One of his links even has a scan of the BMW TSB that states BMW's position on speedometer accuracy (the speedometer can read up to 10% + 2.4 mph over, but may not read under at all)

10% high reading on the speedometer is typical at highway speeds on a BMW.

If you transition from clear payment to packed snow at highway speeds, you will find even more error, due to tire slip on the snow. That one surprised me, I didn't expect it to be so noticeable.

[53helofe]

Thanks guys. I did a search with speedometer but not speedometer+error and would not have thought the thread about "does not feel like 100 mph" would have had anything to do with speedo error but just how darn good these things ride.

A little annoying to be so inaccurate on purpose but good to know it is intentional. I think 5% would be a more acceptable number without the +2.4 mph part. It is a digital system, there is no reason for such inaccuracy. I understand not wanting to ever have it read low but if an owner puts different wheels/tires on than recommended then that is just a result of such a thing.

[X5Sport]

I think the tolerance limits comes out of an international standard, and in the UK the Police should take no action against speeders if the speed they record is less than the 10% +/- 2mph above the posted limit. I also believe that the fixed & mobile speed cameras work the same way. As the UK is going to see a massive expansion of 'average speed' cameras from 2009, which log your registration number and if it appears again on another camera within up to 30 miles calculate the average speed over that distance it is going to be very important. The UK is becoming a real "police" state in regard to this area - personally I would rather see police on the streets than a camera any day.

I also belive that there is a correction system available if the error is 'out of limit'. I had a VW where the electronic speedo was replaced and the new unit was way out (over-reading by 15mph @55mph) according to my GPS. The dealer plugged in a computer and entered a correction factor and from then on the speedo was always within 4mph of the GPS even up to 100mph.

One question I have always wondered about though, is whether the odometer is driven from the speedo (as they always used to be when the were cable driven) in which case a speedo over-read of 10% actually means that your odo also over reads by the same and you have not actually done all those miles. Anyone know the answer as to how this all works on modern all electronic systems above the posted limit

[53helofe]

I think the speedometer is based upon a sensor in the transmission taking a reading off of a gear. I would imagine it is measuring a phase voltage and the voltage amount equals a speed that is then displayed on the indicator.

Reading the BMW TSB about the speedometer issue it did have a note that clearly stated it does not affect the odometer. With that being said, it would be interesting to know how it calculates miles/kilometers, or where the sensor is.

I am sure there is also a bit of secrecy on this because we would not want people to be able to manipulate the actual miles/kilometers driven on a vehicle.

[X5Flyboy]

mine works out to be a fairly constant 5% error, ie, 60mph = 57 actual - which has been confirmed many times by those roadside speed radars

[realchef]

I have used mile markers, travel at 60 and avg your time over a few miles. It's as solid as anything.