My new Passport 9500i has an option to use GPS to give a

continuous realtime display of speed. My 550i speedometer

seems to be 2-3 mph slower[or is it the Passport]. Anyone

else have a similar experience?

 

slower? check again... bmw speedos usually show 2-3 miles faster than it really is.

 

I used a portable aviation GPS* to check the speedo in my 525d. I found it to slightly over read over the range. 80 mph on speedo is actually 78.9 mph for example, in the UK a speedo must never read "low", i.e. say you are going slower than you actually are - some are as much as 10% out. So 88 on the speedo is actually 80.

I was quite pleased, it seemed to be always around 1.5% high. Given more time and a passenger to take the readings, I could probably work out the curve from normal over most of its range.


* I used my portable aviation unit as it uses differential GPS (WASS in the USA, EGNOS in the EU). This makes it accurate to around 2 to 3 feet - the unit can be flipped to automotive mode, to save the conversion from knots to mph (and the warnings about being too low!)

 

Speedo accuracy is also affected by the condition of your tyres.
My Speedo was reading about 6% high according to my GPS. Then I got some new tyres and the Speedo was absolutely accurate.
It doesn't seem possible that a few millimeters off the tread would make that sort of a difference, but obviously it does.

Mike

 

I ran a short test today on the BMW speed error.

I used an aged Magellan Meridian (has WAAS capability) hand held GPS receiver. Ran at some selected speeds and noted the speed on the Magellan. I set the speed on the ACC.

@ 40 mpg the Magellan read 38.2
@ 50 mpg the Magellan read 48.2
@ 60 mpg the Magellan read 58.1
@ 70 mpg the Magellan read 68.2
@ 80 mpg the Magellan read 78.3

The mph read out on the Magellan was not steady (it varied ax .5 mph) so I had to guesstimate a median value. It is curious that the error is near constant in absolute value (1.8 mph) and not a percentage.

This would be difficult with a mechanical speedometer, but not when you ?drive by wire?!

Even more incriminating, the OD for the entire trip was near perfect ? measured 43.9 miles on the Bimmer and 43.92 miles on the Magellan.

 

I've checked mine and it is 2-3mph off between 50 and 80 mph, where the speedometer reads faster than the GPS. It really wouldn't bother me other than the warranty piece. This translates to my 50k warranty running out about 2500 miles early because the odometer doesn't accurately represent the mileage.

A similar issue happened to Acura, and they proactively came back and gave all MDX customer a 5% grace period on the warranty. That was a sweet deal.

 

This is not new news. This is so the manufactures don't get pulled into speeding ticket suits and because they use the same speed calculating hardware over a range of car models and configurations, so they are always optimistic.
If you are driving with a driver only vs. five people plus full trunk of luggage, the speedometer will read differently because the compression of the tires would cause the car to go slower still than indicated on the speedo.

 

I have a 2008 528Xi and have checked the speedo with a digital stopwatch and the mile marker signs many times and my speedo at 72 is actually about 70.6 to 71.0 I have done about 25-30 calculations. It is more accurate than my 2004 A8L.

 

Have you tried to access your hidden screens on the dash readout to see the discrepancy between how fast the car thinks it's going, and how fast your speedo says??

Mike

 

hmm...makes me wondering the limited top speed of 155mph is actually only 135ish??