Speedometer and trip computer speed don't match. Which is correct?? - The Unofficial BMW M5 Messageboard (m5board.com)

mkeetn · 25th August 2005, 18:57

I am loving my "new" M5 and appreciating reading the board for insights into the beast.

Cruising on the highway with speedo reading 65mph (don't ask why so slow... yes I forgot the V1!). Reset the trip computer speed and it came back with 59.5mph. Car has the Dinan differential which from doing a search on this board, should not affect the speedo reading. Also, have 275/30/19 rear tires and 245/35/19 which should have minimal difference compared to stock. According to a tire calculator should be less than 0.3% on rear and 0.1% on front compared to stock.

So, I guess I am confused about why the computer and speedo don't match. Since, I routinely cruise a few over the speed limit (watched over by the faithful V1), I would like to know exactly where I am at, in terms of do I believe the speedo or the computer. If the computer is underestimating speed then I would think distance and fuel consumption might also be off. Does the computer pull data from the GPS in which case the speedo is significantly off?

Any thoughts?? Thanks, Mark.

MINENCE · 25th August 2005, 19:13

Trip computer shows the average speed from the time it was activated for the duration of the trip. So if your speed has fluctuated during your trip then it would effect the reading on the computer.

Need4Spd · 25th August 2005, 19:17

The computer is more accurate. The speedos on all BMWs are purposely set to read optimistically higher than actual speeds, usually by around 5%. BUT! YMMV, so don't blame me if you get a ticket.

mkeetn · 25th August 2005, 20:03

Quote:

Originally Posted by MINENCE

Trip computer shows the average speed from the time it was activated for the duration of the trip. So if your speed has fluctuated during your trip then it would effect the reading on the computer.

I reset the computer while I was crusing at 65mph. So, that should get rid of any averaging for the trip. I figure the speedo is underreading by 10% which seems like a lot.

Thanks, Mark.

MEnthusiast · 25th August 2005, 22:19

Do a search on speedometer error. BMW's spec is something like 3 MPH over and then something like another 5% on top of that....

mkeetn · 25th August 2005, 23:09

Quote:

Originally Posted by MEnthusiast

Do a search on speedometer error. BMW's spec is something like 3 MPH over and then something like another 5% on top of that....

Thanks. Did the "speedo error" search and found lots of data showing that there is some error that seems pretty variable depending on the particular car. Still, can't help wondering whether the OBC is showing accurate data which would help develop a sense of how far off the speedo really is. Since the car has a built in GPS you would think it should be able to pull totally accurate data into the OBC if it was built that way. I guess in the end the best way is to use a handheld GPS and verify both readings for each individual car.

Cheers, Mark.

mkeetn · 25th August 2005, 23:34

Quote:

Originally Posted by MEnthusiast

Do a search on speedometer error. BMW's spec is something like 3 MPH over and then something like another 5% on top of that....

Thanks. Did the "speedo error" search and found lots of data showing that there is some error that seems pretty variable depending on the particular car. Still, can't help wondering whether the OBC is showing accurate data which would help develop a sense of how far off the speedo really is. Since the car has a built in GPS you would think it should be able to pull totally accurate data into the OBC if it was built that way. I guess in the end the best way is to use a handheld GPS and verify both readings for each individual car.

Cheers, Mark.

robord535 · 26th August 2005, 00:11

I set my cruise control speed using the aid of the OBC. The speed limit is 100km/hr, I set the OBC to average (with no speed fluctuation to 102km/hr), the speedo reads about 107-108km/hr; I have oncoming cops check me (detected by V1) and they don't pull be. I have all stock wheels/tyres/diff. The difference was worse in my e46 330i. I'd go with the OBC.
The good thing about that is it in theory improves upon my speedo readings for 0 to 100km/hr which was 5.5secs when inexperienced with the car and playing safe with the clutch.

Richard in NC · 26th August 2005, 03:26

The OBC is more accurate. My speedo is off by 3 mph normally. One way to check is on an interstate drive with mile markers. Use the timer and set it at a mile marker. Set the cruise to 60, or 75 mph. At 60 mph 5 miles takes 5 minutes and at 75mph, it takes 4 minutes. Every 4-5 seconds off is 1 mph of speedo error.

WannaM5 · 27th August 2005, 02:00

Here's an excerpt from a C&D article explaining the discrepancy between the speedometer and the OBC:

"The European regulation, ECE-R 39, is more concise, stating essentially that the speed indicated must never be lower than the true speed or higher by more than one-tenth of true speed plus four kilometers per hour (79.5 mph at a true 70). Never low. Not even if somebody swaps a big set of 285/35R-18s for stock 255/45R-16s. There's your explanation of high-reading European speedometers, with the highest readings on Porsches and BMWs that are most likely to lure owners inclined to fool with tire sizes. Of course, only the speedometer must conform. Trip computers are free to report average speed honestly. Try setting your BMW or Porsche cruise control and then resetting the average-speed function. Unless you've screwed up the tires, the trip computer should show a nearly accurate reading. Even General Motors, whose domestic speedometers are the best, must skew its readings slightly high on vehicles exported to Europe."