armoara

The center trip computer always shows about 5mph less than the speedometer. Which one is right?

Where is the trip computer getting its information from? The Nav+ maybe? Maybe the Nav+ needs to be fixed? Or is it the speedometer?

I have the 17" wheels with all season Continental tires, if that makes a difference. And I am looking to get rid of these to buy some sport tires, the all seasons just can't handle my driving style. Will I need to do anything special since the size of the tire is going to be different than the all seasons?

SFV A4

My 2003 only has an average speed display, not a current readout. Was something added for the B7?

zerinS4

Speedos in every car I've ever been in have read 3-5mph high.

bubbalito

Are you sure that's not the average speed? Could explain the difference ;-)

Jestapilot

While driving you reset the ave speed and it shows 5mph less than speedo. Mine does it too.

armoara

and set the cruise control to 80mph. And my average shows 75mph, when it should obviously show 80mph.

My TT was always accurate. There is no reason for a speedometer to show 3-5 miles higher than your actual speed. So which is right, the average speed or the speedometer?

Are you telling me that when everyone thinks they have gone from 0-60 in this car, they have really gone 0-55? Or if I am going 5mph I am really going 0? Haha. And does the speedometer get more and more inaccurate the faster you are going? If you are doing 130 are you really doing 120?

zerinS4

Uh, every TT I've been in has been 3mph high, as has every Golf, Jetta, etc.

Reggie

compared to the data on the trip computers and Nav packages.

Quattro mag had a article on it a while back.

SoCalB7

If you want to know whether the speedo is accurate, then you need to have the car tested by a speedo shop. (If you live in Southern California, AAA also offers these checks free of charge.)

As an alternative, you could drive a given distance with a stopwatch at a fixed speed, and then check the time traveled to make sure that you've covered the distance in the appropriate amount of time. (For example, using your cruise control and highway mile markers, drive one mile at 60 mph, and it should take 60 seconds.) However, it is possible for your speedo to be more accurate at some speeds than others, so you would need to check it at various speeds across the range to check for accuracy, and speedos tend to become more inaccurate as speeds become higher.

Using an average speed display on your trip computer is not an accurate way to determine the accuracy of your speedo. You don't know how the computer is programmed to calculate average speed.

My guess (assuming that you are doing as Jestnomen speculated below) -- that moment in which the computer is being reset momentarily fools the computer into thinking that the car is traveling at zero speed, leading it to calculate an average speed that is a bit lower than the true speed of the car. That trip computer is meant to calculate averages over meaningful distances, not serve as a real-time display. I wouldn't use a trip computer to determine the accuracy of my speedo.

pyewacket

that, in Germany, there are stiff penalties to the auto manufacturers if the spedometers read low, so they always build in a ~3% margin.

http://www.bimmerboard.com/forums/posts/111606/