So I checked my speedometer about 1 1/2 years ago with Dunlop tires on the car and the thing was dead on accurate up to 80 mph. Today I checked it again with Bridgestones (same size as Dunlops) and it was 2 mph under at 20 and at 30, 1 mph under at 40, right on at 50 and 55, and 1 mph over at 60 and 70. Nothing else has changed. (Well, I've changed a little, but that doesn't count.)

Tire pressure? Tire circumference changes with speed more with Bridgestones than Dunlops? Inclination of the sun with the horizon this time of year? Car has PMS? I have PMS???

Any guesses?

 

Phase of the moon!
My '87 S4 was reading 10 Kph below all speeds...... Wanna bet the needle had been removed?

 

Yeah, come to think of it, the pod has been off a couple of times since the first test. Maybe something has been bumped or moved.

 

Ron, maybe that's whatever error was inherent in the measurement process last time, compounded by whatever the error was this time?

 

What was used to measure your speed, and when was it lastcalibrated .... ... seriously.

 

Palo Alto Speedometer in Palo Alto, CA. They are a VDO service station and did it last time as well. They put the car on a dyno and run it up to the different speeds. I then take the print out to the judge and tell him where to "file" it. Of course the judge doesn't always see it my way. But that's another story.

 

Well, it must be an issue with tires changing rolling radius at certain speeds. This may be due to different tire construction, different pressure, or both. I would expect a bad connection to exhibit the opposite symptoms, e.g. degrading with higher frequency pulses.

Or it could be that you've stretched the spring in your speedo...

 

Funny, that's the first thing I considered. That means Bridgestones are less dimensionally stable over varying conditions than Dunlops. To drive on them, I would suspect the opposite: Bridgestones feel more stable. Dunlops do give more audible warning before breaking loose, whereas Bridgestones seem to just snap. Maybe I could blame them in court......naw, won't work.