Agreed. It's not the tires. It's either inside the speedo or the speedo sender/hall sensor mounted in the transmission case.

Right, so it sounds like a speedo can become inaccurate independent of the tires.
Since it's 30 years old, I'm thinking a free re-calibration can't hurt anything.

Have you actually gone and looked at the tires to see that you are running what you think you are running?

Then, have you given them even a quick diameter measurement to make sure that you didn't get some Friday afternoon mis-labeled tires?

I mean, if I got new tires, and my speedo was 4-6 percent off (we don't know how much yet really), then I'd be examining the first order change for an anomaly.

That's assuming that the speedo was accurate beforehand, which is also an unknown. Maybe you've just noticed now, or maybe it was 2 percent off before, now 4 percent off, but that's enough to notice 2 miles on a measured 50 mile drive?

Took the liberty to bold a passage in your quote.

2% or 4% is "spot-on" in my view.

Thank God for GPS. Told this story before about reaching terminal velocity of almost 140 mph in my driver SC. 2nd run in same location with GPS, mid-120's with speedo reading high 130's.

When my speedo says 80 mph, GPS says 73. Speed is too high, odo is too fast. Odo clicks off a mile before real mileposts are reached on highway.

I am not concerned but I would like to hear from sugarwood about how well his vendor dials it in.

The car had the wheel/tire upgrade before I bought it.
I simply replaced the old tires with same sized new ones.
Speedo was probably off when I got the car.
Right off the bat, the trip odo always seemed alarmingly fast.

Today, I marked off an 8.8 mile route with my DD.
I will drive the 911 tomorrow and compare.

But did you walk out and CHECK that the tire jockey put 245/45s on, instead of grabbing the wrong ones and putting 245/40s on??


Your DD might be 1-5% off as well, I think that the law is 3% error.

Yup. Get an app for your smartphone that uses gps to count the miles and MPH. My DD (BMY 525i) is off by about 2% with OEM size tires.

I verified the 8.8 mile route today. It clocked in at 9.0 in the 911. That's about a 2% error. However, the DD may be miscalibrated also, as it came with a wheel upgrade package.

Today, I verified a 49.3 mile route on Google maps.
Odo showed 50.9 miles
That a little over 3% error.