Better check your car while you're at it. A large number of American cars are off, but less than our ST1300's. Durango is 3 MPH off once you reach 20 MPH. My ST is dead on up to about 25....then goes up to between 5-7% off. Toyota Corrolla are spot on ( all 3 we've owned,so far). Been told manufacturors can be fined if they read below actual speed.
I discovered a few months after I bought my 03 Mustang GT 2+ years ago that my speedo read l
ow. On my first real road trip in the car, from Houston to Ill, I had my Garmin GPS stuck to the windshield and noticed that at 80mph indicated, I was actually going 85mph. Then I discovered that a previous owner had replaced the correctly sized 245/45ZR17 tires with skinnier and taller 235/55R17 tires. I lived with that until I had a flat that destroyed the tire, so I replaced it and the most worn of the three remaining tires with a pair of the correct size tires, installed on the rear - the front tires are still the wrong size, but aside from that are perfectly good tires so I plan to leave them on until they need replacement. A block or so after I left the tire shop with my new tires installed, the traction control engaged at ~40mph - oops, I forgot that would happen. Slowed down, turned off the TC, all was good, I just had to remember to turn off the TC every time I started the car before reaching 30-35mph or so. That worked fine for a few weeks, then the ABS light started to intermittently light up. I suspect that was also related to having two different tire sizes on the car, but don't know why it didn't happen right after I had the new tires installed. There may be something else that caused the ABS failure, because sometimes the light would stay on after starting the engine, before the car had moved an inch. when that happened, the ABS failure would disengage the TC, but then if the ABS light went out while driving the car, the TC would instantly engage if the car was going over 40mph. I eliminated that problem by unplugging one of the fuses for the ABS system (the only fuse I could find that didn't affect anything else but the ABS). Now I'm driving a car again with neither ABS nor TC, just like all the other vehicles I ever owned prior to 2002. It's a little more fun now, since I can once again cut donuts on a parking lot after a rain shower without mashing the TC OFF button first, since the ABS and TC are both disabled now.
BTW, putting the correct tires on the rear corrected the speedo problem as it now indicates nearly spot on, just slightly optimistic.
