I recently did a tire rotation. I seen my fronts wore quite a bit more than my rears. It's been about a 7k-8k since oil and tire rotation. I like to rotate every oil change. I run 215/70R15 tires. They used to read +3mph and a little before the rotation, I thought my speedo was reading more accurate but randomly. Since the rotation, it seems I'm only +1mph now. When I had stock size, it was actually -1mph. I've also noticed my mpg getting a little better. (on the trip counter anyways) I recently found out that a Mini Cooper gets its reading from the right rear wheel. I was wondering if my 2010 Prius 193.5k was the same or maybe I've worn my transmission or just the tires wearing down, etc Thanks, Higgins909.
I really, really, really doubt it's the transmission wearing out. However, don't forget to change the oil in it though as recommended in the manual or sooner. Bigger tires will make the speedometer read slower than your actual speed. I hope that helps. I'm unsure where the speed is read from. I would suspect part of the transmission as the MG2 spins at the same speed as the wheels. But with a speed sensor on each wheel for antilock brakes, who knows, it could average all of them out. Hopefully someone with more knowledge chimes in soon.
Something’s off here: stock tire size will without fail have speedo reading high, say 1~2 mph at 50.
The brake/skid control ECU has four wheel speed sensors, one at each wheel. So it can tell if you're going straight, or curving, or skidding, and it can compute an overall speed for the car based on the ones that don't seem to be skidding. That number computed by the skid ECU is where the combination meter gets the info. It gets the number by asking the skid ECU, over the network. If I remember right, the number you can see by querying that PID from the skid ECU is pretty accurate, and the slight exaggeration shown in the meter display is applied only there, for some kind of regulatory reason. The skid ECU also generates a pulsing signal, at kind of a standardized so many pulses per wheel rev, because that has been a traditional way that various other accessories get the speed signal (like the audio amp for boosting the volume at higher speeds, and so on). That signal is also wired to the combination meter, which buffers it and repeats it back out to any other accessories that use it. But the number the meter actually shows you doesn't come from the pulse, it comes from asking the skid ECU for the actual number. The way most people become aware of the speed pulse signal is because of an apparently common way for the skid ECU to go flaky, where the speed pulse signal gets coupled onto a wire that goes to the hazard flasher relay, and suddenly people notice that the hazards unwantedly flash, and flash faster the faster the car is going.
On the Avalon I have a "mechanical" spedometer, a digital one on the dash and I can see another read off of the OBD II port with a ScanGauge. All three read differently within about 3 or 4mph at 60mph.
