[imjin]

You're dead on with the speed/rpm threshold. The general consensus seems to be running at roughly 65 (max) or keeping the RPM's at 2K or lower seems to really help.

I'm wondering what the size differential is between your original tires and the new set. Your wheel speed sensors are calibrated at the factory to the tire size that came on the vehicle originally, and that calibration doesn't change. I'm not even sure the dealer can tweak that. In using my own tires as an example, with larger than stock tires having fewer rotations, the computer is interpreting the sensor data incorrectly which in-turn throws off the speedometer, odometer and trip meter readings into a negative direction, displaying a slower speed than you're actually going, which in-turn causes the odometer to add fewer miles than was traveled.

Imagine cutting into one of your original and new tires and stretching them out flat on the ground like a piece of spaghetti, side by side. The original larger tire would be longer than the new smaller tire. If you flipped them end-over-end, the larger tire would reach the destination in a shorter amount of time than the smaller tire traveling the same distance. When the computer counts the number of revolutions that it uses to calculate miles traveled, the actual versus the programmed revolution count would be off slightly. Over a period of miles traveled, that "slightly" starts to get multiplied.

With smaller tires, as in your case, they would turn more rotations and cause the opposite effect on the calculations, adding speed and distance to the speedo, odometer and trip meters. So, if you're speedo says you're going 55, you're probably going a bit slower than that and your odometer and trip meters will start to add up more miles than you actually traveled, which MIGHT account for the improved mileage readings you're getting. Your RPM's will probably drop because of the decreased mass the motor & transmission has to turn. If you're using these to manually calculate MPG, they might be a little off because of that. Have you tried using a GPS to see how close your speedometer is reading? Other than a dyno, it's probably the best way to gauge how fast you're going over what the speedo is reading.

Using my truck as an example, I went from the stock size of 265/70/16 (30.60") to a 285/75/16 (32.83") and it threw my speedometer off by about 4 MPH into the negative, so my speedo was showing a speed of 55, but in actuality I was going about 59. I just recently installed a Truespeed calibrator to fix the problem, but I had been driving it like that for about 4 years. I've also been keeping steady fuel data for the last 4 years, from each fill-up and I'll probably need to go back and adjust the mileage numbers to compensate now that I can get a good set of true mileage reading, which affects my MPG averages.