[doviatt]

I was discussing MPG improvements with friends at work and realized most don't even think their odometer/speedometer might be off and so haven't checked it. They rely on the multifunction MPG display. I've never seen one to be correct. Tall tires on my Metro make mine off by quite a bit. I still use my speedometer and odometer but multiply by the constant I came up with during my calibration. I mentally remember where my needle needs to be for my standard commute target speeds of 50, 55, and 60 which of course reads lower. The last thing a hypermiler needs is to drive faster than he intends to. I use my multiplier constant when resetting the trip odo at my fill ups.

My calibration included 4 trips of at least 60 miles each (round trip to work). I took 2 GPS (one on my phone and a portable aviation unit) and also verified my route distance with google maps. I averaged out the GPS trip distances and then came up with the multiplier to try against my odo reading. Worked quite well. I check it again every few months and every time I change tires.
Use the inverse of the percentage delta as a multiplier if your tires are big and your speed reads low. I guess it would just be a percentage if the opposite were true.

Oh BTW good thing to note is any scan gauge or eco trip meter in your car will always be off for the same reasons as it gets its data from the same sensor.