Ok, I have a 06 Grand Caravan SXT with a overhead console. That has the mileage trip odometer computer. I have been keeping track of my mpg's for business reasons. I find that the computer is off by about 2 mpg. I fill the tank full every time. Then I use a hand calculater to figure out my mileage for the previous tank. It shows that the computer is off by that amount. The formula I use is the Miles divided by Gals. = mpg. I also use a GPS a Garvin nuvi. I reset the data to zero every morning before I start out. One day last week it showed I went 372 miles, and the actual vehicle od. showed I only went 366 miles. Also I notice that speed on the GPS indicator said I was doing 70 mph. The vehicle od showed I was doing 68 mph.
Go figure!

My daughter tells me I still have brown hair in the back. Frankly I don't see it.