Was that 20.5 mpg the figure shown on the trip computer -- or was it actually figured by dividing your miles driven by the total gallons consumed?
If you indeed got 20.5 at 80+ mph, that is decent - and would match the EPA mileage for this vehicle (17 city/21 highway) with the V6 engine. I find that my actual mileage (not the optimistic number shown on the trip computer) is around 20 to 21 mpg if I can keep it right around 70 mph on a long journey. However, the trip computer will show 22 or 22.5 mpg.
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Subtract 1.5 to 1.8 mpg from what the computer says, and that will give you a better estimate of your "real" economy. Better yet, keep a little notebook in your 4Runner to record your actual fuel economy and don't forget to reset your computer when you fill the tank. Otherwise, it will keep computing your "average" over more and more miles - and the discrepancy will keep growing as you drive off into the sunset.
Oh, I'll be taking my 4Runner to Michigan again next month. I pulled pretty decent mileage last year, but hope to improve a bit this year. Let's hope that I don't get stuck with "Katrina" prices on the way home this time. When I left VA last year, fuel was $2.49 a gallon and jumped to $3.49 in less than one week!!!!
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