I've said it before in other threads but it's worth saying again:
You have to calibrate your odometer before you can calculate your true MPG.
Most odometers/speedos are off by a little but but adding bigger tires increases the error.
Think about it: Your odometer doesn't "know" how many miles you're going, it only knows how many times your driveshaft is turning. If you have bigger tires you will go further with fewer revolutions of the driveshaft, so the odometer will "think" you've only gone, say, 100 miles, but you've actually gone 105.
Best way to calibrate, IMO, is with a GPS. Most GPS units have a tripmeter on them. Set the GPS tripmeter to zero, then set the 4runner's factory tripmeter to zero. Drive around (the longer the better.) When it's done you will know how much of an error your odometer has.
Then, when you fill the tank and calculate mileage, you will have to multiply the miles by the conversion factor. Say your conversion factor is .032 (that is, it underreports miles driven by 3.2%). So when you fill the tank, you take the number of miles shown and multiply it by 1.032 to get actual miles driven. Divide
that number by the number of gallons put in and you will have the
true mileage.
As for my 4runner, I started using a web site called Fuelly.com to record my MPG. So far it has been consistently around 18 - 19 mpg average, which is right where it should be. I've gotten as high as 22 on long stretches of straight, flat road.
My Fuelly.com "badge" is below, showing my MPG:
and this is my DD Mazda's badge:
Which explains why I drive the Mazda whenever I can!
