The only way to determine mpg is to keep a log of miles and fuel. Filling a tank can vary each time. My '83TD with 4-speed conversion will see 28-30 beating around town and 32 or better on the highway if I keep the speed under 85. I've calibratated the odometer using the mile markers over 50-100 miles and you can also get a real accurate speedo calibration at the same time.