If your speedometer is reading faster than your actual speed by 5% then your odometer is reading 5% more miles than actually driven. Thus your mpg will seem to be 5% better than actual. To correct this divide miles driven (say 350 mi.) by 1.05 ( percent error) equals 333.33 actual mi. For a vehicle like mine with oversized tires I multiply my miles by 1.08 (8 percent error) because I'm going faster than speedometer indicates. I hope that clears it up.
wungun 
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