Incidentally, you will find this on many vehicles. The speedo is often designed to read high compared to the actual speed (to reduce the possibility of it reading low, and getting the driver convicted of speeding offences).
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.