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If your tyres are oversize then your speedo will be reading slow and short of a few miles. That may explain your differences.
If you have the gear ratios in the owners handbook or workshop manual then you can work out what your speedo should be reading. The tyre diameter should be measured as twice the raidius from the hub to the road to allow for the flat spot due to vehicle weight.
Having siad that you should be able to use your gps to work out your distance travelled and your tank refill to work out a corrected average.
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