A difference of 2 MPH sounds like a fault on one of the computers.
There will be a difference between two 26 inch tyres, because the external tyre diameter will vary a bit, but 2MPH is too big a difference for that to be the case.
Richard
p.s. I wouldn't advise measuring in inches then converting to get a millimetre figure because of a loss of accuracy, but there's 25.4mm to the inch.
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Currently riding an MTB with a split personality - commuting, touring, riding for the sake of riding, on or off road :)