Speedo is electronic, accurate to within 1 mile up to 25 miles per hour, above that it shows 3-4 MPH high per international agreement . There are at least 2 threads explaining whole thing with website links and yes, between 25 MPH and 35 MPH the difference is somehow less, but above 40 MPH it seems to be consistent 3-4 MPH regardless of speed since it's artificially introduced error. Probably there may be some differences between cars, tire wear/ pressure etc. but it should always show fast above 25 MPH as long as it is per factory specs.