Will the M25 do for that test?
Guess you'll be doing that one in the middle of the night when it's quiet - if you want to get a constant speed run??
As others say, car speedos are allowed to read up to 10% fast - this allows for tyre wear and not under-reading with new tyres. The only way to be sure is proper calibration on a rolling road.
My old 4/4 actually under-reads, but I know by how much (I suspect it's due to age and a lack of use for a few years before I bought it). The +4 seems reasonably accurate. The MX-5 reads high, but within 10%, as for the 90, I haven't bothered to check it.
Cheers,
