I have the opposite problem; I'll hammer myself to a way-early bonk if I don't have the comp sitting there telling me that I'm still doing 12 when I feel like I'm only doing 8.

OTOH, my $20 Bell wireless says the bike is going 64MPH with a U-lock through the back wheel when I park next to the Coke machine at WalMart. Imagine my suprise when a ride shorter than my 5.6mi round trip commute showed 35 miles. The DrPepper machine at HEB doesn't cause that problem.