Seems unlikely that they'd interfere, but about the only way to be sure is to try it out and see. I have an old Garmin GPS-V that always loses reception when on the dash in my daughter's car when the car radio is tuned to somewhere in the AM 540 - 580 kHz range. That's not anywhere near the GPS frequency, nor would the radio's local osc. freq. be in the right range, but something clearly causes a problem.

It's not uncommon for GPS-based max. speeds to be off sometimes. Lots of things can momentarily give GPS readings that are off a little (a switch in which satellite signals are being used, one or more satellite signals blocked from direct line-of-sight to a bounced signal, etc.). It generally recovers quickly so the odometer and average speeds aren't affected much, but a momentary glitch can give you an unrealistic max. speed.