[brucepick]

Yes, it has a correction adjustment for speed/distance. It's on pg. 17 of the manual. Follow Menu > More > Setup > Speed.
You can increase/decrease the speed/distance in 1 % increments.

I haven't had a need to adjust this correction (I don't do rallys).

Apparently it reads out current speed while you adjust the correction so if you have a GPS running along side, you can get it dead on, within the 1% increment limit.

Otherwise, my understanding is you need to determine the correction you want to apply, then enter it as + or - the % needed. That is, if you're stopped, the speed shown is zero but you can still enter a correction %.