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I like to use a Garmin 305 during races for pacing.(Let's take that as a given.) In other words, I set a target to complete the distance in a certain time, in effect running flat splits (On the Garmin this can be set and then seen directly using a virtual partner.) How do you compensate if your GPS or footpod measuring device indicates a different distance to the real distance run, as a race or event gradually unfolds?
Twice now I have run a marathon in which the 305 gradually showed more and more discrepancy with the km markers. By 21 km on my last mara, it was showing 21.6 km. When I woke up and realised this meant I had actually been running slower than my target, I had to speed up by 3% for the second half just to get back on target, something that proved too much on the day. Plus mental arithmetic is difficult at such times.
I just wondered how people cope with this: do you look at every marker to check, and adjust speed accordingly? Maybe you add a bit onto the target distance before you start, to allow for any inaccuracies? I'd probably try the former next time, but would be interested in any experiences or solutions.