I used both a cyclometer (10 dollar Schwinn from Walmart) and a GPS on my tour. I used the GPS on a flat surface to calibrate my cyclometer. Since I like to use the GPS for more then just bike riding, I find both compliment each other. Like the previous poster said, the GPS measures distance horitzontally so you're lose a few pecentage of accuracy on steep hills. And in dense woods, the GPS can lose it's signal or get a weak signal which decreases accuracy. And also once in a while, the GPS will show you moving slightly even when stopped which will give false MPH average and false moving times. What I like about a GPS is watching my elevation climb or drop and seeing where I am on their map. I use the Garmin Etrex Legend which has a basemap.
It's nice to have a GPS but it is not a critical item. I love mine as you can tell by my nym on here. It was the most expensive item I took on my poorguy trip (worth more than the bike itself) but like I said, I use the GPS for more than just keeping track of mileage.
Have fun.
Cheers.
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