GPS Configuration Settings
My GPS has a lot of configuration settings. I really like that. It allows me to optimize it for my use. I would love for it to be even more configurable. Here are a few settings that seem to make a difference for use with a bike.
Lock on road This option puts your position in the middle of the nearest road. It's nice when driving, especially around tall buildings in a city where you may not have great reception, and you don't need a precise track log. When cycling and looking at the track log afterwards, I would get wild speed changes as the GPS "corrected" my position to the center of a road. Turn this one off for cycling if you want the most accurate track log.
Optimize route for bike This one is hit or miss. Sometimes it puts me on really wild routes to skip a section of road that's great for cycling. I usually leave the route optimization set for "Car/Motorcycle" and just have the unit recalculate my route when I don't get on the freeway.
Track log setup This one's a bit tough. It's a balance of having the best number of track points for accurate post-ride analysis, but not having so many that as the GPS accuracy varies you get track points wandering around while you're waiting at a stop light. For most rides, I usually leave the "record method" set to "auto" and the "interval" to "most often". For the ultimate in precision, try dividing the total number of possible track points (10,000 for my Legend) by the distance of your planned route and then setting the "record method" to "distance" and the interval to that distance. I suggest avoiding setting the "record method" to "time" because you're forcing the GPS to write a data point even when you may not have moved - you'll end up with horrid track logs.