Well, since mosquitos can only fly 2 mph max, I suspect one of three things: 1) Perhaps you are slowing down more than you claim to be. 2) You are getting bit by some other insect. Deer flys would target your back but I'd guess you would be able to tell the difference between a deer fly bit vs. a wimpy skeeter. Or 3) It may be that there are just a lot of mosquitoes where you ride and instead of chasing and catching up, you are just encountering successive clouds of them.
Anyway, to respond to your post: I've used DEET successfully for everything except chiggers. I don't really like the oily skin feeling though. I've started using permethrin treated clothing and it seems to work better, though it's a hassle because you need to wash separately.
Good luck.