Dunno. What are you riding? It sounds like you might be "mashing" too high a gear on the flat or might have some aerodynamic or tyre problem.
This guy goes fast: https://fitrecovery.wordpress.com/the-noobs-guide-to-cycling/ - I don't think it's worth it or much fun, but he seems pretty accurate on the few bits I've tested.
Im trying to figure this one out:
20+ mph is where “aero” makes a decent difference and 25 mph is where it’s a huge advantage… How much? If you went whole hog with aero, bike, 50 mm deep rims, aero helmet, tight fitting clothing… At 25 mph, over one hour you’d save three minutes (that’s more than a mile folks, it’s significant)
So at 25 mph, you will cover 25 miles in an hour on a normal bike, but you will do it in 57 minutes on a TT bike (Which I assume is either made of carbon fibre, or dark matter).