I had that problem before. I’m not sure if STRength actually is taken into consideration in order to calculate the max bike speed (well, it uses MUSCLE fuel, so it’d make sense) but my experience seems to suggest that you can only get to max possible speed if your character has 10 or more STR.
My guy with 8 STR wasn’t able to outrun shocker zombies, but another 10 STR one was able to do so without problems. By outrun I mean that you can outrun them, but by a very small margin, and they always catch up with you when making a turn, not to mention that you’re always at zapping range. That particular character was finally shocked to death while riding the bike. While in full health, mint condition, etc.