I want to get a flowmaster 40 series for my 2001 Harley Supercrew and want to know if i will see any noticeable gas mileage gains.

 

Welcome ! You may get noticeable power, but this is not a mileage improving modification.

 

i see that magnaflow says 10% mpg increase, is this not the same for flowmaster? if i get one to two miles more i will be happy.

 

If you searched the zillion posts here on this topic, you would find that some say they gained 1-2mpg, some say 0, and others say they lost gas mileage. The bottom line is that muffler exchanges are not the place to expect increased gas mileage. It's VERY, VERY common for those in sales to BS product benefits. Most of what's blabbed out could easily be cut in half. How else do you think they find people to buy throttle body spacers and air intake wind spinners? Try finding someone that logged mpg for 10,000 miles stock, then logged 10,000 miles with ONLY an upgraded exhaust, and ALWAYS averaged a 10% increase.