Originally Posted by Kuakeye

I recently purchased a Trek FX 7.1 just to commute to school and also to get back into exercising. I usually bike around 20 miles once a week on relatively flat terrain. Hopefully I will eventually be able to bike much further.

One gripe I have about the bike is that the gear ratios are tailored towards mountain bike style gearing, and I find it way too slow for me. I am usually always on the highest possible gear ratio and still pedaling too fast. I can hardly go faster than 20-25mph without being completely winded from pedaling too fast. My legs at that point don't feel tired at all.

The configuration on my bike is stock: 28/38/48 on the crank, 7-speed 14-34 teeth cassette with a HUGE first gear (I never use this)
So I want to upgrade to something that will fit my needs better: making this hybrid bike act a bit more like a faster road bike than a hill-climbing mountain bike.

I found a few Shimano 7 speed cassettes with a lower sprocket of 12 teeth online, but I'm not sure if I can still keep my chain, or if I need to do anything with my derailurs, or anything else to consider. I figured that the lower teeth number on the cassette would help me to be faster whole maintaining a more comfortable cadence. If I went from 14 teeth to 12 teeth, how much faster would I go? Is it even worth the upgrade at that point?

Shimano CS-HG20 7-Speed Cassette, 12-28T (Amazon, apparently I can't post links until I've posted 10 times in this forum...)
Shimano CS-HG200 7-Speed Cassette - 12-32 (Amzn)
SRAM Pg-720 7Sp 11-25T Cassette (Amzn)


Also a question before buying anything, do I have to use Shimano cassettes since I have Shimano Acera derailurs?

Any tips or recommendations would be greatly appreciated. I'm a complete beginner to the bike customization world. I've been riding old cheapo bikes for as long as I can remember. Thanks in advance!