I'm messing around with a mtb that I use for family rides and turning it into more of a city bike. Mostly flat not to many big hills on the ride I use it for. Would it be better to run a 12-25 or 14-28 rear cassette? The 12-25 is a spare shimano ultegra I have from a road bike and the 14-28 is what came on the bike. It's a very cheap bike I picked up mostly to tinker with and have an alternative to my road bike for rail trails with the family.

14t and cheap sounds like a freewheel and maybe not compatible with your cassette.

14t on a mountain bike also sounds really short.

I'm guessing its an 8 speed out back?

No matter what cassette you look at, you have to factor in the chain rings up front. A safe guess (since its a MTB) is that you have maybe a 42, 34, 24 up front. That being said, and that you want to make it more street friendly for flatter terrain, I would run an 12/23 cassette.

Overall, that gearing would give you lots of gears but the ratios would be relatively close for flat ground riding. I did this on my Giant Innova. The original gearing was 42/34/24 x 11/28 giving me a huge gear range. I found that I never used he very low and very high gears so I switched the cassette to a 12/21. The gear ratios were much closer and the bike is really fun to ride now.

Hope this helps. Let us know what you decide and how it works for you.

Thanks clarkebre, I think I'm going to swap it out and see how it goes. Darth Lefty, it is very cheap. But the parts I have available are compatible, I'm hoping I can get away without changing the rear derailleur for a road derailleur. I only have 2 of them in my spare bin and one is very well used wore out emergency part and the other won't index shift (properly) with shimano sis, friction only.

Edit: they are both 7 speed.

May not even be a cassette, but a screw on freewheel .. whole different type of Hub..