I'm skinny but still very heavy. Where does this put me?

Yeah, i figured it was a verbiage thing. Hopefully the OP returns to share some more details. :thumb:

So I guess I am completely wrong on the gear inches thing. ThermionicScott, what details are you looking for exactly.

Hey, welcome back! Gear-wise, the 72.5 on your MTB and the 74.6 on your proposed FG compare well, but it would be interesting to compare the crank lengths on the two. FGs often use 165mm cranks, and MTBs generally use longer cranks. If you look at the inboard side of your cranks, it should have a marking like "170" or "175". If you could find that out, we could use gain ratios to compare the overall leverage. :thumb:

That said, the approach I've taken is just to buy a handful of cogs and inexpensive chainrings,and then let my riding tell me which gearing works best. ;)

I did that today before leaving school, its a 170 crank.

Ah, cool. For gain ratios, I get 5.42 for the MTB, and 5.75 for your proposed fixed-gear if you use 165mm cranks. I'm not sure how big of a difference the crank length makes here, since we already knew your gearing was going to be a little higher. ;)

My advice would be to get a flip-flop hub and put an 18T cog on the other side of your 17T. You may find that you prefer a slightly lower gear for start-stop city riding. :thumb:

Alright, I think I'll end up buying some cheap cogs to try out instead. Thanks for the help :)

Gear inches is a formula, has to do with front tooth, rear tooth count and wheel diameter. So with a 26 inch mtb, vs a 700c FG/SS, the tooth count will have to be different to get exactly the same GI calculation. Weight becomes a factor when it's dead weight, whether it's unfit cyclist body weight or the actual bike itself. And then there's rolling resistance from the tire itself. So same gear inches with one bike is definitely going to feel different on 2 types of bikes. I can go much faster on 700c tires than I can on 26 mtb tires. I have several bikes and each has a setup that pedals & feels very similar, if not identical, but that doesn't mean that with the weight of the bike and every other factor considered, that I can go the same speed, even pedal it for the same duration without burning my legs out. That's the beauty though of having several types of bikes, I can use one to train and get more physically fit pushing myself on one bike and then when you ride the other, a benefit accrues when riding another. Naturally, you may out-fitness a FG/SS GI setup in a couple of weeks of hard riding on a bike that is capable of more.