Replace crankset or cassette for better road performance on an MTB? I few years ago I got into cycling again to help keep fit. I still had my mountain bike from when I did some easy trail riding. 

I'm mainly riding on the road now and I was wondering if I can change the crankset or cassette might give me a little more speed. A couple of months into the warm weather and I top out the gears even on a slight downgrade.

There's one very hilly ride I do and the toughest hill I'll sometimes go as low as the second or third lowest gear in the beginning of the season until I get used to riding again. I only ride when it's warm out. If anyone's in the NYC/NJ area it's the hill by the Englewood Cliffs boat basin in PIP.

I have an old Gary Fisher Gitche Gumme (specs in link I think the same year) which has:


Shimano Altus, 24/34/42 teeth crankset 
7-speed, 11 - 28 teeth cassette. (Actually cassette might be 8 spd 11-30)  
Shimano Altus top-swing, top-pull front derailleur 
Shimano BB-CT91E, 116mm spindle bottom bracket


In a year or so I might get a road bike but hoping I can get a little more life out of this one. I saw the Shimano FC-M131 48/38/28T 175mm which I like because I'd prefer longer cranks and also the FC-M311 48/38/28% 170mm which both seem like reasonable upgrades to make to a bike this old.

I have no idea if these will fit the bottom bracket I currently have (which seems fine) or if I'll need a new one. Or would I be better off changing the cassette?

I'm not looking to win any races or set any mileage records just don't want to keep hitting the top gear and not having any resistance while pedaling.

I don't know enough bike parts to know if the 2 cranksets I mentioned are easy to swap on my bike or not and would appreciate input from someone who knows more or recommendations on other cranksets that would fit better. The 2 currently are going for 30USD and I don't want to put more than 50-60 into this old bike.

Update:

I haven't changed anything on the bike. I have been working on my cadence and that seemed to be the best approach. With my increased cadence I'm rarely topping out the gears on level roads. Keeping the tires at the upper range of pressure helped too.
Since you already have an 11 tooth smallest cog you're not going to be able to swap that for a smaller on. Well, you can get a 10T cassette but I don't recommend it, they're designed for small wheel bikes where the faster wear on that cog is just part of the deal.

Swapping to a road triple crankset should be do-able, and with any luck your front derailleur will work with the new one. A cheapish 30-39-50 is about $60 on ebay or amazon, so probably a bit over $US100 fitted if you go to a bike shop. But since you'll need extra chain to wrap around the bigger chainrings, best budget for a new chain and rear cassette (normally those get replaced together. See here and here for why) which will probably be another $100 or so. Call it $250, but ask for a quote because this is a pretty standard swap. 

If you're unlucky you might need a different length bottom bracket ($50-ish), or a new front derailleur (plus gear cable, $100-ish). Those are things the bike shop probably won't know until they start fitting parts to your bike, so the only way you'll get a certain quote up front is if they decide to replace them regardless.

That will give you noticeably higher gears across the board, but obviously you lose the low gears (you'll effectively lose your smallest chainring and gain a much bigger one at the other end.

Or fit a slightly larger MTB triple. That will give you less gain in range but is more likely to work with your current derailleur and bottom bracket. You can easily get 28/38/48 sets, and the difference in the top gear between 48T and 50T is small. But that's the problem - you're going from 44 to 48 or 50T, and while either will be noticeable, it's not as much as you might think.

The difference is in how easy it is to go even bigger. On a road triple getting a 52/54T chainring is easy, but with an MTB crankset those chainrings get pricey if you can find them at all. We have a local manufacturer who CNC cuts chainrings, so we can get anything we like. I expect people like that are on the web as well, but you'd have to look (and try to buy local since you're fairly likely to want to swap them when they don't work on your bike because they're too big).

A completely different approach would be to fit fat slick tyres. Or even fat knobbly ones. If you're riding for fitness, the extra drag (and very slightly higher gearing) will be just what you want. Schwalbe make their "fat frank" tyres as big as your bike will take, or "Big Apple" out to 2.5". The big tyres will be noticeably harder to push along at the same speed. And remember, as the cops here keep telling us "the faster you go, the bigger the mess". Going fast just because you can, rather than because you have somewhere to go, doesn't make a lot of sense. I commute ~20km each way on a nice comfortable upright commuter bike that will go 30kph if I push. So I get fitter faster for the same distance ridden. And when I'm lazy I pull out my drop bar racer and do the same ride in 3/4 of the time (33% faster average speed). But I'm more likely to crash on that bike, and if I do it'll hurt more.