How much more speed would you get out of a MTB with semi-slick tires such as the Kenda Kross tires? I will buy some when my factory tires give in, which shouldn't be anytime soon. I don't expect it'd be much, but would it be less strenuous on your legs for pavement/gravel? Thanks guys

Edit: 26" x 2" tires currently, forgot to mention

You may gain a bit but not a whole lot unless you have some serious knobbies on there now. If you want to keep some offroading capabilities, I'd go with these:
https://www.nashbar.com/bikes/Product...2_10000_202470
I've used them (or the Performance version of them) on every MTB I or my family have owned for at least a decade. They ride nice on pavement but still do OK on dirt and gravel.

If you are going to stick to the streets, go with a narrower, higher pressure slick tire. Something like these:
https://www.nashbar.com/bikes/Product...1_10000_202472
if your rims are narrow enough.

It's not a huge amount, but it's definitely noticeable. I put slicks on all my mountain bike commuters. (Specialized Fatboys on the Hardrock right now.) I'd guess you end up with an extra 1-2 MPH with slicks, probably something similar with semi-slicks. If you don't go off-road, slicks are the way to go in my opinion. It's been a long time since I converted over to slicks, but the difference was immediately obvious & appreciated.

Yes, it is totally worth it to got with slicks for pavement. Semi-slicks give you a compromise, but also some of the worst of both worlds. They are seldom buzz free on pavement, and cornering on the side knobbies is dangerous. I like: https://www.performancebike.com/bikes..._20000__400237 for the money. If you want high end, go with Schwalbe Kojaks.

You should notice quite a difference if going from full knobbies to slicks, semi-slicks, or inverted tread tires. At least I did. All the above mentioned tires would be good choices, IMHO, depending on your particular application. I have these in 1.5 on my mountain bike. They're great on road, gravel, and grassy and hard packed stuff. I haven't had them on anything else, and I don't use that bike for commuting. I would probably go with something like that Nashbar, or the WTB, or even a pure slick, if commuting any distance, and all on-road.

IMHO tires can make a huge difference. Also it's not just a matter of choosing tread type or tire width, rubber compound and the quality of the casing also comes into play. In my experience a high quality tire like Schwalbe Fat Franks roll better than a much skinnier and less expensive tire like a Kenda Kwest.

Schwalbe marathon plus.. the serious puncture protection adds weight ,
but , fixing flats, on the way in, is not something to help get you to work on time.

I have a set of 26-1.75, they've been excellent.

+1 for Schwalbe

I run a Schwalbe Big Apple in the front and a Marathon Racer in the back. For winter use ( in Montreal, Quebec) I use a pair of Marathon Winter.

Slick on pavement offer a great avantage over knobby tires.

Fat city tires are incredibly cushy for pavement riding. I would go with that if I had a mountain bike. I would not worry about speed too much. Aerodynamics are going to kill you once you get to a certain point anyway. To hold 20mph on a mountain bike with slick tires for me feels like about the same amount of effort as holding 25mph on a road bike, so it's a pretty big difference.

I'd recommend a pair of Kenda K-Rads.

I've been using a set of 1.95" on my main bike. They'll give you good tread on most anything, but still seem to roll fast. I swapped them out the other day for a set of 1.25 Specialized Nimbus and didn't notice any difference in rolling resistance. If anything the narrower Nimbus's felt slower on the overpass climb. The KRad's don't advertise any puncture proof belts, but I haven't flatted yet.

Three words: Schwalbe Marathon Pro

I have them on my old MTB, they feel a heck of a lot faster than the knobbies it came with, and they cope reasonably well with gravel and that sort of thing. Only one puncture in 8 months (can't tell you how many miles, quite a few) and that was from a thorn about half an inch long, and the tyre went down very slowly until I dug the thorn out. If my commuter wasn't 700C, I'd have them on that as well.

In my experience, semi-slicks will make you feel faster, but when the novelty wears off, you'll realize that you've gained very little speed (if any). I like running semi-slicks on my MTB just because of the significantly reduced noise.

I gained about 1.5-2.0mph on average switching from knobbies to Schwalbe Marathon Supreme's.

Not sure if I'm faster, but I like the reduced noise and vibration from the knobby tread. The bike feels so smooth now!

I swapped out the stock tires on my wife's comfort bike (26 x 1.95 similar to the Kenda Kross tires the OP mentioned) for the 26 x 1.25 tires I mentioned in my earlier post. She instantly picked up roughly 3 MPH average speed (going from 12 MPH to 15 MPH was a huge difference). Maybe it was all in her mind but it showed up clearly on my cyclocomputer.

I put WTB Slickasauras tires on my MTB to replace the knobbies and I'd never go back. Knobbies are for off-road and serve little purpose on pavement. I never tried semi-slicks. That said, my MTB isn't anywhere near as fast as my road bike.

I got Schwalbe Marathon Plus Tour 404s for my bike. Can't say that I've tried a lot of other brands, but it sure is better than the thin-walled Bontragers that came with the bike.

This seems right to me. I cannot imagine it's 'just feel'. Big freaking knobbies vs. nothing? How can you not be faster?

Schwalbe Hurricane tires are what I am going to install on my cruiser when the factory tires finally die. Since I ride on the road and on gravel they seem like a decent compromise.

https://www.schwalbetires.com/node/1355

I have them on my bike too, in the 1.75 x 26 size. I like them and they seem indestructable though I probably have barely over a thousand miles on them. I wish they made them in the 700 sizes like in a 40mm - 45 mm width.

That sounds about right. I use 26x1.0 front (heck I have a front shock) and 1.5" conti slick in the rear (for better gearing, and flat protection).

I used to ride with road bikes on my mountain bike - from 400 mile tours to 55 mile race training. With the right tires, it is possible. One of the biggest weaknesses of MTB tires is the weight - they can't accelerate from a stop, or climb a hill as well, and all of those make a difference in your average time as well as your mean cruising speed.

Yes, knobbies take a lot of energy away from you, weight is killer in wheels, and Aerodynamics (2.0" vs 1.0" tire) is pretty important if you are getting anywhere near 20+mph. Below 15mph, it isn't much of an issue. It all adds up if you are riding strong. If you are doing 10-15mph on a hybrid or upright bike, probably not so much...

I got a faster, rougher ride when I switched from 26x1.95s to 26x1.5s. No desire to go back either.

Wow, so many good answers, thanks guys! Another question, still very nooby... how do I tell how thin my tires can go? I'd probably go no lower than 1.5, but how can I tell for future reference?

More than you ever wanted to know is right here:

https://www.sheldonbrown.com/tire-sizing.html

Sheldon's site has tons of excellent reference material.

+1 for the SM Plus Tours.