Joe123

Hello guys, I'm new when it comes to bike-building, I bought a bicycle without much thought, the guy in the shop (decathlon) showed me a few bikes and I was like "yeah this one's black and looks cool". This was a few years ago, and now I'm really into cycling and I love it, I ride 1-2 hours every day. I'm also pretty fast, when I go to the park to ride with other cyclists, I regularly overtake people with road bikes (I actually just learnt that these are the bikes with the slick tyres and they run pretty fast, and it turns out my bike is a fat and slow mountain one). However the majority of people with road bikes overtake me, even if I keep up with them and maybe even pass them for some time, eventually they'll catch up to me and it feels like I'm out of breath, I'm sure if I had a road bike I'd blow them out of the water (I am like really fast, I drive on the highway to work about 25km every day), right now my budget doesn't allow for a new bike, so I read a bit in the internet and it turns out I can swap my fat tyres with slick ones and I'm halfway there to a road bike, now the problem is, even though I'm really into cycling and everything, I can't read specifications of bikes and I don't know what most parts do (like I said, all my bikes I've bought so far have been bought based on looks and what the seller guy tells me, not based on research or something like that), I need help. I live in bulgaria if that's relevant, what tyres should I buy for my bike, rockrider 520? I want it to be real fast

HTupolev

From what I can tell online, Rockrider 520 uses 27.5" wheels, also known as 650b. So you'll need to get 27.5"/650b tires.

The 27.5" wheel size is mostly used for mountain bikes and gravel bikes, so while there are road tires available, very few are skinny. It might not be a bad idea to stay with a fairly wide tire, to keep your bike's handling consistent.
If you really need to go skinny, and if you can find it, the Hutchinson Confrérie des 650 is a decent tire.
For fat road tires, my personal go-to is Compass. They've got a 38mm, a 42mm, and a 48mm available in 650b. They're not cheap, but they ride extremely well, and have pretty good wear life (often over 5,000 miles in the big sizes).
Soma sells a similar 42mm 650b tire called the Grand Randonneur, in both an EX and SL variant. The SL is extremely light for a tire of its size, at the cost of a thin tread that won't last so long.
Panaracer makes a few 650b road tires, the Pari-Moto and a 650b slick-tread version of the Gravelking.

Joe123

Ok, so I think I'm finally gonna buy them now, I chose the hutchinson tires, I think they look cool, like old-fashioned cool and they are 32mm, and I read that smaller tires are faster, plus they're relatively cheap. Still about a sixth of my bike, together. My bike has 24 speeds, and I always use the 24th, smallest 8th on the back and biggest 3rd on the front, the hardest one, but I feel like it's getting a bit too light for me, especially when going downhill and going full speed, I've been riding only on 24th speed for years now and I'm used to it, that's why I'd like to get a harder speed, so it'll be tougher for me to pedal, as now that I'm buying these new tires I'll be able to pedal much easier, supposedly. How can I do that, to upgrade my bike speeds? What do I need to buy? Preferably something cheap if possible