I recently ordered 1.5 slicks to replace my city commuter 2.1's. It is a hardtail mountain bike. Only 1 came in the mail but I decided to put it on anyway and try it out (rear). Today I already noticed that Im increasing in speed a pretty good amount, even when pulling a trailer. I know that on the rear it plays the biggest part since most of the weight is on the tail. So how much does the front tire plays in terms of helping speed? Or does the front only effects handling?

Your increase in speed is from the lower rolling Resistance from the narrower tire and smaller contact patch.

I got that part but how much does the front tire play in terms of speed like 70% rear% 30 front?

Not sure on that but,

I noticed with a nice "road" tyre on the front when on the road the bike handled a lot better and thus i was naturally inclined to go faster .

Did the company charge you for both? Is the other one still in the mail?

Depends on your weight distribution, which is going to depend on bike geometry and your position on the bike.

If you really want to know weight distribution, you could measure it. I'd say put the other tire on and be happy with more speed.

ThermionicScott - originally yes the company was going to charge me twice cause it was two separate orders since I thought it was a pair (there website said tires) but a quick email fixed it.

Yeah im not complaining about the speed or nothing just curious how much it would affect the current speed once I do make the last switch. As far as weight go all weight is pretty much in the rear panniers and all.