It's like the cogs on your mountain bike. Stay on the smallest front ring and largest rear ring and you are pedaling super fast. Then stay on the largest front ring and smallest rear ring and you can barely make it up hills.

What you want for daily street driving is keeping the overall diameter tire size the same as stock. Remember your tires and wheels are like gears since they are part of the transmission. Let say you have 195/55x15" (overall diameter is 23.4" and tread width is 6.8") tires like they came on my 00 Civic Si. If I wanted to "+1 PLUS SIZE" my tires I would go up to 205/45x16". "+2 PLUS SIZE" would be 215/35x17" but there is no such size so I have to use 205/40x17" (23.6") which is 1/4" larger than stock. This in turn makes the car accelerate a bit slower since it increases my gear a bit. On the highway if your speedo says you are going 65mph then you would be going 67mph according to the police. Very minor. Also your mpg suffers because your car has a higher gear (in a sense) and takes more energy to turn the wheel to get you going. Sort of like starting off in 2nd instead of 1st.

But you don't have to follow the recommended PLUS SIZING. Like most guys do we run 205/50x15" which is like 1/4" shorter than stock and tread width is 7.3". So it gives a bit better acceleration but doesn't really throw off the speedo by much. Say you are going 65mph the cop will clock you going 63mph. It's wider than stock so you have better cornering grip.

If you really want to widen your track you could use a 215/40x16" (overall diameter is 22.7"), so the tire is very short but wider (tread width is 7.8") So you gained about 1" width, which makes a HUGE difference in cornering, but now you are 3/4" shorter in height. So at 65mph your speedo will read 75mph or so. I remember asking my friend when I pulled along side him and he was going 80mph and my speedo read 90mph! And my rpms were like 5500rpms with an N1 exhaust.

So there you go