I`ve got myself 17' 5ziger rims(i dont really know which ones) with 205/40 tires,I didnt put them on yet.
CAN THEY SOMEHOW MAKE MY 1993 HONDA CIVIC SI COUPE GO SLOWER???? If yes then how big can this problem be? Can I do something about it????

Well in most cases larger wheels and tires weigh more which create parasitic drag. Sort of like adding 100lbs to your car.

The other thing that can go wrong is buying the wrong tire size. Picture your car like a 10 speed bike. Start off in the highest gear (which is also the fastest) is sort of what it's like.

For most Civics the stock tire size is 185/65x14". To +1 size you would go to 195/55x15". To +2 you would go 205/45x16" and +3 is 205/40x17".

Yes They Can Make You Slower. And Chrome Even Worse. With You Set Up Though You Wouldnt Notice It Too Much. Do You Have Auto Or Stick. If You Have Stick You Wont Notice It As Much.i Got Chrome 18's On My Accord And In The Winter When I Put My Factory Wheels Back On It Flies.but Its For The Look Not Speed. Decide What Your Goin For,for Speed So Nice Si Wheels Should Do It, Or Even Gsr Wheels, I Think They Look Go On Civics That Are For Speed.

There are two reasons that larger tires make your car slower
1. They are larger and take more power to turn and also will raise your top speed which kills acceleration.
2. They are heavier and have a greater potential energy...they carry more weight when they turn and are harder to slow and stop.

There are two reasons that larger tires make your car slower
1. They are larger and take more power to turn and also will raise your top speed which kills acceleration.
2. They are heavier and have a greater potential energy...they carry more weight when they turn and are harder to slow and stop.

Your #1 is almost right. It all depends if you keep the overall diameter the same. Gearing will only change if you get a tire size that is not the same diameter as OEM.

That's what i was alluding to. I wasn't meaning plus sizing the OEM tires and keeping the same diameter. I should've clarified that a little better.

There are two reasons that larger tires make your car slower
1. They are larger and take more power to turn and also will raise your top speed which kills acceleration.
2. They are heavier and have a greater potential energy...they carry more weight when they turn and are harder to slow and stop.
what is OEM??? i put 205/40/17 on my 93' civic si coupe.

http://www.miata.net/garage/tirecalc.html

use that to calculate if your new overall diameter is the same as your OEM.

what is OEM??? i put 205/40/17 on my 93' civic si coupe.
Iy says that speedometer reading is going to be 0.1% too fast.

Iy says that speedometer reading is going to be 0.1% too fast.

That means your overall diameter is well within (actually, almost exact) the OEM specs.

Original Equipment Manufacatured. It Simply Is An Item That The Car Company Makes. Oem Parts Are Generally Purchased In The Dealer. But When You Here Something Like Oem Specs, That Just Means As Close To The Factory Specs As Possible.