i downgraded from my stock 20s to some aftermarket 18s and i was wondering how bad the speedometer will be off and which way...

 

It depends on the overall diameter of the tire. Discount Tire has a handy tire calculator on their website. just type in your previous tire size and your new tire size, and it will tell you all the differences including the speedometer difference.

 

overall diameter of the tire will impact you, not the wheel..

the best way to determine if your speedometer is off is by using a GPS to map your speed.

in theory, though, if the tires you're running are shorter, the speed will read faster than you are actually going..

 

You can use a reliable gps to tell how much different it is, if at all. As said above, it's the difference in the tire size, not the wheel. I went to a smaller rim but a taller tire and at 60mph, I'm a mph or so off on the speedo (going faster than speedo says). I have a gps driven speedo app on my iphone I use on my golf cart and I also have the typical garmin portable gps that shows highway speed.

FYI, I'm fairly sure it throws the computer that displays avg mpg, etc. too.

 

http://www.1010tires.com/Tools/Tire-Size-Calculator

check out this site. Put in stock tire size, then put in new tire size, it will tell you exactly what the % difference will be.

 

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What they said - overall diameter affects speedometer and odometer. With 285/70x17s, mine reads 4.3% low.