how will my speedo be affected by 20 inch wheels? if im going 30 will it say im going faster or slower?

 

depends on your tire size, here's the tire calculator i use:

http://www.discounttire.com/dtcs/infoTireMath.dos

type in the stock tire size and then the size you have now and it'll tell you how fast your going at whatever speed you select
-GP-

 

My stock size are only 0.6% off from my 245/35/20 and ~1% off from my 275/30/20's so if the 07's tires are near the old sport tire diameter you will be fine.

 

Marcel if you are replacing your stock 245/45R18 tires with 275/30R20 tires then your speedo will read .7% too fast, which means when your speedo reads 60MPH you will actually be doing 59.6 mph. As Tim says, you will be fine .

 

Check out this calculator link in the title box. It should help you out...

 

Here's another good tire calculator.

 

I'm sure it doesn't matter to some people but i just thought i'd point out that the two tire calculators posted has difference measurements/results on a same tire size. Even after I chose both calculators to give me the results in Inches. I was using 275/30/20 in comparison to the stocks 245/45/18.

 

Ok, i tried another tire calculator site, and 1010tires.com's calculator is off compared to the other two.

 

How do you calculate the accuracy of the speedometer when our front and rear oem 18" tires are of different diameters? I mean, what would happen if you go smaller/larger on only either the front/rear? What if you go smaller on the front and larger on the rear and vice versa?

 

I'm thinking the speedometer is reading the rear wheels on a coupe since that's the drive wheels. so making the overall diameter in the front and keeping the rear as stock size would still give you an accurate reading on the speedometer

 

crap...i left a word out.... making the over diameter in the front "smaller"...