with my new rims/tires 275/30/18's how far off ill it set the speedo

 

275/30-18 is too small.... the outside diameter is only 24.50". More than 1" smaller than the stock 25.66". That means the tires will be spinning 4.5% faster than the stock diameter, and the speedo will read higher than actual speed by that amount. At a true 60MPH, your speedo will read almost 63MPH. When you drive 1,000miles, your odometer will rack up 1,045 miles. 275/35- or 285/35-18 is the size that matches the stock diameter. Worth correcting in the PCM.

Doesn't that make the gap between the tire and the fender opening even larger?

 

yea its 275/35/18 sry