i must be reading the tire calc results in the wrong way or something. where exactly is the 1% number you are talking about? im assuming its the diameter difference.

Stock Tire - 225/40R18 Search Tires Tire 1 - 255/35R18

Revs per Mile: 829.3 (225/40)
Revs per Mile: 831.3 (255/35)
Actual Speed: 60 mph 100 km/h
Speedometer1: 60.1 mph 100. km/h
Speedometer Difference: - Speedometer Difference: 0.241% too fast
Diameter Difference: - Diameter Difference: 0.24%


Stock Tire - 225/45R18 Search Tires Tire 1 - 255/40R18 Search Tires

Revs per Mile: 800.9
Revs per Mile: 799.0
Actual Speed: 60 mph 100 km/h
Speedometer1: 59.8 mph 99.7 km/h
Speedometer Difference: - Speedometer Difference: 0.237% too slow
Diameter Difference: - Diameter Difference: 0.24%


so going by these results the number are almost exactly the same, but the 2nd setup actually is slightly closer to equal then the 1st setup. thanks for the help

Yes the 1% difference is the different in diameter. It's best to keep it under 1% if possible. Even with 20's, you can get it to .9% which papahoff ran without problems.

I personally just prefer to stay closer to 0.5% or less but there's no need to.

Remeber your stock size is 225-45-17 not 18. So if your not comparing it to your stock 17's you could end up wrong.

Stock Tire - 225/45R17 Search Tires Tire 1 - 255/40R18 Search Tires
Section Width: 8.85 in 225 mm
Section Width: 10.03 in 255 mm

Rim Diameter: 17 in 431.8 mm
Rim Diameter: 18 in 457.2 mm

Rim Width Range: 7 - 8.5 in
Rim Width Range: 8.5 - 10 in

Overall Diameter: 24.97 in 634.23 mm
Overall Diameter: 26.03 in 661.16 mm

Sidewall Height: 3.98 in 101.09 mm
Sidewall Height: 4.01 in 101.85 mm

Radius: 12.48 in 316.99 mm
Radius: 13.01 in 330.45 mm

Circumference: 78.44 in 1992.3 mm
Circumference: 81.77 in 2076.9 mm

Revs per Mile: 832.9
Revs per Mile: 799.0

Actual Speed: 60 mph 100 km/h
Speedometer1: 57.5 mph 95.9 km/h

Speedometer Difference: - Speedometer Difference: 4.242% too slow
Diameter Difference: - Diameter Difference: 4.08%