anybody using 18's have any comments???

 

Wheel/Tire diameter does NOT change rear end gearing but your OVERALL TIRE DIAMETER WILL!!

If you stay within the stock tire overall diameters (ODs) of 24.8 to 25 inches, there will be no change in rear end gearing. If you increase your rear tire ODs to 26 or 27-inches, then you WILL heighten your rear end gearing, but it won't be 3.90, unless you plan to run tractor tires back there

 

A 285-35 will result in a 3.94 effective axle ratio. A 295-35 gets you right at 3.90 (tire is 26.13"). A 275-40-18 is worse at 3.82.

I use 285-30s which are actually a bit smaller in diameter, resulting in a 4.12 effective axle ratio.

Simple to calculate. Simply divide the stock tire's diameter (24.86" is 225-60-16) by the one in question and multiply by 4.1.

If you don't know the diameter, use the following:

((wheel diameter (in) * 25.4) + (2 * aspect height))/25.4

where aspect height = tire width (mm) * aspect (mm) * 0.01

For example, 285-30-18:

aspect height
= 285 * 30 * 0.01

= 85.5

diameter
= ((18 * 25.4) + (2 * 85.5)) /25.4

= 24.73

Ratio:
= 24.86/24.73 * 4.1

= 4.12

Gene
www.purplemantis.com

 

Only thing to consider regarding Gene's post is that different tires have different 'specs' for the same given 'size'. Extreme example is Pirelli PZero Rosso in 285/30/18s is measured at 25.2" dia when mounted on a 10" rim (the regular PZeros being 24.9"), and Kumho Ecsta V700 measures 24.6" dia on the same size rim.

Every tire manufacturer has their own idea what size 285/30/18 (or any other size) really is (even among different model ranges from the same manufacturer), hehe. Tirerack.com has lots of neat measurements for each given tire under the 'Specs' tab on each tire model page.

 

Yeah, that's absolutely right. Use the previous equations as a general gauge.

I've found several 245-40-17 tires that vary considerably in diameter and treat/section width. There are many 245-40s that are actually wider than 255-40's.

Gene
www.purplemantis.com