I am kinda lost (no really lost) when it comes to tires sizes. Say a 285x65R.18 tire would be comparable to what 33" tire. I am not sure on the conversion for the tires could someone let me know, thanks.

 

To determine the "advertised" size, take the crosswidth spec, 285 in your case and divide it by 25.4 (converts mm to inches). That gives you 11.22", which is the crosswidth of the tire. If you multiply it by the aspect ratio of 65%, you get 7.28". Add that twice 7.28" +7.28" = 14.56", and then add your rim diameter of 18" to the product and you get 32.56". But the actual diameter could be more or less than formula. Most tire manufacturers have that data on their website.

 

There is a very good tire calculator and sizing chart on www.discounttire.com

 

Hey, welcome to the site!

 

Awesome guys and thanks.

 

Welcome. http://www.1010tires.com/tiresizecalculator.asp is a good one too.

 

Welcome!

This is a good one and one I use often. It also shows how much the speedo will be off.

http://www.miata.net/garage/tirecalc.html

 

That's the one I always use. Good luck!

 

Really, the only reliable data to use is the manufacturers revolutions per mile data to determine speedometer error. Published tire diameter is inaccurate information for that purpose. I'll just give an example. I have BFG Rugged Trail T/As on my truck. The revolution per mile data is 660, which is the same data programmed into the PCM to make calculations. If the tire turns 660 times in a mile, then it turns once every 96". Divide that by pi and you come up with 30.56" as the effective diameter. BFG claims the tire diameter is 31.9" and it probably is the diameter with no load. That difference is 4% alone. Consequently the metric sizing indicates it to be 32.1". Hope this helps.

 

I'm sure it's not exact but a good reference point when tire shopping. It might be different if you compare half of this data with the other half of the data from a tire manufacturers site or half of it by your calcs but comparing all the data in the calc should be pretty darn close with all variances being equal and accounted for. Besides, my Tirepaq allows you to enter tire diameter by 1/4" increments, so if I measure it and my eye sees 33.25" and your's sees 33" there has to be a fairly big margin of error. I know tires vary from manufacturers, even in the same size...So in other words, adding tire load can't change the tire height by much so figuring out *exact* tire height, while fun , probably isn't necessary.

 

Loaded or unloaded the tire will be the same circumference. i.e. the tire still travels the same distance in one revolution.

 

That's not correct. Make a chalk mark on the pavement at the particular point on the tire and payment where they intersect. Roll the truck back and make another mark where the tire contacts the pavement. That measurement will be different from the unloaded circumference. Trust me, I've programmed several PCM to get the speedometer correct. If I didn't have revolution per mile data (which essentially proves my point to begin with) I would take the measurement described above and see how may times that length went into a mile.

I also use that information to determine rpm at various speeds to determine final gear ratios. You guys don't have to believe me, but next time you're trying to program tire size changes in your programmers, remember this post when your speedometer is still off.

 

You speak from experience so I will defer...but where does the "material" go? Ever try to make an slightly oversize or stretched O ring fit? Doesn't work, you can't make the circumference smaller.

 

The material just buckles under the weight of the vehicle. It's the bulge in the cross section and the flattening of the threads for the contact patch. Measure the radius from the pavement to the center of the hub and then measure the radius from the top of the tire to the center of the hub. They're different. You have to look at this measurement as the rolling circumference as opposed to the inflated circumference. Air press alone inflates the middle section more than the edges of the tire. It's really the loaded radius that determines tire height because it's the only thing touching the pavement.

 

Anyone use this one? http://www.dakota-truck.net/TIRECALC/tirecalc.html