I have a 1995 F350 PS, 4X4. I just bought it less than two months ago or there abouts and I am fixing a ton of stuff and working on a bunch of other stuff and pipe dreaming about fixing even more stuff...you get the idea. This truck has been rednecked just about to death. With that background and the info that I am trying to get it as close to factory specs as I can so I can safely pull my gooseneck horse trailer, here is my question:

The tires on it now are some big mudder types, size 315/75R16. The door says 235/85. OK, I am so cool with smaller tires, because right now I am several inches shy of getting the beast under the tongue of my trailer. But how small are we talking here?? I mean...OK, well, first of all if someone would be kind enough to explain EXACTLY what those numbers mean I would be so appreciative. I know the first number (235) is the width, and the second number is some kind of ratio of height to width and the third is the wheel/rim size. But how do they work?? Because I saw a Ram2500 today (not 4X4) that had 245/70 and they looked TINY. And would look retarded on my truck. And I thought that the larger the second number, the shorter the height of the tire...please say I'm wrong.

Someone care to help a clueless girl out here?

 

You are correct about the first number being the width of the tire (in millimeters). It is not a direct measurement of the width of the tread, but of what the manufacturers call "sectional" width. Actual tread width is usually quite a bit less. The second number is the "aspect ratio". It is the percentage of how tall the sidewall of the tire is compared to the width. So on the 245/70 tires you saw, the sectional width is 245mm (about 9.6") and the sidewall is 70% of that, or 171.5mm (about 6.75"). If that tire would have been a 245/85 instead, the sectional width is still the same but the sidewall is now 85% of the width or 208.25mm (about 8.2"). So, the larger the second number is, the taller the tire is in relation to it's width.
Did I make that as clear as mud?

 

Thanks! Yeah, that helped a ton, I don't know why I was thinking that the larger the second number the shorter in height the tire was. Sooo...check my math here, would ya? The tires I am getting are 235/85r16s, which will be 235 mm "wide" and about 31.98 inches tall. The ones on it now are 315/75r16s, which are about 34.9 inches tall. So new tires will be narrower by about 3.2 inches and shorter by about 3 inches? Looks and sounds right, but I AM an English major... ;-)

 

That's it.