I have 285/R75/16 tires on my new to me F-350. I input this info into a web tire size converter and it spits out a number almost 33 inches. I just read the latest Petersons 4wheel drive magazine and in it they mention a 285 tire and in parentheses they state its a 31 inch tire. I took a tape measure to the tire (with the truck sitting on the ground) and its about 32 inches. Whats the scoop with converting metric to english tire sizes?

The Ford forum is lame on 4x4wire as nobody hardly posts <img src="/forums/images/graemlins/sleeping.gif" alt="" />

Based on my calculations, a 285/75/R16 should be 32.83in (say 33in). Did the article say it was a 15in rim? That would make it 31.83in.

My .02 cents.

It's fairly easy on the conversion but outside diameter is dependent upon the manufacturer. Finally a non-mechanical expertise question <img src="/forums/images/graemlins/smile.gif" alt="" />

285 is the width in millimeters and there are 25.4mm to the inch.
75 is a ratio indicating that the sidewall height is 75% of the total width of the tire.

So 285 divided by 25.4 = the width in inches = 11.2205"

and 75% of 11.2205" would be the sidewall height = 8.4154"

The overall height of the tire (Diameter) would then be 8.4154"(sidewall 1) + 16"(wheel height) + 8.4154" (sidewall 2) = 32.8307"

This is in theory and is purely based upon math and not the variation of manufacturing. Brew <img src="/forums/images/graemlins/cheers.gif" alt="" />

Brew, thanks for the detailed explanation. Makes sense to me. I was wondering why the rpms were so high on this truck in overdrive 2100 at 65 mph. The kid I bought it from put 33s on and did not change the gearing. Now I need to check the diffs and see if I can read the tag for the gear ratio, although I think the spec is 3.73 for the Ford 10.5" rear. Guess I need to get taller gears and some lockers while I'm at it (at least the wife cant see that stuff so won't know I spent the money). I already said I wanted a WARN front bumper with the new 15000 winch/air compressor, gee thats only about three grand <img src="/forums/images/graemlins/scared.gif" alt="" />

I put 265-16's on my 93 T and there is a 10% spedometer difference (based on the portable radar displays they use here). 55 is 60.5.

265/70-16 (stock ~31" size) or 265/75-16 (slightly taller)? And what did your truck originally come with? I'm guessing something less than 31's.

In my experience, Toy speedos seem to read a bit high with the factory sized tires/gears...

Going to a larger diameter tire without re-gearing will reduce your engine rpms at any given speed (unless it keeps it out of o/d). It's like changing your rear end from 4.10 to 3.90. Perhaps the rpms seem higher because you're actually doing 70 instead of 65.


You want just the opposite ("shorter" gears; higher numerical ratio) if you wish to restore stock performance, and speedo accuracy.

DOH <img src="/forums/images/graemlins/rolleyes.gif" alt="" /> Well heck fire, I sure got my stuff all bass ackwards now didnt I. Gonna take my GPS with me tomorrow to see how far off the speedo is. Checked the diff tag today and it is 3.73. Found a chart online that showed I need 4.27 to get back to stock arrangement. I did spin the rear wheels and both roll forward with the back in the air so there is a lSD in there. Didnt seem like it last week when I got stuck in the mud in 2wheel drive. Looks like the only option for the rear is a detroit locker, while the front is a Dana 60 so could go with an e-locker there.

The rpms went down 10% at the same speed. Ran about 2250 at 55 before, just over 2000 now with the 265 15's. No idea what the rear end is but who cares. As long as I know that when I pass the cop with the speedo at 60, I am at 66........mileage is up as well if you use the 10% factor.

Ran with the gps today and the speedo was pretty close to dead-on going 65 on the speedo said 66 on the gps, same at 70, only one mph higher. The rpms were about 1800 at 65 and 2100 at 71 mph. Hard to believe that the speedo is so close to accurate. Perhaps the gears were changed previously but I didnt get that record.