Originally Posted by chadstickpoindexter

Thanks for the link. But, I'm still not exactly sure where to start? When choosing an F250 with 6.7, 4x4 and 3.31, the XL trim starts with a 245/75R17 tire, and an XLT (my trim) lists the first tire size as a 275/65R18 tire. I understand that the actual (hard cut steel) gear ratio is a 3.31, but (if this makes sense) which tire size would this most relate to to effectively be 3.31 at the tire? Or maybe I am (still) over thinking this?

For example:
When using 3.31 as the stock gear ratio and I start with the smallest XLT 275/65R18 tire option and go to the 275/70R18's, the effective gear ratio is 3.2
When using 3.31 as the stock gear ratio and I start with the smallest 245/75/R17 tire option on the XL F250, and go to the 275/70R18's the effective gear ratio is 3.14.

But:
When using 3.31 as the stock gear ratio and I start with my 275/70R18's and go to the 285/75/R18's, the effective gear ratio is now 3.15.
When using 3.31 as the stock gear ratio and I start with the 245/75/R17's and to to the 285/75R18's, the effective gear ratio is now 2.9.

So, I would assume that when we pick the 3.31 rear end, to get this effectively at the tire, Ford would base this on the smallest diameter tire that is an option?

I don't guess it doesn't really matter at this point... I already have the truck, these are the tires I have on it, and I can't say I have any issues. When it comes time to replace them I will go with either the same size on it now, or bump up to the 35's (like the DuraTrac's that come on the Tremors), but I have been curious, and then when seeing this thread made me decide to ask about my own situation. Just trying to figure out what my effective gear ratio actually is with my set up. Thanks for any help.

Originally Posted by chadstickpoindexter

I have a hard time wrapping my brain around this... I went with the 3.31's and the GY 275/70R18 wheels which have a 33.2" diameter and 630 revolutions per mile. I have the PSD and only pull a 6000 lb GVWR camper, so didn't figure I'd really even need the 3.55's... but I did start wondering what my actual (effective) ratio is with the slightly larger tires, and what it would look like if I decided to go with a 35" tire when it comes time to replace them. But I'm guessing, like mentioned above, the effective ratio on mine isn't really 3.31, and it's probably quite a bit lower than that?

Originally Posted by speakerfritz

Very confusing

i have a 4:30 which came with 19 inch rims and I believe 33 tires.

i put 22.5’s and 37’s on it….does that mean my effective is 4:10.