Ok, so it is time for new tires again. Now towing near 10K so was thinking about the advantage of axle ratio affect of the two sizes difference. Thinking that larger tire would be less advantageous, but using a calculator online, it seems to tell me I actually get better ratio for towing with a larger tire?

I have the 3.73 ratio now.
New Tire Diameter: 32.8
Old Tire Diameter: 31.6

The calculator says the affective ratio is now 3.87 with the larger tire.

I know, not much difference, but did not think larger would go better for towing? What am I missing?

 

RobertH, 3.87 can not be correct. When you put the larger OD tire on the Excursion, the final ratio must go numerically lower. Possibly, around 3.60?

Good Luck.

.

 

Looks like either the ratio calculator you used was working backwards or you may have entered old/new tire sizes inverted. When I plugged in your tire sizes to this calculator 4Lo.com :: Tire Size Change, New Gear Ratio Calculator and flipped the old/new sizes the result was your effective ratio of 3.87. When the sizes are entered correctly the effective ratio comes out as 3.59, which would be slightly worse for towing heavy.
I'm in the process of collecting all of the goodies to swap in a new set of axles with 4.88 gears on my 3.73 equiped EX. I'll be going from my stock 265/75R16's (31.5") to 305/70R18's (35") to wind up with an effective ratio of 4.39, that should make pulling the heavy ToyHauler much more fun.

 

Wow... I have 245's installed by the p.o. And 4.30 gears, that means I have a new ratio of 4.44. No wonder my mileage sucks....but it does pull great. Which is why I bought it.

 

245 what? (245/??/16)

The second number is the height of the tire as a ratio of the width.

 

245/75/16 load E

 

Is is actually 4.47 for my new ratio

 

Thanks Tom! I thought it did not sound right.
Seems that the website I was using must have their formula backwards.

That seems to be significant in reduction of the ratio. I think I need to go back down to the 265/75/R16 to benefit for towing.

 

I have 3.73 gears on my truck with 265/75/16 tires. When it is time for new tires later this year or next, if I step up to an 85 tire does anyone know how much that will improve my mileage or how towing will suffer?

 

I think an 85 series tire is a bit odd. You will find much more selection in the 75 series. You could go up in tire width, which also increases diameter. Going from, say, a 265 mm width to a 285 mm width (285/75-16) would increase the diameter from 31.7" to 32.8" (3.6% increase). I don't think it would increase mileage measurably. Effect on towing depends on diesel/gas and your towing speed as matched to aero drag.