Basically yes. The 18" rims wear tires that are 33.2" tall and have 628 Revs per mile
The 20" rims wear tires that are 34.4" tall and have 606 revs per mile

The difference in revs per mile is about the same as the difference in rear end gears. With a slight edge to the 3.31 for economy and to the 3.55 for performance.

I've driven both trucks and on a 1000+ mile trip at 80 mph, I got the same mpg.

 

I've got 18"s with 3:31. I've seen 23-25 mpg on 55mph hwy drives. 80 mph on I-40 & i75 see 18-20 mpg.

Took a 2300 mile trip last month pulling 10k enclosed 10.1 mpg. (8.6 was the lowest I got driving way 2 fast).

Anyway the 3:31 had NO trouble w that weight.

I bought a set of factory 20"s off a guy that went with after market 22. I'm going to see how the truck does w the 20/3:31 comb after my current tires wear out.

 

thnk you - that is my experiance. Some of these threads had me thinking 18-22 was the norm.

 

If I don't give up towing performance with the 331/18s I might as well order that axle with the hope of better mileage empty. If there's little difference in mileage I may go for a bit more performance with the 355/20s. I really don't want to order the 331s only to regret not going with 355s.

On a different note - when does Ford shift production to the 13s? I plan to order in the next few months so I might wait to see what the 13s have to offer.

Best Regards,

Doug

 

i no this may be out there a little bit but i got 4.10 and i get 15 around town and interstate is where it kills her if i keep it around 70 it will get 18 but if u get up to around 80 and 85 it drops big time talking bout down to like 14 so from what i see the gear ratio u pick aint really gonna change ur mileage by much its how you drive it
just my 2 cent

 

pretty much spot on with the last statement. here a good article that explains it all pretty well.

AXLE RATIOS

 

I'll buy that statement 2

In the article when talking about towing I'd say thats right on with smaller trucks. But with the power of the 6.7 and that low low low 1st gear I don't think it matters much


How about this.....
Anyone raced 3.31 vs 3.55 a few kicks? LOL I'd say its whom ever hits the fuel 1st :-)

 

great article...

 

Really good article !! Many thanks.