Taller tires tend to screw up the aerodynamics though. So you'll get more wind drag along with the effect of taller gears. So it'd be hard to know exactly what causes gave you whatever effect you ended up with.

 

According to the gear calculator I used, going from the stock 235/75/15 /tires to say 245/75/17 tires reduces the effective ratio from 3.55:1 to 3.26:1. (https://tiresize.com/gear-ratio-calculator/)

That's about a 9% difference. Would I get 9% better fuel economy on the highway? That would take me from 17mpg to 18.5.

 

I've been at both ends of the spectrum. I had a '88 302/aod with 3.55's. It got 14 around town, 17~ on the highway(65 mph). I had a '95 with 3.31's. It got 17 around town and would break 20 mpg on the highway(level ground). My current '95 has 3.08 and it gets 14~ around town and 18~ on the highway. It constantly unlocks the converter on the highway with any kind of small grade. Around town it isn't bad, but it needs more gear to get moving easier(thus better mileage). A few more RPM isn't going to be as hard on mileage as lugging with your foot in the throttle to keep speed.

I am going to put 3.55 or 3.73 in truck at some point. I was really surprised how well it towed a trailer with an HHR on it. I locked OD out and it went 55 mph without a grunt or downshift on the highway. Still got 15 mpg too.

When I was looking for this last truck I was hoping for 3.08 gears, and just happen to find it. Now I see that was a bad decision. Just not enough gear for a pickup, especially on the highway(even empty). Just lugs it too much. To prove the point, I need to install a vacuum gauge and compare it between the two gear ratios. Less throttle=better mileage.

 

Only if it's flat in every direction where you are and there is never a cross or head wind.

 

No. Stretch that thinking out farther and with 1.78:1 gears you'd be getting 25.5 mpg. It's no where near linear.

Personally I think if you got 9% taller tires you'd be very lucky to still be at 17 mpg. If there's any gain at all from the effective gearing difference it would very likely be entirely offset by the additional wind resistance and the additional power to accelerate more rolling mass.

 

I bought a '95 F150 new, 4.9L 5-speed, with a 2.73 axle, longbed with a tall cap, and I almost never put it in overdrive. Less than 18 mpg city or highway.

 

As you can see nomad, it's not that simple. Add all the potential variables and even the factories could not always get it right simply by raising the differential gear ratios.
It all depends on where your truck's engine is happy. And how balanced the different factors are:
1. Aerodynamics
2. Engine dynamics
3. Tire rolling resistance (it's more than just size that changes this)
4. Weight of everything.
All those (and probably more) can change how an individual vehicle benefits, or is hurt by a gear change.

It was already mentioned, but using larger tires to mimic a gear ratio change will only tell you what rpm you'll be at for a given speed. Otherwise all bets are off on fuel mileage (other than it will likely be worse) due to the tires being totally different on the road.
And you can use a calculator to figure out engine rpm anyway.

For your truck to benefit from the taller gearing, you really would need to be rolling along on flat ground with little need to push on the gas.
If it did, would it be worth the cost of new gears in the rears? Not from a monetary standpoint certainly. But some higher mileage could happen!
Do you do your own gear changes? What's a shop charge these days, including parts? Maybe $500 at the absolute low end? Perhaps as much as $1000 at the high end? How many miles would you have to drive at an additional 1-2 mpg to make that work? And if it gets worse, there goes the investment.
Plus you still are going to lose your acceleration. And even someone that slogs around town easily can use some zip now and then, right? I know I still do!
I know the hope is that it's a worthwhile trade-off to lose some (a lot?) of your power to gain an mpg or three. But that's the whole gamble.

Different vehicle and setup, but when I changed from a 35" General Grabber M/T tire with an actual rolling diameter (with full vehicle weight on it) of 33.5 to a 35" BFG M/T with an actual 34" rolling diameter on my Chevy 4wd truck, not only could I tell immediately that my rolling resistance had gone up (much more than a simple 1/4 radius would normally have indicated, by the sluggish acceleration) but my fuel mileage dropped by 2 full mpg! Consistently even on long I-5 cruises.
Just in one seemingly minor tire change. The tires were free as a company gift, but I more than paid for them in the end with the additional fuel costs over the next 30k miles.

The best way for you to tell if your own personal truck and driving habits will benefit from a lowergear at least, is to drive around with overdrive locked out, like was suggested.
This won't tell you if using a taller gear ratio would help, but it would definitely tell you if a lower one will help. Or hurt.
When I did that test on my Chevy (the only OD pickup I'd had to that point) I did multiple tests late at night with low traffic to get in the way. (Yeah, I had more time than good sense at the time!) and it made zero difference in that particular truck.
This was both with stock 265/75/16 Goodyear (rock hard pos!) and the 33.5" Generals and 4.10 gears. I tried 55, 60 and 65 mph with overdrive, and the same without. Filled up each time at the same gas station right on the freeway on-ramp, and drove a 45 mile loop on a relatively straight and flat freeway with very little traffic to cause me to speed up or slow down.
End result for that truck (a '96 K1500) was 17 mpg give or take about .25 mpg with the stock tires and15mpg give or take about .25 mpg with the 35's. Very disappointing as you can imagine!
Maybe the taller gearing would have made a difference with the stock tires, but certainly not by much I'm thinking.
But with the larger tires? I doubt it! And the loss of usable power around town would have made life miserable and perhaps the fuel mileage even less.

With the '79 F350, old tired 400, C6, 31-ish rock hard traditional truck tires, 4.10 gears, doing that same routine but with only changing the speeds between 55, 60 and 65 mph (leaving it in Drive obviously) netted me 12, 11 and 10 mpg respectively as speed increased. Can you say "aerodynamics"!
With the old Bronco and 302, 4.11's and 31-ish tires, I would get 17.5, 16 and 15 mpg by changing the speed. Both old trucks would get about 8-9 mpg around town no matter what I did.
All this while driving like an old man, even though I wasn't one at the time.

By the same token, to show what a difference an engine and transmission can make, the Bronco with a stock Ford Explorer engine, with full smog and even a catalytic converter, with an overdrive trans and ultra low gearing in the diffs gets 20mpg if I try.
Now this engine vecicle combo I'm willing to bet WOULD indeed benefit from going to a taller ratio. Right now it's biased to off-roading, but that engine's sweet spot even with the poopy aerodynanics of the Bronco would easily pull it along at a much lower rpm. Right now I'm at 2300-2400 rpm at 65 and 2000 at 55 mph. That engine would easily pull the lighter vehicle along at a few hundred rpm less. And the result would be, I bet, better mpg in this case.
That's all with 4.56 gears and 31" BFG's (rolling diameter a measly 29.5") and 4R70w trans. In this case, I bet 4.11's or even 3.50's would benefit fuel economy at least on the highway. But I'd almost certainly lose the ability to do wheelies!

"Old man wheelies" of course!
Sorry for the typical long-way 'round of basically saying "Your Results May Vary" as always.

Good luck though. If you do opt to go to a numerically lower gear, please let us know the results.
But I would never go all the way into the 2's however. Too far for any normal truck in my opinion. Stick into the 3's at least.

Paul

 

Thanks Paul. Nice write up. I plan on doing some mpg tests with & without OD at various speeds. I'll post my results.

My truck has a lot of giddy up from a dead start which made me think about changing the gear ratios to hopefully tame it down & add some top end. I'm 71 years old and have zero interest in speeding with my remaining hair on fire.

 

Well you're only 10 years ahead of me then. So at the speed I drive, I'm catching up to you!
But hey, that's what I have a supercharged sports car for... Ok, so "sports car" perhaps isn't the best descriptor for an old-man-Buick. But hey, Regals are nice sleepers!

Can't wait to hear if yours responds well, or poorly to the higher rpm of the 3rd gear instead of 4th.
Since you say it's got plenty of pep, you probably could get away with some taller gearing. It's just an expensive experiment if it ends up not working. Or if you go too far!

Good luck.

Paul