You probably need to regear it with those big tires. You'd probably see fuel economy go up if you went to 4.56 gears.

Find out what your current gearing is.

 

My advice? Take off those big tires and put small ones back on it. I can only assume you're using this thing as a daily driver which is a poor choice. Put the big tires back on when you need them, like when you plan to go offroad. I used my truck as a winter DD for several years and used little tires (you can see what I've got in the description, I think you'll find that my truck is very similar to yours). It may look goofy, but I saved several hundred dollars in fuel over the course of a winter by doing it that way.

 

Must do a lot of driving if you can save several hundred dollars in gas over the winter by changing tires up or down a couple sizes.

 

5000 miles isn't all that much in 4 months time.
Let's do a little math just to show, shall we?

5000mi/12mpg (12mpg is what I get with little tires) = 416.6
416.6*$3.6 per gallon of gasoline= ~$1500 over 5000 miles

5000mi/10mpg (which is what I got with 35" tires) = 500
500*$3.6 = $1800

I'll take that $300 extra cash just from swapping tires thank you very much.

 

5000 is more than I drive over the winter. Didn't say what the small s ize was, guessing stock. I have tried going up & down a couple sizes and never saw any difference. The ol 460 does its thing, doesn't seem to care much about tires or hills. Suspect some folks forget that the odometer will be off when tire size is changed.

 

Yeah the odo will be off if it wasn't corrected. But for those interested in correcting it (really should if you have an auto trans) check out this link for a how to---no tuner required: HOW TO RECALIBRATE THE SPEEDOMETER ON 92-96 FORD F-SERIES AND BRONCO .: Articles

And yes, those tall tires would pair well with deeper gearing. 2000rpm for the 351 on the highway seems to net me the best mileage: I think lower (especially with the big tires) and it will "lug" or be at a significantly higher load (thus holding injectors open longer...more fuel consumption) for a given speed. That can only be remedied by deeper gear (assuming you keep the big skins).

This is probably magnified in the city...think bicycle stuck in top gear, it takes all your might to get it going from a stop sign (you keep your human injectors open as long as you can till you get up to speed) versus if you leave a stop sign in a deep gear (you may push hard briefly but gears allow you to comfortably accelerate). You get to work in your more efficient physical output with the proper gears. Same for your truck. Putting those big tires on & leaving a stop sign is analogous to starting out on your bike up a hill, it really sucks if you're in the wrong gear, but if you start in a deep gear it really isn't so bad. Can you tell I commute to work by bike? haha

Definitely need an accurate baseline fuel mileage though. And if you don't drive often, a gear swap probably is not very economical.

 

Yeah stock sized, 235/85R16. My speedometer and odometer were correct for all of that.

I'd expect you to not see much difference with a 460.