Gas milage issues
 

I recently purchased a1985 Ford Bronco with the 300 I6 and Np 435. I was getting about 18-23 mpg at first. Now I get about 7-9 mpg. The only thing I changed was the tires. At first I had 235/75/15. Now I have 31x10.50/15. The tires are about a 1/2" taller than the factory tires. I have never been much on Ford but I fell for this one . Would the tires change the mpg that much? Is there anything I can do to get better gas mileage that won't cost over a few hundred dollars. I bought the truck for $700 and have invested $4000 in small repairs. Windshield, clutch, tires, wheel bearings, fluids, etc. Any info will be greatly appreciated. Thanks for your time.

235 is 9.25 inches
6.9 inches high, 15 inch wheel
15+14=29 inches diameter

So you went from 29 inches to 31 in the diameter
and you went from 9.25 inches wide to 10.5 inches wide
and your new tires might weigh significantly more than your old ones.
29 pie=91 inches, 63360/91=696 revolutions to go 1 mile
31 pie=97 inches, 63360/97=653 revolutions to go 1 mile

odometer reads 1 mile at 696 revolutions
Odometer with new tires reads 1 mile but actual mileage is
6 percent more

Thanks for the info. So it's the tires that are screwing my mpg up. If I add a 2 barrel carburetor would that give me better mpg. I like the truck alot and I finally have a dependable vehicle but the mpg's are killing my wallet especially when pulling anything. Another thing I noticed was I can smell gas when walking around and sometimes driving the truck but can't find a leak anywhere. Any ideas. Thanks again for the info. It's greatly appreciated.

what gear ratio do you have? I bet its 3.08's in which case you could probably swap over a later year bronco front/rear diff to get 3.55's and see a great boost in performance. anytime you increase the tire size though your going to loose mpg not just due to tire size but also the vehicle being taller.

Thanks for responding. How would I tell what gears are in the rear end and front end? Would I have to change the front end gears also? I was thinking about rebuilding the carburetor. It looks a little wet at times. It maybe leaking idk? I have been told to get 2 Ford 9" rear ends for the front and the back. That's alot of work and I'm in the whole now I will never get the money I have in it....lol. But I really like the truck. Just want better gas mileage. I may just go back to stock tires. They just don't give it that mean look. Idk what to do. Thanks for the info. It's very useful.

I dont think you can attribute just the tire change to that drastic a drop in mpg. You say you smell fuel and carb looks wet sometimes. I am betting you have more problems than tire size.

start researching

Just bought a 2 barrel carburetor. What do I need to do to swap it out. I have read to use efi manifolds. What's the difference in the manifold that I already have. I was just planning on using a flat plate 2 to 1 adapter. The carburetor is a 9T motor craft. I'm hoping I can just swap it with no problems. Any suggestions are greatly appreciated. Thanks for your time.

I have an 83 Bronco, 6 cyl, NP435, 3.55 gears (stock) and 31" tires. I'm still getting in the high teens on the freeway.

You have something wrong with your truck and its not the tires. Too large a drop in mileage to be from tires alone.

If your truck is not running pig rich you must have a fuel leak somewhere. Brakes dragging?

The brakes do clang when applied. I let my wife drive it and the park brake doesn't work anymore. It feels a little weird like a thump under the seat from time to time. Could the brakes be stuck? I'm gonna check them out and get back with the outcome. Dang why didn't I think of that 😒...Thanks for the info.

power valve
 

If you dont see a large leak on the exterior, check manifold vacuum for base gasket leak which may cause power valve to stay open. Check power valve anyway, depending upon age of carbs, replace power valves anyway.

Thanks for the advice. I have a 1648 alumacraft boat. I pulled it to the lake for the first time the other day. The lake is in the mountains of Henry county. The hills are terrible, by the time I got to the top I was in first gear. Do you think a 2 or 4 barrel carb would help out on the pulling power.