I had a 300 in a '78 E250, since new, drove until 2002, almost 200k on it. Never burned oil, wouldn't stall, like a tractor, great engine! Had 3.73's with 31" tires, 3 on the tree, could tow anything and got 15 mpg. I thought 3.73 was great as I could still cruise at 70 -75 and wouldn't go 4.10 with out overdrive. I picked up an Offy intake and liked that a lot, I wish I had known about the EFI exh. manifolds. I tried a glass pack but preferred the sound of a turbo muffler. I didn't need the 4 bbl so made a plate to adapt a 2bbl carter from a 318 mopar and ran that on the secondary side of the dual-plane. Eventually swapped a C-6 into it and got 12 mpg . Never needed any thing but routine maintenance to keep her going.

('78 3/4 ton burned regular, no cats! It was a simpler time)

 

U may wanna look into a 500 cfm edelbrock or holley makes a 390 cfm 4 bbl that works too..there's your limiter as for gearing..there's usually a tag hanging off a bolt or stamped to the rear end to tell ya somewhere what gears ya have. I only have 3.25 in my 68 f100 but with the t18 behind the 300 ..my truck will still haul ***** and get decent on the highway. If in fact u do have 4.11s..is switch to 3.50s or 3.70s..you'll get better mpgs plus u won't sacrifice a whole lot for towing.

 

For 70 mph on up you can't beat 3.08's. 3.55's would give you a little bit of both worlds though and I would recomend 3.55's.

I had 3.08's in my truck and I loved them on the highway. I would top out my speedo in no time.

 

A 4bbl can often get better gas mileage than a 2bbl IF you learn to keep your foot out of it and only use it when you need it. If you're looking to keep the power away from lead feet, a 2bbl may be the way to go.

Do you have a tachometer in the vehicle? Also, if you open your driver's door, what does the AXLE code read on the sticker? (Granted, it's always possible someone swapped them out, so what the door jam sticker is only a guide.)

You can also find what gears the axle has on the tag on the axle itself.

Last, but not least, you can jack it up, rotate the drive shaft, and count how many times the tires turn. You can't argue with straight up proof. =P


3.08s are good for the highway but if you get too large of tires on there, you may find the carburetor liking to continually dip into the powervalve as engine vacuum gets low. That's an issue I deal with frequently with my 31" tires.
Even though 1500 RPMs @ 65 MPH seems like it'd be nice for gas mileage, my engine vacuum frequently drops to around 5 - 7hg, which opens the power valve and completely negates all my savings... Just a thought.
If your tires are smaller, that may not be an issue at all.

I have 3.55s on my '84 Bronco (with a 300 I6 and a 4bbl) and it is great on the interstate. However, I don't do much over 65 in it because the RPMs are at 2500. They'd be at 2950 at 75mph since I don't have an overdrive. Those RPMs would be even HIGHER with smaller stock tires. 215 75 R15s would be at 3250 RPMs @ 75. That's gonna kill gas mileage.

4.11s without overdrive are pretty much for off-roading, around town, work vehicle use only, or if you have big, big tires.