i've got a 93 f-150 with a 300 I6, 5speed and 308 gears. understand that the I6 is ideal for towing but with 308 gears, with any kind of weight it won't get out of it's own way. i'm thinking about swapping for 373 gears but i don't know if it will make enough of a difference to justify the cost. what i'm looking to haul is a 1,500lb slide in camper with a 4X8 harbor frieght trailer and 4wheeler. as it sets, i'm doing good to hold 55 in 4th gear. i have to downshift to 3rd to make it up any hills. i realise i'm asking alot of my truck but it's gonna be 3 years before i can purchase somthing bigger. does anyone happen to know if 373's will make enough of a difference or should i stick with what i got untill i can upgrade in truck?

 

Don't know if this will help, but back in 1996 I had an '85 Bronco XLT with the Straight Six. I had the C6 3-speed automatic and 3.54 gears(not a typo- Ford 9" has 3.54 gears and 8.8 has 3.55 gears), a 4-barrel carburator, 4-barrel aluminum intake manifold, long tube split headers(first three cylinders into one collector and last three cylinders into another collector) and true dual exhaust with no cats and just glasspacks. And, it was bored .010 over with higher compression rings. Also, it had 33" tires.
Anyways, that truck was more powerful than my friend's '87 bronco with a 351 Windsor. I used to haul a lawn maintenance trailer all over the place and burying the speedometer (happens at 80 mph) was not a problem. Never weighed the trailer, but it had to weigh at least 1200 pounds with all the equipment loaded.
Also, I had another friend with the same tranny and motor you have (Straight Six with fuel injection) and he had 3.73 gears. He was running 40" Ground Hawgs and did not have a problem getting up to 80mph.
Obviously, you have a 2wd so you should be making more horsepower to the wheels than a 4wd because you have less parasitic loss. Therefore, in conclusion I think that going up to 3.55's or 3.73's will drastically improve your towing performance.

 

The Mazda M5OD-R2 isn't a towing transmission. It's barely strong enough for an empty truck. You're talking about going 3x over the truck's rated cargo capacity, AND towing a trailer behind it. You need either a heavier transmission (ZF S5-42, NP 435, BW T-18/19), or a heavier truck.

But your FIRST step should be some serious maintenance. Get a Haynes manual & go thru the whole truck. Plugs, wires, cap, rotor, filters, fluids - everything. Then test the vacuum at idle, check compression & fuel pressure, have the injectors cleaned professionally, & AIR UP THE TIRES.

Mine is a bone-stock '95 4.9L in an '83 Bronco w/NP 435 & 3.08s on 32s, and I can tow the world, or set the cruise over 100mph.