I have been away for a while, because work has been grueling. However, I have had time to perform an upgrade to my truck to significantly improve the fuel economy and overall drivability of my truck without diminishing it's hauling capacity. I have a 1996 2wd F-150 with an NP-435 and an EFI 300. At one time I had a 3.08:1 axle ratio in my truck. These gears worked well, and my truck got about 16 MPG average, and on the highway it would get 20 MPG at 55 MPH. Also, when towing, I could easily pull 8,000 lbs and I only needed to use 1st gear when starting a big trailer from a dead stop on a steep hill. At 260,000 miles or so, my original axle went bad. I didn't have time to lay the truck up while I fiddled with it, so I called the local wrecking yard for an axle, and they had one pulled and ready to go. It was a 3.73:1 axle. I wasn't thrilled but I needed to get to work, so I bought it and installed it. My mileage dropped severely; at 55 MPH I was only getting 12.9 MPG average. It actually hurt my pulling power, because at highway speeds I was so far above the engine's sweet spot, I would loose speed on hills, and the truck worked hard to pull the same trailer it had pulled so easily before. But then I had a stroke of good fortune. I found a really nice 1989 F-150 4x4 with a bad engine that I bought for $500 to fix up and sell. I also found a T-boned 1994 F-150 for parts, that cost me $300. So while swapping the engine from the 1994 into the 1989, I realized that the '94 had the 2.73:1 axle ratio, and I decided to swap it. I installed the 2.73 axle into my truck 2 months ago, and began keeping records. My fuel economy jumped dramatically and is even better than with the 3.08:1 gears. My average for the last 2 months has been 18.1 MPG, and at 55 MPH with no load I am now getting 22.4 MPG. I towed the 8,000 lb trailer with the truck in Pennsylvania on some steep hills in the Poconos, and she did it with no trouble. In fact, on a few steep hills I did have to gear down to 3rd (which acts like 4th would if I had 4.90 gears) and maintained 55 MPH no sweat. I do need to use first gear to move off from a dead stop with a heavy load on uphill grades, but 1st gear is actually usable now, and I still only need it with a heavy trailer. I am extremely pleased with this swap and have to say that my truck is much more pleasant to drive. This might not be the ideal gear ratio with an overdrive transmission, and definitely would not pull anything with an M5OD, but it might be something for some of us to consider if we are running HD 4 speeds. The nice thing is that someday, when I install a 4BT Cummins, I'll already have the right gearing in place.

 

Towing 8000 lbs with an F150 might have something to do with why your axle went bad. It's just not a good idea. The published GVWR and GCWR have more to do with getting the load stopped when the trailer brakes fail than getting it moving.

The reality is that you're overloading your truck and putting other drivers around you at risk.

Go find a regular, old analog bathroom scale and jump on it from 4 inches up. See what multiple of your weight it is. Now think about what that 8000 lb trailer is doing to your inadequate suspension and driveline every time you hit a bump....

Jason

 

Without an overdrive transmission axle ratios in the 3.00 range are whats needed to maintain sensible fuel consumption.. assuming relatively stock sized tires, but throw in that OD gear and you get the best of both worlds, better pulling power and highway fuel milage. BTW a 4.9l F150 with a 2.73 axle is technically only rated to tow 1700lbs and only 2600lbs with 3.08.

 

Just out of curiosity, did you regear the speedometer gear for the different axle ratios? I know switching from say, a 3.42 ratio to a 4.10 ratio (I did this on a GM) will result in more turns through the speedo cable with the speedo reading faster than normal and clocking up more miles. For your situation, if you still have the stock gear or programming (since I'm not sure if you're cable driven or electronic) you may actually be getting better MPG. Best thing to do would be to check it against a portable GPS unit to see how true things are. If you figure out the % difference you can use that as a factor in your MPG calculations.

In the end, I think the 2.73:1 ratio is the way to go being that you have a granny low. Probably pretty similar to having a mazda 5 speed with a 4.10 axle, except the NP435 is a million times stronger.

 

The max towing capacity listed for this truck with the 8.8" axle is 8,250 lbs. according to my owners manual. Granted, that was with the 3.55 axle and a 351 V8 and an automatic. However, the NP-435 was not available in that year, I swapped it in a long time ago. This truck is spec'd weird. It has the high GVW springs and suspension, and it came with a Mazda 5 speed and 3.08 gearing originally, which meant it was rated to tow 0 lbs. As for towing 8,000 lbs I only do that with a weight distributing hitch and trailer brakes. No F-150 is rated to tow 8,000 lbs without that.

jroehl: If I did what you suggested, the scale would undoubtedly break... It already cries and begs for mercy when I just stand on it. Also, as listed above, I do it with a weight distributing hitch and trailer brakes. The suspension is adequate, since the truck already has the heavy duty springs and brakes. the GVWR is 7,650, which is 50 lbs below the base 3/4 ton. Also, I have been towing this trailer for the entire life of the truck... 260,000 miles does not seem to me like a low figure for a half ton axle even if it never carried a load.

Conanski: That is true except for the fact that the NP-435 wasn't offered in the 1996 model year F-150s. The transmission ratios have as much to do with it as the axle ratio.

91chevywt: The curious thing with this truck is that the rear axle is where the speedometer picks up it's signal. The RWAL sensor in the axle housing provides the speed signal to the ABS computer, the powertrain computer, and the speedometer.

 

And the transmission too of course, the low rating the truck gets with 2.73 and 3.08 gears is beause of the M5 transmission.. which is notoriously weak and those gears put too much of the load on it.

Yep.. and that means the only thing that affects speedo accuracy is tire size since the ring gear is the same size(8.8") no matter what gear ratio is in the pumpkin.

 

Good to know, that makes things easier. Mine's cable driven in the t-case so I just assumed they were all fed from the transmission or t-case

 

Good tidbit to know about the speed sensor being in the axle thanks. I have a 96 4x4 with M5OD and supposedly 3.08 gears and the best I get is 18mpg and the worse I get is 16mpg unless running locked in on snow. I wish I could get it up there with what you are getting, would be nice these days, congrats on your success

 

I want to put a Cummins 4BT in someday, so I wanted a taller ratio anyway. The 3.08 gearset is the tallest available for a 4x4 because they don't make a front gearset taller than 3.08:1. The 2.73 works for RWD only. That was the deciding factor: Cummins Swap with 2.73 gears and 2WD or 3.08 gears and 4WD, because I was going to swap 4WD in on my truck. The problem with that is even with 3.08 gears and 31" tires, my top speed with the Cummins would have been 75 MPH, which isn't good, because I'd be almost maxed out at 65 MPH on roads with a 65 MPH limit. With 2.73 gears and 31 inch tires, I would max at 85 MPH, which gives me some breathing room. Not that this is happening any time soon, but some day, in a few years perhaps.

 

That would be a cool experiment. I'd be interested to see the absolute best mpg a full size truck like this could make without sacraficing useability and practicality

 

I have not filled up yet since last thursday (May 5, it is now May 15) and my rear tank is still full, the front tank is reading at 1/2 full (to be fair, that's like a 3/8 tank in actuality), and my trip odometer is showing 256 Miles. I am sure that the Mobil 1 Synthetic Gear Lube in the rear, the Mobil 1 5W-30 in the transmission and the Rotella Synthetic 5W-40 in the engine are helping things along. Before the gear swap, I'd have exhausted the front tank and been well into the rear one by now.

 

I just went to the filling station and refueled. I was shocked with the results. Even though I am doing mostly rural driving (not highway/interstate) and some around town driving, I got 18.91 MPG this go around (344.5 mi and 18.217 Gallons). I intend to take a highway trip this summer and am eager to see what the mileage will be then!