The truck is a 1995 F150 4.9 with the E4OD, 3.55 rear open diff and it came with the towing package.

Playing with the online RPM calculators with a 0.70 OD ratio, the engine would be turning 1584 rpm at 55 mph, the national speed limit when it was built while running stock 235/75R15 tires.

When I am running 70 mph for the 90 mile round trip to my parents house and back that I do a couple of time a week, the truck is turning 2015 rpm according to the calculators. I know that it no where near red line on these engines, but it is enough to pull the fuel economy down tremendously compared to 55mph.

The truck is my current daily driver, only towing a boat and trailer that weighs maybe 3500 lbs and a car carrier with a 2400lb compact tractor. I'd say it spends less than 5% of its time towing anything.

If I go to 3.08 gears, my 70mph rpm would be 1749, closer to doing 60mph with the 3.55 gears. That would help bring the MPG numbers back up some.

Right now, the truck is fun in the city with the low gear to come off the line, but I have faith in the inline 6's torque to not become a total slouch with the gear change.

Anyone else done this type of swap? I know most people go lower in the gears to help with bigger tires. I want to go the opposite to help the truck get closer to its 55 mph mileage in this newer 70 mph world. I know the increased wind resistance at 70 will skew the #'s a little. Though, I would feel better with the engine in a lower rpm range for extended amounts of time.

 

Don't do it, mine has 3.08 and I'm gonna swap to 3.73 this spring. It constantly hunts when trying to run 55-60 on backroads very annoying and have to use way to much throttle. It also stays in open loop a lot because of load hurting mpg.

 

Expensive change to make. Unless you can do the gear change/set up yourself, it doesn't pay off from a mileage savings point of view. If you can handle the work, sure go for it.

 

That engine will be a lot happier cruising 2 grand than 15-1600. Your mileage will go down if you do the swap.

 

It would be easier to change the axle. There would have to be some 3.08 geared axles in the junkyard.

I agree with the other though. This isn’t a step in the right direction. How much mileage do you think you are going to gain? I would expect you to be pressing the throttle down further trying to make up for the lost torque.

 

Ditto on this.

I bought my '95 with 3.08's thinking it would be great mileage. My other '95 had 3.31 and it did better and did not have to downshift on hills going 65 mph(with no load).

You would be surprised how much more throttle it would take to keep it running at 65 with faster gears. Thus it would want to downshift more, where no it just floats along with minimal throttle.

 

Yes, if the op can't install gears, complete axle swap would be cheaper.

 

I'm almost in the same boat. 3.08 with a 302 4r70w. Not impressed with the mpg's. I had a 302 3.55 and a 5 speed. I'd like that performance/mpg back.

What is "OPEN LOOP"?

 

I have 3.08 with an NP435, direct. 70 mph is pushing 26-2700. My truck is geared perfect for 55-60 mph, but that's not the speed limit anymore. I try to hang around 65 mph or less.

Under 2000 rpms these engines don't have the power to run a higher rear. I wish I had a 5 speed HD transmission in mine, but it would need to be a short OD ratio for it to work.

 

I say do it. I had a 300 Six in my 1991 originally and it had the 3.55 gears. I swapped the entire rear-end to a 3.08 hoping to improve MPG, which was not what I was told to expect from the Six (about 16 is what I got). The MPG stayed about the same but the engine was much happier with the taller gear. Y'all flame away ONLY IF you have actually done this swap yourself and are not just bench racing.

Agree 3.55 is better for the 302 and even my current 460 but the Six likes a tall gear in my experience.

You do understand that max torque for the Six is *at* 2000 RPM, right?

 

I should understand. I've had this truck since 1991. I live next to the mountains, in a hilly area. I just know what works where I live, and a tall gear would not be it. With what I do, which is 95% two lane roads, what I have works. I don't like the interstate rpms though.

 

You need 3.73 or more depending upon tire size.

Gas guzzling mode.

 

I live in southeast Louisiana, plenty of bridges, but no hills for a few hundred miles in any direction.

The truck is also a regular cab, short bed so it's about as light as one of these trucks can get.

Right now, going over some of the taller and steeper bridges that go over navigation channels, the truck never down shifts, drops out of OD or unlocks the torque converter.

In a perfect world, I would just put a Gear Vendors behind it and have the best of both worlds, but I don't have the cash laying around to throw at that instead of the gas pump for a long time.

I've built 9" third members, but that was on a work bench. The 8.8 would be new to me.

If I went to the Pull A Part, what year trucks would I be looking at for a direct bolt in swap?

 

Living where you do you should be in pretty good shape switching out ratios, until you cross the Mississippi in Baton Rouge on I-10, lol... You may shift back a few gears on that bridge pulling a boat.

 

I have a '96 1/2 ton 4x4 with the M5R2 manual & 3.08 gears. I highly recommend you don't bother swapping gears. The M5R2 has a pretty low first gear, not sure about the auto. Considering that you tow anything at all with the automatic transmission, you will not gain any real advantage by swapping in 3.08s. My $0.02.