Considering changing rear end ratio
#1
Considering changing rear end ratio
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.
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.
#2
#3
#5
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.
#6
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.
#7
Trending Topics
#8
#9
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.
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.
#10
Join Date: Feb 2002
Location: Greater Austin, Texas
Posts: 7,300
Likes: 0
Received 355 Likes
on
285 Posts
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?
Agree 3.55 is better for the 302 and even my current 460 but the Six likes a tall gear in my experience.
Under 2000 rpms these engines don't have the power to run a higher rear.
#11
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?
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?
#13
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?
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?
#15
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.