Ideal Cruising Speed
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5th Wheeling
Joined: Feb 2023
Posts: 48
Likes: 11
From: Eugene, OR
Ideal Cruising Speed
I have a '79 F-150 4x4 short bed. Manual 4 speed. 4.9 (300) I6 motor. 3.50 gears. What would be my ideal cruising speed? 50 mph? 55 mph? 60 mph? I'm trying to find the best speed for speed and fuel economy. Thank you!
Lead Driver
Joined: Jun 2006
Posts: 5,663
Likes: 985
From: Near Yosemite CA
What is your tire size? My 79 F250/4x/4sp/4.10's w/33's at 55-60 avg. 18mpg when I drove it to MO, my 66 F100/2wd/3.70's and 265/70/15's avgs 16-18 at 55-60, I would have to go drive it to see what my rpm's are, but I believe around 2300... I have found on all of my older rigs, they like 55-60 for best mileage.
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5th Wheeling
Joined: Feb 2023
Posts: 48
Likes: 11
From: Eugene, OR
What is your tire size? My 79 F250/4x/4sp/4.10's w/33's at 55-60 avg. 18mpg when I drove it to MO, my 66 F100/2wd/3.70's and 265/70/15's avgs 16-18 at 55-60, I would have to go drive it to see what my rpm's are, but I believe around 2300... I have found on all of my older rigs, they like 55-60 for best mileage.
Tuned
Joined: Jun 2018
Posts: 458
Likes: 123
From: Arkansas
My f100 with a NP435/3.55 rear and ~32.7" buckshots will do 60-65 at 2200-2300 RPM. To me that's a pretty happy cruising RPM for not having OD. Shooting for 2000 RPM would probably be ideal for MPG and still having some acceleration but a stock 300 at that speed on the highway may be gutless. Although I find 18 MPG to be hard to believe with any stockish engine in these trucks
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5th Wheeling
Joined: Feb 2023
Posts: 48
Likes: 11
From: Eugene, OR
My f100 with a NP435/3.55 rear and ~32.7" buckshots will do 60-65 at 2200-2300 RPM. To me that's a pretty happy cruising RPM for not having OD. Shooting for 2000 RPM would probably be ideal for MPG and still having some acceleration. Although I find 18 MPG to be hard to believe with any stockish engine in these trucks
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Lead Driver
Joined: Jun 2006
Posts: 5,663
Likes: 985
From: Near Yosemite CA
Yes, more than likely, the other option was the T18 I think, my 18mpg avg while driving 1800+ miles was in 79 F250/4x/NP435/4.10's/33's, I also had an Offy 4bbl intake and Holley 390cfm carb, EFI exhausts y-pipe into 2 1/4" glasspack out the rear. My current 66 had the og 240/1bbl/T19/3.70's and 265/70/15's and it avgs 16-18, yes, both gps verified, my other 66 F250 I had was pretty much 16, of course the faster you go, the worse my mileage was..
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Cargo Master
Joined: Nov 2007
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Fuel economy comes from the lowest RPM you can run the engine without lugging it when you accelerate easily. It has nothing to do with speed. A quick search shows a stock 4.9 has peak torque around 1600RPM, so you'd want to be a few hundred RPM lower, at whatever speed you're cruising at. Ideally you want to be at an RPM where there's just enough torque to maintain speed and accelerate smoothly under part throttle, if you need to accelerate quickly you can downshift.
Taller tires will lower your RPM at a given speed, but will make acceleration slower. It's essentially the same as going from a 3.50 rear gear to a 3.30, or 3.00, depending on how tall you go. My truck with 4.10s and a 34.5" tire was just about the same rpm as 3.55's and stock sized tires. Just don't put on substantially heavier, or wider, tires and don't change your driving habits to make up for the lost acceleration, or you could end up with worse fuel economy.
Taller tires will lower your RPM at a given speed, but will make acceleration slower. It's essentially the same as going from a 3.50 rear gear to a 3.30, or 3.00, depending on how tall you go. My truck with 4.10s and a 34.5" tire was just about the same rpm as 3.55's and stock sized tires. Just don't put on substantially heavier, or wider, tires and don't change your driving habits to make up for the lost acceleration, or you could end up with worse fuel economy.
Tuned
Joined: Jun 2018
Posts: 458
Likes: 123
From: Arkansas
Having owned an EFI 4.9 truck before with a tall rear end, a straight 6 is miserable on the highway at 1600 RPM. You wouldn't be able to climb a grade without losing speed. I think peak torque is closer to 2000 RPM but it makes like maybe 100 HP there.
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