Lower RPMS at 65mph
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Lower RPMS at 65mph
I am new to old trucks. I have a 1979 f250 supercab lariat with a 460 it is 2wd. I know that Gear Vendors is my best option for lower RPMS on the highway. But i am wondering what R&P would be best for highway, i dont intend do do any hauling with this truck since i have a 3/4 ton newer truck. I am running 285/75/16 on the 79 and at 65mph it is doing about 2600rpm will changing the rear gears help or is it fruitless. I havent had time to check what my ratio is but what would if i had one be the best R&P to change to? to drop that highway travel without loosing to much acceleration?
Senior User
Joined: Apr 2010
Posts: 248
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From: Farmington, New Mexico
If you have a manual trans, swapping in a ZF5 speed would be the way to go. Cheaper than the GV if you can find a good ZF.
I'm guessing that your truck came with the 460? If so, you must have a C6...?
Definately neeed to get those rpms down by about 700.
I'm guessing that your truck came with the 460? If so, you must have a C6...?
Definately neeed to get those rpms down by about 700.
Post Fiend
Joined: Dec 2007
Posts: 8,521
Likes: 16
From: Lost
A GV unit or swapping to 3.08 or 3.55 gears would help, as most older F250's came with 3.73 or 4.10 gearing.
Niether option is "cheap", and would take quite a while to give a return on the cost in fuel savings.
Niether option is "cheap", and would take quite a while to give a return on the cost in fuel savings.
Fleet Owner
Joined: Dec 2009
Posts: 20,678
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From: NorCal
I recommend doing a "Return on Investment" calculation based on what mileage it gets now and what gains are expected with the planned modification (rear gear/GV overdrive/5-speed swap). Remember, the calculation is based on the marginal gains and the miles driven when it is actually paid off. Only after that will you actually be saving.
Tuned
Joined: Mar 2010
Posts: 491
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Why would you swap transmissions, when a simple change in the rear end will get you out the door for a lot less time, money, and hassle?
I've got a 460 with a C6 in my truck. According to the VIN tag (which I have no clue if it's correct or not, previous owner was a jerk) the truck has 4.10's in the diffs. I'm running a very similar size tire (33x12.5R16.5) and I see similar RPM's on the highway (50-55mph = 2,200rpm)
I would drop down to a 3.55 ring/pinion if you want to slow down the motor but keep some of the power. Going much past that and you'll probably hate the way it drives around town!
-Brad
I've got a 460 with a C6 in my truck. According to the VIN tag (which I have no clue if it's correct or not, previous owner was a jerk) the truck has 4.10's in the diffs. I'm running a very similar size tire (33x12.5R16.5) and I see similar RPM's on the highway (50-55mph = 2,200rpm)
I would drop down to a 3.55 ring/pinion if you want to slow down the motor but keep some of the power. Going much past that and you'll probably hate the way it drives around town!
-Brad
Senior User
Joined: May 2010
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Why would you swap transmissions, when a simple change in the rear end will get you out the door for a lot less time, money, and hassle?
I've got a 460 with a C6 in my truck. According to the VIN tag (which I have no clue if it's correct or not, previous owner was a jerk) the truck has 4.10's in the diffs. I'm running a very similar size tire (33x12.5R16.5) and I see similar RPM's on the highway (50-55mph = 2,200rpm)
I would drop down to a 3.55 ring/pinion if you want to slow down the motor but keep some of the power. Going much past that and you'll probably hate the way it drives around town!
-Brad
I've got a 460 with a C6 in my truck. According to the VIN tag (which I have no clue if it's correct or not, previous owner was a jerk) the truck has 4.10's in the diffs. I'm running a very similar size tire (33x12.5R16.5) and I see similar RPM's on the highway (50-55mph = 2,200rpm)
I would drop down to a 3.55 ring/pinion if you want to slow down the motor but keep some of the power. Going much past that and you'll probably hate the way it drives around town!
-Brad
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Cargo Master
Joined: May 2009
Posts: 2,587
Likes: 47
From: Eagle River, AK
I would not recommend a trailer tire as a truck tire, they are constructed differently and designed to handle different forces.
Maybe a highway tire, but certainly not a trailer tire.
Maybe a highway tire, but certainly not a trailer tire.
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Senior User
Joined: May 2010
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The word used is "TALLER" not "TRAILER"... meaning going from a 33 inch tall tire to a 35 inch tall tire will reduce gear ratio and rpms...
Cargo Master
Joined: May 2009
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From: Eagle River, AK
Understood, sorry about that. I miss-read it on my little computer screen.
Senior User
Joined: May 2010
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lol... Old Trucks and the new internet devices that let us talk about em... Thought it was my accent that got ya.... lol... 
Senior User
Joined: Oct 2005
Posts: 271
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From: Gresham Oregon
So if you have a newer truck, why are you concerned about economy? What do you use the truck for? Lowering the rear ratio may cause your trans to kick down on hills more often negating any fuel economy you might get. Your feeding a 460, so you can throw a lot of money away to get 2-3 mpg. Ditching the C6 would help. EFI would help. But what is the cost to MPG savings?
Cheapest thing for fuel economy: Vacuum Gauge.
Cheapest thing for fuel economy: Vacuum Gauge.
Senior User
Joined: May 2010
Posts: 246
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Taller gears is not the best answer. My other ride is an 86 Bronco with a C6, 3.50 gears and 35" tires. Yes I have a great highway gear and am around 1900 rpm at 65. But I have to put it in low range to get my aluminum boat up the ramp. It's like I am starting in 2nd gear all the time, no takeoff at all. An overdrive, which ever type, will give you a good starting gear and a good highway gear.
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highway
Its not really about MPG cause i realize having a 460 engine will never give that however its about not wanting to have the engine churning at 2600 rpm to be on the freeway. I would like to drive it to tucson when i visit my folks every now and then and be able to be going 65-70 and not being at 3k rpm or higher.
