diff change for economy
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Freshman User
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diff change for economy
Hi,
At the moment I am using my 78 F100 with a 302 and NP435 100% on the highway at 70mph.
The diff has a 4.11 ratio and I would like better economy because I travel so many miles. It is currently sitting at around 3000rpm at 70.
Would a 2.47 ratio diff centre (lowest I can find) be better for me? Or would that put the revs too low and cause me to use more fuel since the engine makes hardly any power at such low revs? What engine speed is the sweet spot to cruise at for hours to gain max mpg? I drive only at 70mph. It is a stock 302 with a 350 holley and dual exhaust. Original cam as far as I know, idles smooth. I am using 235/75/15 tyres.
I would also be able to get back use of my granny 1st gear with the taller diff gears (even though 1st is non synchro)
Thank you
At the moment I am using my 78 F100 with a 302 and NP435 100% on the highway at 70mph.
The diff has a 4.11 ratio and I would like better economy because I travel so many miles. It is currently sitting at around 3000rpm at 70.
Would a 2.47 ratio diff centre (lowest I can find) be better for me? Or would that put the revs too low and cause me to use more fuel since the engine makes hardly any power at such low revs? What engine speed is the sweet spot to cruise at for hours to gain max mpg? I drive only at 70mph. It is a stock 302 with a 350 holley and dual exhaust. Original cam as far as I know, idles smooth. I am using 235/75/15 tyres.
I would also be able to get back use of my granny 1st gear with the taller diff gears (even though 1st is non synchro)
Thank you
Postmaster
Joined: Nov 2011
Posts: 2,553
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Too much gear will lug the engine and force you to use lots of throttle to keep going - hurts mileage plus makes the truck slow. I'd suggest something in the 3.50 range for your setup which should drop cruising RPM to 2600-2700.
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Freshman User
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I did think about that. Trying to find the balance between high revs/small throttle opening and low revs/more throttle
I need to find out what rpm will make the engine use the least fuel given the 70mph cruising speed and taking into consideration the drag and wind resistance etc etc. I can get pretty much any center ratio for the same price so I have to make an educated decision if I'm going to aim for better mpg.
I have also thought about rejetting the 350 holley. Anybody experienced doing this?
Thank you
I need to find out what rpm will make the engine use the least fuel given the 70mph cruising speed and taking into consideration the drag and wind resistance etc etc. I can get pretty much any center ratio for the same price so I have to make an educated decision if I'm going to aim for better mpg.
I have also thought about rejetting the 350 holley. Anybody experienced doing this?
Thank you
Moderator
Joined: Jul 2011
Posts: 6,510
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From: Sioux Falls Area
maybe others can help you guess how much better mileage you will get with a gear change, but i would strongly suggest running the numbers to see how long it will take to pay back the money spent on the swap.
you say you drive alot, so it might be worth it, but gears are expensive, and your savings may only be 1 or 2mpg (hopefully others will chime in), but unless you gain significantly in the mpg department, you'll have to decide if you want to spend that much for a long ROI.
you say you drive alot, so it might be worth it, but gears are expensive, and your savings may only be 1 or 2mpg (hopefully others will chime in), but unless you gain significantly in the mpg department, you'll have to decide if you want to spend that much for a long ROI.
Fleet Owner
Joined: Dec 2009
Posts: 20,678
Likes: 82
From: NorCal
With that engine combo and highway driving, I'd look for a 3.00 rear gear and a used one at that. Easy enough to swap the pumpkin. A 302 isn't very torquey so a 2.47 will be a dog on hills and dales requiring downshifting.
The NP's 6.69 first gear and a 3.00 rear gear still ends up with an effective ratio first gear of 20.07 (6.69x3.00) making it still very useable for towing & pulling. Passenger cars will typically be between 10 and 14.
Your current combo's effective first gear is 27.49... you can prolly walk faster than that. Dropping down to a 2.75 makes it 18.39 - still decent for towing and your highway mileage will increase.
My brother's 360FE, 2bbl, c6, and 2.75 rear gear nets 15.7 highway mpg . That's pretty amazing for an FE.
The NP's 6.69 first gear and a 3.00 rear gear still ends up with an effective ratio first gear of 20.07 (6.69x3.00) making it still very useable for towing & pulling. Passenger cars will typically be between 10 and 14.
Your current combo's effective first gear is 27.49... you can prolly walk faster than that. Dropping down to a 2.75 makes it 18.39 - still decent for towing and your highway mileage will increase.
My brother's 360FE, 2bbl, c6, and 2.75 rear gear nets 15.7 highway mpg . That's pretty amazing for an FE.
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