I have a 1965 f100 manual and was wondering if i could get better mpg by changing the diff, and if doing so would have side affects.

 

Yes, you can change out the gears in the diff. to improve better mpg and drivability at freeway speeds. Check to see if the tag is still attached to one of the bolts on the diff cover, that tells you the gear ratio, I think that is where it should be. You may also be able to tell from the VIN data plate, if you post a photo, someone will decode it.

 

The nerd in me says this needs a break-even analysis!

Have you priced out getting a new gear set installed? Let's say it's $600

What mileage are you getting now and what mileage do you think you'll get with the gear change? Let's say you get 10mph now and maybe you're really lucky and get 11mpg with the change.

How many miles do you drive the truck in a year? Let's say 5000.

So thats 500 gallons of gas for a year at 10mpg and 454.54 gallons for the year at 11mpg.

With gas at $4 a gallon you save $181.82 per year with the gear change. So your break even point to cover the cost of the gear change is 3.29 years.

Obviously this is all hypothetical and you can enter your own figures to figure it out.

 

Interested in other member's experience: I have two 65's where both have the same axle ratio, the 65 with the 390 and 3 speed w/ O.D., affectionately referred to as the 'gas hog' gets approx. 12 mpg highway; whereas my 65 with the 302 and T18 gets approx. 17 mpg highway?

 

Kinda depends on if changing the gear will put the engine in it's sweet spot rpm wise or result in lugging the engine.

 

SixtyFour is kind of right. But only if you don’t mind your truck howling at anything over 50 mph on the freeway. Got rid of the 4.56:1 gears on my old 59 F250, to something like 3.73:1, and that made it pleasant to cruise. Before, it was not, so I will eventually do the same to my 66.

 

Welcome to FTE.

Maybe, yes.

What engine, transmission, tire size, axle ratio and usage of the truck do you have?

 

Yes it is assumed the owner needs a re-gear otherwise whats the point? Even if he doesn't actually need a re-gear and is simply looking for the max MPG, the only change acceptable might be a 3.73 to 4.10 and in that case he wont even notice a difference.

In any event with the break even analysis i really dont see a non-essential gear change worth it.

 

If this is a 240 I-6 w/a 3 M/T .. it'll have a 9" rear axle with a 3.50-1 or 3.70-1 or 3.89-1 ratio.

You do not want to change any of these ratios to a 3.00-1 or 3.25-1 because the 240 won't have enough torque to pull a sick trollop off a porcelain device!

Even if you do change the ratio, only the highway MPG will improve, but very slightly. These trucks were not noted for MPG .. Expect 8-12 city, 13-15 highway if you drive 55 MPH

If you want better MPG, buy a used econobox.

 

Even with a 352, the stock 3.25 gears in my truck were pretty sluggish. I went with 3:70s and a 5 speed. How many miles will it take to break even? Probably well past my expiration date, but who cares? How do you put a price on fun?

 

I have 3.00 in my 63 and it will run smooth and effortlessly at 80mph on the free.
I don’t really care about MPGs.
I have smaller tires on it now but for 20 years ran 31-10.50-15 All Terrain T/As, I’m seriously considering going back to the taller tires.
Taller gears might not be a good idea with a six cylinder.

 

It isn't that I care or don't about the MPG, but want any engine to run to spec with optimum performance and efficiency, and those two pretty much go hand in hand.

There is never a performance benefit to wasting fuel, it will usually run worse in fact.

You're most likely far better off by simply tuning it carefully, than swapping out the third member, there is a typically a lot of wasted fuel going straight out the tailpipe with the by-now degraded ignition system, poor distributor ignition advance curve, and the usual carburetion problems. At least try a good tune-up first, with special emphasis on distributor curve and carb jetting.

 

Welcome to FTE, as stated earlier, what engine/trans/axle ratio/tire size do you have?
My 66 F100 when I bought it, had 240/T18/3.00's and I put 265/70/15's on it. Even before the larger tires it was a slug, I actually bought a 302 for it figuring the 240 was just tired, it couldn't pull a hill in 4th to save his life.
Rear end was making noise so I dropped the 9", tag said it should have been 3.50's, it was 3.00's, I found a 9" w/3.70's, swapped it in, TOTALLY different truck! Lot's of pep, pulls hills in 4th and gets 16-18mpg driving to Fresburg and back. Basically it put the engine back in it's sweet spot where the torque is, AND my speedometer is ***** on accurate with GPS, that was about 8-9 yrs ago, and the 302 is still in my garage.
I would drop your 9", count teeth on R&P so you know what you have, if you have 3.50's or higher, I would not change them, odds are, like mine, someone changed to a lower ratio thinking it would be better, when in fact it made it worse. Also, a good tune up, plugs, cap/rotor/wires and points will help too.

 

I can do a similar equation for Smiles Per Gallon ;-)

 

The way I look at it is a hobby. What do you want to spend and how much. My 1965 F100 390/4v 3 speed manual with 3.50 gears will get sometimes
14 mpg depends how I drive it. It will also get 10-11 if I want to play around with it. I enjoy it just the way it is plenty of pep when you want it.