I have a 66 f-100 300 I6. It came factory with the 240 and 3.70:1 butt gears. It was painful to drive. I put 3.00:1 butt gears and I have less than 1/2 MPG increase in fuel economy. What should I look at next guys? Should I have seen at least 2-3 MPG?

TIA Todd

 

What was your mpg to start with? What percentage improvement is 1/2 mpg? Did you calculate the mpg over several tanks, using the same procedure each time? You can easily have 1/2 mpg error if you do things just a little differently.

Assuming 29 inch tires:

65 mph, 3.7 diff = 2786 rpm (near redline!!)

65 mph, 3.0 diff = 2259 rpm

527 rpm drop, apprx 19% drop.

Your mileage has varied....

 

There's a reason it came with 3.70 gears. The 240 probably needs them to lug the truck around. If it has to work a lot harder to pull it around with 3.0's you won't see any mileage increase. I used to have an 86 GMC that had 2.73's in it and it got 12-13 mpg average. When the rearend blew up we replaced it with one that had 3.08's and the mileage went up to 18.

 

Chk the first post, it appears he is running a 300 now.

 

there are many more variables to a change in average fuel economy than just a rear gear swap. chances are if you know how to swap a 300 for a 240, and 3.70s for 3.00s, you can identify most of them (and certainly the significant ones). I think i speak for all of us wheni say you'd need to give us more info than just the rear gear ratio....

 

3.00 gears, too high. 3.70 gears too low (I may beg to differ on that, though). 3.25:1's are common for your style of 9" (57-72). One thing that will help is to put a 5 speed (think ZF5) or automatic with OD. Or do what I do...drive no faster than 55mph. with your engine, I'd think a 2bbl adapter and Motorcraft 2150 might give you better fuel mileage, at the same time better response. It's also a cheap way to go without buying a brand new Holley or Edel. Your truck came without EGR. If it had EGR, you could expect another 1 mpg. If you went EFI...you'd improve alot. But it'd be a major PITA putting that in. Just food for thought. Good luck.

 

I use to average 13.8 mpg over several tanks with the 3.70:1 R and P. This is driving the same route to the same place and back again for weeks. I did the same thing again and on the first tank I came out to 14.3. I would have expected much better than that? It drives like a dream, lots of rpm and throttle to spare at cruise. I like that. It does not lug at all from a stop. BTW I am using a NP-435 trans.

The above figures are basically spot on for rpm Vs. R and P ratio.

The truck came to me with a 300/435 combo. I cant take credit for that. As far as swappin gears out, 'bout anyone can do that.

Since I bought the truck, new dizzy, new PCV system with NO vac leaks, rebuilt Carter Yf, and single 2 1/2 out to flowmaster(annoying as hell, it blubbers like a SOB on decell.)

I have thought about a different intake and carb with EFI exhaust manifold, but I just dont know.

I see guys claiming 15-20 MPG on this forum, and I want some too. I drive under 60 as I never see the freeway.

I thought the EFI guys got worse mileage than the carb guys? I got a 94 E-250 with a 300-6. I even thought of swapping that in, but again thought the EFI guys didnt get enough better to make the swap worth the while?

Todd

 

One thing's to be said...the NP435 will let you get away with a higher gear ratio. With my 79 Bronco w/swapped in 300..rebuilt Carter YF, no EGR,DSII, EFI manifolds, Walker downpipe into 2 1/2" single, C6, NP203 fulltime 4x4, 3.55 gears, 32" tires, I got 15 mpg at best on the highway doing 55. I only use mine to plow snow since the rear leaf's snapped. So I haven't done any tuning or carb swapping to try to get any improvement. However, the way I look at it is, my 300 gets 1 and a half times the mpg than my 400 did, lol.

EFI is more hp friendly and less torquey, but gets better fuel mileage.

 

What tire are you running. You might be able to go 2.79s but the cost of another diff swap may not be in the cards....

 

im assuming when you got the new dizzy you timed it correctly.

i have done 23 mpg highway before with my '83. its a 300/435/3.55s/32"s (2wd). it was definitely circumstancial though. usually 15-20.

whats the condition of the tune-up parts? how about wheel bearings? honestly at this point, it might be time to analyze your driving. vacuum gauges are $50. i bought one for mine. youd be surprised what youre doing without even knowing it. also if you never see highway, your mileage will always be in the basement. no one claims 20 mpg city.

 

New tires, front wheel bearings, and rear axle bearings too. I have a vac gauge in the cab. It reads 17"hg ad idle. I put it on an exhaust gas analyzer. The thing said I was at 13.8:1 air fuel ratio based on my gases. Stochiometric combustion occurs at 14:1 so I think the carb is dialed.

I am assuming I got it timed right too? I always run on the ragged edge of detonation with points ignitions.

Todd

 

well, youve hit all the major points man. i'd say this is as good as its gonna get. Youre near the peak of a pretty flat efficiency curve, so any improvements wont yield any major improvements. Might wanna try new rear gears, maybe 3.55s or whatevers in the 3.2 range if you can find em, but it wont be significant. welcome to owning a full size truck! ive done 9 mpg with my 300 before. Carb died and was running pig rich, and i was pissed so i was driving the **** out of it. im normally about 14-15 mpg city though.

 

Gas is 14.7 :1, you are a little rich.

Your tire size, like if you are running really big tires, will decrease mpg even if they provide a taller overall gear ratio.

 

13.8:1 A/F ratio is just fine.

 

Agreed. 14.7:1 is the theoretical ideal. With a carb you won't get near it and still have decent manners on the road at part throttle. You could probably get there at WOT, but you'd be giving up a lot of power. A more powerful engine will actually eat itself when running that lean. You need extra fuel at idle anyway because of the poor efficiency of the engine with the throttle closed.