Fuel Economy Question

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1979 F100, 300, 3 speed, 2.73 rear end, 235/75 tires.

Vacuum readings/cruising speed on flat ground:
17/45mph
15/65mph
13/70mph

Original tires were equivalent in circumference to a 225/75 tire. Engine has 200,000 miles with frequent servicing. Compression readings 144-148 PSI--engine runs smoothly with good acceleration. 15 mpg Hwy./13 mpg Cty.

Are the vacuum readings listed above normal at the speeds shown?
Would my mileage improve with a 3.08 or 3.55 rear end?
Any driving techniques that would improve fuel economy by monitoring intake manifold vacuum?
 
I can't offer any comments on the "normal"vacuum readings. But as far as the rear axle ratio, going to a numerically higher axle ratio will decrease the fuel economy. A 2.73 ratio implies that the driveshaft is turning 2.73 revolutions for each wheel revolution. Assuming the wheel rim diameter is the same, your 235 tires would be slightly larger than the 225 in circumference. Therefore, your larger tires are covering a little more road than the stock 225 tire for each engine/wheel revolution. A transmission with an overdrive top gear may be your best bet at improving mileage.
Doug
 
I can't see how changing to numberically higher rear end gears would increase mileage. They require the engine to turn higher RPM's for the same road speed. More fuel consumption...

How's the fuel delivery system? EFI, carb? If you have a 4bbl, you can increase economy by delaying the opening of the secondaries. Generally, a healthy, clean fuel system yields gains to mileage and performance over a gummed-up system.

Your mileage numbers don't sound that far off of my old 302 Falcon. That may be the best you can do without an overdrive gear.

Can't speak to the vacuum stuff.

Syntehtic oil in your driveline may be good for 1-2 mpg, but don't expect miracles there, no matter what they advertise. Keeping tires properly inflated can increase mileage as well.

The wider tires will reduce your mpg a little due to increaded rolling resistance. This may be offset a little by the increased circumference of your new tires, but not noticeably. They throw off speedo and odometer calibration sligtly, becuase they take fewer revs per mile. You are actually going faster/farther than indicated, but by a very small percentage (2% or so in your case).

General driving tips- Driving like your name is Mario hurts mileage. Driving like your name is Maude helps mileage. No surprises there.

Adding headers to free up exhaust flow can also help. Less of the engine's work is used up pushing out exhaust gases. Header companies claim up to 20% increases, but I doubt it. I'd expect cloer to 5-10% on the optimistic side.

--mikey
 
The vacuum numbers - if the gauge is accurate - sound pretty good. I've seen plenty of motors unable to pull 13 at idle. Driving with a readily visible vacuum gauge will tell you when you're operating the motor at best efficiency.

If you're looking for maximum fuel economy, then a five speed box with a good spread of ratios may be the ticket. However, the cost of purchase and installation will take a long time for the fuel savings to pay off.

Regards, Adam.
 
I will agree with Adam with regards to the payout of the overdrive transmission. Unless you rack up a large amount of highway miles per year on the vehicle, it is likely to be a very long payout.
 
Those rolling vacume readings look good, but they are misleading. It is not only the higer reading when you are off thottle like crusing, but the percentage of on throttle to off throttle time spent. ie; how fast are you getting up to speed? how many hills you climb and how steep are they?
also too low of rear end gears make you spend too much time in secound or worse yet lugging and useing more open throttle to keep the same speed.
Things thet help my 300 get 17 average around the county in these hills are 2v carb, keeping the bed covered, keeping my tune ups up to date, changeing my on time, fairly open exhust, water injection. This is in an '83 F150 mit a flat bed thet is wider then the cab by six inches on both sides. Talk about 'pushing a brick, milege drops to 11.5mpg when pulling my camper and hauling the horse wagon thet sticks above the cab by two feet.
It also has a power eating c-6, but it is a farm truck and I like thet big trans.
I don't have the vacume gage in cab so I can't watch it while driving. But it reads between 20 and 21 at warm idle.
I keep a log book on gas fills and other things. My averages are over several years and I still have the records to copare thing mit. One tank will not tell you anything go for a month or two then look and see if anything helps.
 
What's your altitude? What's your idle vacuum reading?

The compression numbers are good, but to tell the truth those vacuum numbers seem a bit low, and with that rear end ratio your mileage should be better as well. A little more info about where you live would help...
 
Watching a vacuum gauge while driving is an excellent idea. It will almost certainly improve your mileage if you keep the readings as high as you can for whatever you're doing. Sometimes that number may not be so high, like pulling a load up a hill, but even then you can still keep it as high as possible and still get up the hill without slowing too much.

Essentially high vacuum readings mean you don't have your foot in it. The gauge lets you see what's going on and maybe lift your foot a little at times you might not have without it. Or not pushed it down so far in the first place. Ever floored it and had nothing happen except maybe a little change in sound, or let up on the gas a little going up a long hill and not slowed down any? A vacuum gauge will show that right now.

A vacuum gauge is one thing that would not have a decades long payback time, unlike almost everything else. Kind of boring, but smart. And they are kind of interesting to watch, to a gearhead anyway.

As mentioned the numbers themselves are relative to a number of things, even themselves.
 
always a good tool to check if the engine is in good operating condition or if your carb/timing is out of whack
 
Thanks for all the replies. I'll throw out some answers in no particular order:

Speedometer drive gear was changed to compensate for 235 tires size.
Altitude--sea level, roads flat
Carburetor, 1bbl Carter, remanufactured.
Idle vacuum--20
Synthetic engine oil, transmission fluid, rear end fluid.
Stock exhaust w/no leaks, EGRvalve new and functional.
Mileage when new was 13cty/16hwy.

I have been experimenting with minimal throttle position in order to maximize vacuum reading, as rmt commented on. At speeds greater than 60, this method doesn't work.

If I went with the 5 speed to get an overdrive, what rear end ratio should I use?

This Carter carb has only two adjustments, throttle plate position and idle mixture. I've minimized the throttle plate position at idle to prevent "dieseling" at shut down and used the idle mixture screw to get proper curb idle setting. Is this method OK?

Does the idle mixture setting affect how much fuel is used during cruise conditions? That is, if the idle mixture is rich at idle, will the fuel mixture at cruise also be too rich?

Thanks for your help,
Alan
 
Idle vacuum looks good... but...

alanvng":2d71lzm3 said:
This Carter carb has only two adjustments, throttle plate position and idle mixture....
Click to expand...
There's a third adjustment-- inside the float bowl (you'll need to remove the 5 or so screws that hold the top on). The adjustment will be a screw sticking up, can't miss it. This is the part-throttle/cruise circuit; the screw contols how much of the tapered metering rod sticks into the main jet. Remanned carbs are notoriously mal-adjusted.

Turning the screw counterclockwise leans the mixture. Try 1/2 turns at a time until you get a lean stumble or too much pinging, then go back 1/4 to 1/2 turn. But be careful not to go too lean in the adjustment...

Engine vacuum pulls that plunger down... When making the adjustment, be certain to physically push the plunger down all the way as a check to see that the needle isn't bottoming out in the hole before the plunger seats all the way.
 
I may have been able to say it better. You want your vacuum as high as possible when possible. It's not always possible. But at least you're aware of it and can try to get a higher reading when you can, and that will improve mileage. It probably won't make a heck of lot of difference unless you were a sloppy driver to start with. Then it could make a big difference.

Going across country in an early 70's New Yorker my friends and I tried to see who could get the best mileage out of it while taking turns driving. I use the word "best" loosely here. There could be a 30-40% difference between drivers. Bad habits make a difference and it shows up quick in a car like that with such poor mileage. Everything is amplified.
 
Alan;
Here's one thing that may help overall; make the hole in the EGR valve smaller. Some valves come with a washer for this purpose, like if the valve also fits smaller engines.

Reducing the amount of exhaust gas in the intake manifold's mix will improve the actual combustion of the fuel you use, instead of letting the catalytic burn it off because of the little mismatches that occur without computerized mixing. This will translate to better MPG overall. DON'T simply kill the EGR valve unless you also recalibrate the lower range of the carb, about 15% leaner, to make up the difference. This does not mean "change the main jet", because that won't work: rather, adjust the range for 1/4 to 1/2 throttle mix. Not always an easy task... :?
 
I always thought that in order to get the best mileage, you want the engine turning as slowly as possible with the lowest vacuum (i.e. highest manifold pressure) possible. That's why overdrive trannies are so nice, they slow the engine down, reducing friction losses, and increase your manifold pressure, giving higher cylinder pressure and a more efficient burn. Your high vacuum/low pressure readings are great for low power settings, everyone knows that the less power you make, the less fuel you burn, but i'd think for mileage either stiff gears or a tall overdrive would be the best way to go. That and driving like Maude, as mentioned earlier.

here's a fun story, 6th grade my family went on a trip from northern MN/Lake of the Woods area to Duluth. 1988 buick LeSabre with the only driveline offered: 3800 v-6 and 440T4 transaxle. that tub with 4 people and all their stuff for a 3 or 4 day vacation got over 30mpg because my parents drive like old people.
I have a 1989 Bonneville SSE with 50,000 on the engine (using mobile1) that gets a best of about 28 when I drive like an old person, this is after putting in a K&N air filter and replacing the intake snorkel with 3" aluminum flexy pipe for much cold air. usually i see 26. same body, same driveline, however, my car has 215/60R16 tires, and the old Buick had 185/75R14s. interesting, no?

Nice to be back online, been squatting for two months.

Ern
 
The GM 3800 is one hell of an efficient engine. The wife's 98 Bonneville will easily knock down 30-32 mpg on the road at sustained speeds of 75-80 mph.
 
There's a danger with putting overdrives in, without checking tire size and ratios. The 300 max torque is around 1800 rpm, which is probably where you'll be making the most efficient burn. You put an overdrive in, and lower it below that, you might be lugging the engine in normal driving conditions, in which you will be using more gas because the throttle will be open wider!

You need to do some calculations on vehicle weight, wind resistance, gearing, tire size, and grade and get a good average of where you want the rpms, and how to gear for it. If you live in Kansas you probably would want to calculate for level ground, in the Rockies, maybe not so much. My father once told me a story which is probably a myth, but when he was younger cars that where sold in the west, on the plains, were higher geared than cars sold in the east with the hills....
 
I can't believe no one has mentioned timing! Ignition timing has a BIG effect on gas mileage. Cam timing does too if you want to get technical about it.
Vacuum readings doesn't reflect most power per-se, but most efficiency! Efficiency=power=gas mileage.
For whatever rpm you travel at the majority of the time, strive for the highest vacuum reading at that rpm. You may even go to a steeper rear ratio and get BETTER gas mileage if it raises your vacuum reading and eases the strain on your motor.
I noticed your vacuum was dropping at the higher speed. You're moving out of your powerband then or your jetting is off. But I don't know your driving habits or rpms you travel at most of the time.
I'm blueprinting a 144(smaller engine=less gas) and planning on a roller cam along with roller rockers. I'm already running a SROD and I put in 108 miles a day round trip just going to work. Right now, I have a 200 in the car and I'm getting around 30mpg at the least. With gas already at 2 dollars a gallon, I'm even looking at injecting my 144.
 
Hmmmm.... 30 mpg with a 200 :) What rpm's are you turning at cruise with the SROD? You may convince me to start looking for an early Falcon 8)
Joe
 
30 mpg is impressive, but you might need to make sure the speedometer driven gear is correct for the transmission speedo drive gear, tire size and rear axle.
 
Howdy Dragrcr62 and All:

Hey Dragrcr, what have you done to the 200 to get 30 MPG. I don't doubt it, I just would like to know the details.

Also I'd like to know the details of putting a roller cam in a 144 block.

I hope to hear from you soon as I'm about to leave for a 3 week trip to the Puget Sound Area. I hope Mt St Helens doesn't blow!

Adios, David
 
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