Alright guys, here's the deal. My 1980 F100, with 302/C4 and approximately 2.7x gears, sucks in the literal sense right now. On my last fill-up I put a little over 16 gallons in and now after only 160 or so miles, it's back to 1/4 tank where it was when I filled her up. Now I know there's no way such a light vehicle with a considerable power plant should be making only 10mpg. Any tips from some more seasoned drivers to help me boost this a bit? I've heard that adding dual exhaust will increase power and mpg by a bit. Now this truck has no catalytic converter, and I notice if I remove the muffler and just run a single straight pipe, I can get up to 15 or a little better, but how much difference would duals make? (Please note I was running with a muffler this last run, and I can immediately tell a difference in fuel consumption)

Another thing, I do most of my driving on the highway, and this thing has plenty of power for as old and as many miles as it has, but running at highway speeds the RPM's climb towards the redline which is about 4200 on my truck, what can I do to help this? Usually doing between 50-55 on the highway isn't too bad, getting up to about 2100 RPM's, but running 70+ on the interstate really burns through the gas quick. This thing tops out at 83 MPH just to give you an idea. I was thinking about adding a 5 speed in place of the C4. Any thoughts or suggestions? Thanks in advance.

 

I see some mis-information in your post(things don't add up).

1. If you had 2.xx gears, you would not be turning those high rpms. Either your judgement in rpms is wrong, or your assumption of your rear gear ratio is wrong.

2. You can't judge fuel mileage by the fuel gauge. These fuel level systems in these trucks are notorious for being off, just be glad yours even works, most need a new sending unit eventually. Most all will not read evenly down the scale, even when they are new. Just because it says you have 1/4 tank, don't believe it. You should check your mileage by topping off the tank at the same pump if possible, drive it till you get to 1/4 tank on the fuel gauge, and then go top it off again, and record the gallons and the miles. You can then figure your fuel mileage.

3 Taking the muffler off is not going to give you 5 more mpg gallon, unless there is something wrong with the muffler.

 

Dave is spot-on with everything he said. I can tell you that with a C6, 3.50 gears, and 30 x 9.50 tires gives 2900 RPM at a true 65. The C6 and C4 will slip essentially the same, so discount that variable.

And, the above combination with a recently rebuilt 351W got 13 MPG on the highway at 60 - 62 MPH. Everything was fresh, including plugs and carb, and its timing was at 6 BTDC.

As for the exhaust, again Dave is absolutely correct. I changed the exhaust on my pickup from single with cat to dual w/o cat and did a TON of porting work on the exhaust ports in the heads and got no appreciable change in gas mileage. NONE. But, I did get a very significant improvement in the ability to wind the engine with the same 2 bbl carb and stock intake, as that rascal would RUN.

So, your 10 MPG may not be as bad as you think. Yes, a 302 should do better than a 351 all else being equal, but if you are running 70+ on the interstate you won't get even the 13 MPG that I got because the wind resistance of these trucks is basically that of a barn door. And, since drag squares with the speed rather than being linear, the drag on your truck at 75 is 1.56 times greater than at 60.

Given that, there may be nothing wrong with your truck. But, give us more to work with like good RPM readings at multiple speeds as well your tire size as that makes a huge difference. And, do as Dave says on filling the tank and average it over several fillings.

 

I have a factory tach to measure the RPM's, and connected it in my dad's truck to verify that it was reading correctly, and the readings were fairly equal. As far as the gear ratio goes, I did the old fashioned raise one wheel off the ground and turn it, and the drive shaft spun about 2 3/4 times with one revolution of the wheel. The tires are the same size as what came out from the factory.

As far as the fuel gauge, I did top it off at the last fill up, and the gauge read exactly 1/4 tank, so I figured if I let it get that low again it would give me some kind of idea, but I'll top it off again to verify.

Now, the exhaust system is all caddywhompus, as the previous owner replaced the pipe coming from the crossover with a straight piece of aluminum pipe, then connected it and the factory muffler with a short piece of flex pipe. I simply slid the flex pipe off and tied the muffler up to experiment with how it sounded, and actually noticed a full tank lasted just a bit longer, so I re-attached the muffler to see if I could spot a problem. Now, the flex pipe doesn't seal well, creating a small exhaust leak, and by putting my hand in front of the pipe, I noticed that the exhaust came back out with some force. Perhaps the muffler is too restricted?

And for the RPM range while driving, around 25-35 MPH in town, it pulls around 1500, speeds from 50-60 on the highway pull between about 1900 and 2300, and speeds 70 and over pull 2900 or more. By the time it hits 80, it's pulling at least 3300 and doesn't seem to have any more "oomph" left in it.

 

These things were designed to run at the national speed limit of 55 MPH.

Add to that, a truck is designed for low-end torque - getting heavy weights moving - not high-end horsepower and top speed.

Things like speed, tire size, RPM, etc. can all be found with mathematical calculations:

Automotive Calculators offer to you by Ford Trucks Enthusiasts | www.ford-trucks.com

 

Your RPM is consistent with what I had with 3.50 gears, assuming that your speedo is reading a bit high, which is normal. At most you have 3.25 gears, but not 2.75's.

And driving the truck the way it sounds like you do you won't see much better than 10 MPG. The only way to get a good reading that will compare to what others of us get is to drive a distance of something like 100 miles at a consistent and reasonable speed of maybe 60 MPH, with no hard acceleration. Then we have a comparable test. As it is your driving is completely different and that makes any comparison null and void.

 

If you are going to do it that way, you have to lift both wheels off the ground, and get someone to help you turn both rear tires the same amount. That will give you a accurate reading. If the other guys are correct, you will get about 3 1/2 turns out of the driveshaft with one turn of the rear wheels. The 3.50 or 3.55 rear gear ratio is a very common rearend ratio in these trucks.

Top it off, then get in the truck and write down the odometer reading on the speedo. Drive it till you get 1/4 tank or so. During this time it's up to you a little bit on what you get. You can baby it along and see what you can squeeze out of it, or you can floor it and run 70+ down the interstate, and see what you get. Or you can drive it normally, it's your choice. When you go to fill it back up, when the nozzle clicks off, write down how many gallons it took. Then go in and write the odometer reading down again.

Take your last odometer reading, subtract it from your first odo reading, that will be how many miles you drove. Take this number and divide it by the gallons number you got off the pump on the second fill up. That's your miles/gallon number you are looking for, this takes the fuel gauge out of the equation.

P.S. You don't have to wait to burn the tank down to 1/4, you can use 1/2 tank or whatever. But the longer you wait and the more fuel you burn, the more accurate the calculation will be. And you can do this routine several times also, and you will find your calculation might vary + or - a gallon or so.

 

Just for comparison, I took my truck out for the first time yesterday. I filled both tanks ($100 later) and drove it to Sioux Falls, SD. It is approximately 100 miles round trip. I put in just over 10 gallons. Calculating fuel consumption in warming up I figure I got just over 10mpg. This is running at around 60mph at 2500 rpm. Keep in mind I have an 8700lb truck and a 460 under the hood so I'm very happy with those results. I hope this goes up a bit when I get the new carb on Thursday.

 

The only reason I suggest a 60ish MPH cruise of 100 miles or so is to get things on the same basis as others of us, or at least me. That's the way mine was tested. Any other driving style will negate the comparison, although at 70+ and blasts to 80 means 10 MPG is probably about right in comparison to my 13 at 60ish.

 

8700 lbs is running very heavy. I'd guesstimate 2500+ lbs in the bed or trailer. 10 MPG is not great even with that amount of load.

My lifted '86 Supercab weighs in at ~6000 lbs with 400+ HP 460 it will manage 10.8 MPG towing a 5500 lbs trailer at 65-70 MPH and unloaded it'll do 12.5+ MPG at 70-75 MPH. This is turning ~2000 RPM at 70 MPH.

OP, something is very wrong with your MPG. Get real numbers and post back with them. MPG = miles / gallons. Save the measurement for a highway trip where you can do a steady 65-75 MPH.

Either your muffler is plugged or your measurements are wrong. For a stock 302 through single exhaust muffler or no muffler will make no significant MPG difference. To increase MPG eliminate EGR and other smog stuff if it has it. Set initial timing to 12*-14* and total to 36*. Set vacuum advance to manifold vacuum. after doing this lean out your main jets as much as you can until you get a lean stumble at steady cruise at any speed, then go up two sizes. This will give you a good start for better MPG and performance.

 

You are turning 2000 at 70 mph, he is turning 2500 at 60 mph. Gearing is obviously a big difference between your truck and his. When I went to overdrive in my truck, I gained 3 mpg.