I have a suspicion you are going to be disappointed in the little to no improvement after lots of work. First of all, axle gears can be very time-consuming and frustrating to set up right to get the correct tooth contact pattern. The wrong pattern can cause noise when cruising, when coasting, or both.
Second, the bigger issue is if a lower gear ratio would make a noticeable improvement, why not go to a 1:1 ratio? Sounds silly, but I ran into a similar problem years ago with my 1972 Dart. It had a 2:72 axle that developed gear noise. It also had the smallest physical axle Chrysler offered, and it only came in the Darts and Dusters. I switched to a larger axle assembly with 3:23 gears and found that while my acceleration increased a pile, the gas mileage stayed exactly the same. I was making a two-hour trip at highway speed back and forth to college every week, so I had a good opportunity to check fuel mileage. I did improve mileage later by playing with a spring and jets in the carburetor, but you already have the best you can get in that respect with fuel injection. After giving this some thought, it occurred to me that while the higher gear ratio made the engine run faster, I was not pushing the accelerator as far to keep the car speed up. Think of peddling a ten-speed bicycle up a steep hill. You will get less tired and use less energy in first gear than if you tried to do that in tenth gear, even though your legs are going too fast to see, like the wings of a hummingbird. With a lower gear ratio in your truck, the engine will have to work extra hard to get up to speed. It will need to develop the same horsepower to keep it cruising at a steady speed, regardless how fast the engine is running.
Back in the mid to late 1970's, a lot of my friends and I were driving cars with 318's, and we were all getting around twelve miles per gallon. Someone smarter than us pointed out these were all high-mileage engines and the likely cause of the poor fuel mileage was a stretched timing chain. That would cause camshaft and valve timing to be late. If you remember "L" for "loose", "L" for "late", and "L" for "low", late valve timing causes a big increase in low-end torque, and the trade-off is a decrease in high-end torque and fuel mileage. You could buy a special offset key for the camshaft sprocket to advance the camshaft. In effect, that would be the same as having an overly-tight timing chain. Remember "T" for "tight", "T" for "top", and "T" for, ... Uhm, ... "advanced", a tight timing chain gets you more top-end torque when the camshaft is advanced.
The next thing I did was buy a special "gas mileage" camshaft for my 318. I do not remember exactly how much the fuel mileage improved, but I found out it was just the same camshaft they put in motor homes. It was also advertised as improving low-end torque, and in fact, now I was able to squeal the tires all over the place. My instructor had two motor homes built on Dodge truck chassis, and one of them had a 318. He said they used that slightly-retarded camshaft to increase low-end torque for good power when pulling away from stop signs. The trade-off is you needed to have a really good plan if you wanted to pull out and pass someone. One of the clues to a stretched timing chain is you will have lots of tire-squealing power on the low end.
For the other half of this wondrous story, I had a 1977 Monaco ex-police car with a 440 c.I. Engine, and the camshaft in that engine was advanced from the factory. This car went from 0 to 60 mph like any other 318, then it came alive and went from 60 to 90 so fast, it just about tore the seats off the hinges! That top-end torque was what they wanted for highway patrol cars. I used both of those cars on the racetrack for amateur racing where they had to be street-legal, and to get there, the fuel mileage was still quite respectable for such a big engine.
There is more to this part of the story because the increase in low-end torque does not agree with the increase in fuel mileage when the camshaft is late. There has to be some other modifications to the cam lobes besides simply retarding them.
To boil this all down, rather than looking at the rear axle, look at the timing chain first, then consider changing the cam timing. When they used a key between the camshaft and sprocket, you could buy an offset key of 0.5 degree, one degree, or two degrees, and you could install it either way to advance or retard cam timing. Two degrees is a lot and is not desirable, but I cannot remember how that affected engine performance. GM engines used a dowel pin between the camshaft and sprocket. For those engines, a special sprocket was available that had three dowel holes in it. One was for no change over factory settings, one advanced cam timing, and one retarded it.
Also, do not overlook a dragging brake as a cause of low fuel mileage. Stop on a slight incline, shift to neutral, release the brakes, then see if the truck creeps downhill on its own. If it does not, we need to look closer at the brakes.
The last thing is to be sure the transmission is shifting to the highest gear and that the torque converter is locking up. If you have a tachometer, you should see engine speed drop 200 rpm when the engine is up to normal temperature, you are in third or fourth gear, and you are up to around thirty five mph. If you cannot tell if that lock-up occurred, cruise at around forty five mph, hold the accelerator pedal perfectly steady, then lightly tap the brake pedal with your left foot. As soon as you do that, you should see engine speed increase by 200 rpm, then it will drop back down two to three seconds later. Along with the huge list of Chrysler's innovations that actually benefited car owners, they invented the lock-up torque converter and first used it with their big-block engines in 1977, and all their cars and trucks in 1978, to improve fuel mileage. Those were hydraulically-controlled. Yours is controlled electronically by the Engine Computer. The brake light switch is one of the inputs that tells the computer to disengage the lock-up feature, presumably in preparation for coming to a stop. If that part of the brake light switch is sticking, the lock-up clutch will not engage and fuel mileage will suffer. The cruise control will not engage either.
Friday, June 15th, 2018 AT 3:09 PM
