This is something I've been pondering for a while and thought I would throw it around. There is no question that if you are hard on the throttle all of the time, your gas mileage will go down. But what about "normal" acceleration? I'm usually pretty light on the throttle when accelerating, but I was wondering if I hit the gas a little harder, like between 1/3 to 1/2 throttle to get up to speed, if I could improve my gas mileage. My idea is that although I would be accelerating quicker and using more gas, I can back off the much throttle quicker because I will hit cruising speed faster. There is no doubt that the throttle setting required to maintain cruising speed is much less than what is required to reach that speed. There has to be a limit where too little throttle during acceleration will give you poorer mileage. The reason for this is that your engine is working at lower rpms where it is not as fuel efficient (where you are lugging it) and you are also spending too much time in less efficient lower gears.

This theory also applies to the drive versus overdrive issue. I've noticed especially when towing my pop-up camper that if I have it in O/D and hit a slight grade, I have to press the throttle much harder and will loose speed. If I kick it out of O/D, I will require much less throttle but will maintain my speed.

The issue here comes down to the amount of fuel used over a period of time and distance. In the higher gear, the truck has a lower rpm, but a higher fuel delivery rate due to the increased throttle position. It's just the opposite when you kick it out of O/D. One of the two will give you better mileage.

This would be an interesting test if you have an overhead computer, which I don't have. Drive at a speed slightly higher than where your truck kicks into overdrive. Watch you fuel mileage on the computer. Then without moving the accelerator, shift the truck out of overdrive and see if the fuel mileage changes and check your speed. The speed should go down, but it might not if your truck is heavily loaded or if you are going up an incline.

I was always under the impression that it is best to run in the highest gear possible. But I am starting to rethink that idea now. I live in an area with a lot of rolling hills. I've noticed how much harder I've had to press the accelerator in the higher gears and how much longer it takes for my truck to get up to speed when it shifts into the higher gears too quickly. It's not an issue with my light car, but I notice it more with the heavy truck, plus I have the ability to control whether or not it goes into O/D. Lately, I've been keeping the O/D locked out until I can get to 55 mph or better on level roads. Just wondering if anyone else has some opinions of facts on this matter.

 

It may help to get your truck dyno'd to find out at which rpm your peak torque occurs. The more time you spend at or around that rpm, the better your gas mileage will be. It is an interesting point that you make about accelleration. In racing around town, I don't think that just stomping it off the line is what costs mpg. I think you lose mpg when you stomp it and leave it stomped. Your best mpg may occur when you get up to peak torque as quickly as possible, and let the gears 'hover' around that rpm. I have done some calculations lately (just looking at dyno models to try and guess where mods have moved the torque curve). My truck is in the shop right now getting new gears. I decided that with 4.10's I will get better mpg. If I slow down ~4-5 mph on the highway from what I usually drive, I will be around max torque. Around town, The decreased work load in conjuction with gearing up to max torque should improve mpg. In theory........

 

Good point about keeping the truck at peak torque. That's the point I was trying to get at but I didn't describe it well. Some other things that brought me to this theory is that my father-in-law just got a new 2006 Carvan that has the overhead computer. He said that he gets better mileage at 55- 60 mph than at 45 mph, and it will only drop off slightly when he goes to higher speeds. Again, it could be because the engine is at a higher but more efficient speed.

I towed my camper from central PA to Washington D.C. about a month ago. It was all highway miles, but hilly and I was following my father-in-law, actually tailgating him since I didn't know the route, and on and off the throttle quite a bit. I ran most of the time with the O/D off at speeds between 60 to 70 mph. When we got back I filled the tank and was surprised that I got 13.35 mpg. Around home I am getting around 12.5 mpg travelling on back country roads with average speed of 40 - 55 mph. But until recently I've been running with the O/D on. So I think I am going to leave it off and see if my mileage comes up, plus try acting more like a kid again and getting up to speed a little quicker.

Also, I've thought about gearing, but can't justify putting that much into that modificaiton. When I was looking at buying the truck, I was happy that it had the 3.55 gears because I thought it would give me better mileage. It probably does on the highway, but since a lot of my driving is at lower speeds with a fair number of stops and starts at stop signs, I might have been better off with 4.10' or other gears.

 

The V10 has a 2000 rpm torque peak, from 1500-3500 rpm I definitely get better mpg at the lower end of the torque peak- 15.2mpg at 55mph in OD vs 11mpg driving 75-80 freeway speeds (also in OD)

 

higher revs will cost you more fuel, there's no question about that. your best fuel mileage comes at the lowest rpm possible without lugging the engine.
just a couple quick examples... take for instance a one cylinder FI engine, for simplicity's sake. at 2000 rpms, you are injecting fuel into it 1000 times a minute. rev it up to 6000 rpm's, now you're pumping fuel in 3000 times a minute. not ony that, remember that engines run best at a certain air/fuel mixture (13.7 to 1 or somethin). when you hit the gas, you are allowing more air into the motor. the computer sees this, and injects more fuel to keep the mixture correct. so now you're injecting fuel more often, and you are injecting more fuel per shot.

take for example a viper or a 6 speed corvette. the 6th gear ratio for both of those is incredibly high (low numerically). they can do over 80 mph and still be around 1800 rpm's. the 6th gear is not used for power or passing, but just to keep the engine turning as slowly as possible to save fuel. look at their mpg ratings. (i know the epa estimates suck, but its still comparable) the vette gets in the mid teens in the city, when you are revving the engine and losing fuel quicker. but people have gotten over 30 mpg's on the highway in them, by staying in the very fuel efficent final gear.

and i used to have an automatic '98 corvette, which had an instant mpg gauge, to show me what i was getting at every moment. the lower the revs, the higher the mpg's.
and going slower also improves your mpg's if you have the opportunity to drive 40-50 mph, as opposed to 60-70, do it. not only is your engine turning slower, you are also cutting wind resistance (and in a brick like a full size pickup, everything helps).

 

Drag coefficient and weight play a big part. I was playing around with a fuel mileage predictor program. At 75 mph (using my truck as the model) there was virtually no difference in mpg between 4.10's and 3.55's, but, if I lowered the weight or drag coefficient signifigantly, the 3.55's were better. The software would not let me play w/ the torque curve or engine tune as I downloaded the trial version. I think that most of the mods you do to make the motor breath better raise the torque curve. At 75 mph, I was tached at just below 2k. I think that the 2.3k the 4.10's will tach is much closer to my peak torque (especially with the M1). I was having to use alot of throttle to maintain anything 2k or less and the tranny was downshifting on grades that would be considered way less than steep. When it comes a vette and a truck, there is really no way to compare what gears or rpms will do to mpg unless you strap a piece of plywood to the grill of the car and throw 40 bags of concrete in it.

 

That's what I think is important when considering whether or not to use O/D. Sure, running down a level highway you're better off in overdrive. It's when you start putting a load on the motor such as encountering a hill that gearing makes a difference on fuel economy. That's why I suggested flipping the O/D off on a hill and seeing if the mpg changes on the computer.

Another thing I thought about was hooking a digital multi-meter into the TPS and observing what the readout is traveling down the road. I'm not going to do this, but it would be interesting to observe how much harder you push the on the gas to maintain speed on the same hill the the O/D on and off. One thing I have done is to keep my foot steady on the throttle and watch how much my speed changes when going up and down hills. It's interesting to do as long as no one is behind you. Since I travel the same way to work every day, I have been playing around with the O/D on and off. What I am finding is that I use less throttle on the hills with the O/D off. But again, does this mean better fuel mileage?

Mopowar - You mentioned making the truck breathe better raises the torque curve. Do you mean the torque peak occurs at a higher rpm with an increase in torque or that the whole curve is shifted straight up with the peak torque occuring at the same rpm? To my way of thinking, anything you could do to the engine to cause an increase in torque at a lower rpm would help mpg. I may be wrong on that, but that is why I want to discuss this issue. I'm here to learn and share ideas.

 

From the stuff I looked at, I think that the mods I have done moved the curve over (more rpms) and increased torque some while maintainig the same characteristics (shape). I am only guessing though. As you can imagine, I could not find a dyno on a truck with the exact mods that I have so its all guess work at this point. I was able to figure out that things like the M1, 2.02's, and exhaust move the curve higher into the rpm band. IMO The most straight forward way to increase low end torque is through the pcm. I am pretty sure that in 99 they had not started the heavy duty torque management that the pcms have now. From the research I did, they started putting that stuff on in the later half of the 2000 production year, but there is probably some to be had there. If you want to know for sure, you could call B&G Chrysler. I am pretty sure the guy there would know. I have read that 1.7rr's will yield some low end torque, but I cannot say if this is true or not. I did heads, rr's, double roller chain, and headers at the same time.

 

Here is the link for the fuel economy calculator demo:
http://www.performancetrends.com/dow...CalculatorDemo

If you need help with any of the parameters, i.e tranny ratios, let me know.

 

Looks like some interesting programs, thanks for the link.