cgettel

My $0.02 after a couple of months of working very hard on fuel economy in two vehicles - one a GMC pickup with the 8/4 cylinder engine, 4-speed auto, and more recently a 2006 Forester X, automatic. The only mods I've made were to switch to a K&N air filter, and run tires at 42 PSI. When I traded in the truck I was running a fraction over 19 MPG overall, with about 75% in-town driving. Given a full-size pickup with an 8 cylinder engine driven mostly around town, that's pretty good. Right now, I'm running about 28 MPG in the Forester with the same kind of driving, and slowly learning how to increase it. The truck had an instant MPG readout, and I found I couldn't live without it so I got a ScanGuage II for the Forester. Very helpful! It's taught me some things that are counterintuitive to say the least, and always interesting.

One of them is that different vehicles need to be drivien quite differently in order to approach their potential best fuel economy. Another is that we're not necessarily looking for max efficiency of fuel energy-to-propulsion at any moment but *overall*, and that matters. One of the ways it matters is that the guideline then becomes "moderation in all things". The learning curve is long, not so much in the general principals but in exactly how to apply them at any moment.

The questoin about how much acceleration is a case in point. I've tried it both ways in both vehicles, and I can say that in both of them, slow easy acceleration results in significantly lower MPG, and hard acceleration does too. I think it's very simple - in general as long as the engine's running, the slower you go the lower the gas mileage. If you stay at low speeds, the lower the MPG and the more gas is used per unit increase in speed. This has to do with RPMs so it'll make a difference whether it's stick or auto, but the principal is the same. Watching the ScanGuage makes this very clear. In general, I think hard acceleration will result in a little better gas mileage than creeping, as long as you don't floor it out of a dead stop. But moderate acceleration is usually the best combination of staying in low MPG the shortest amount of time and not letting the MPG get too low at any point from accelerating hard.

Accelerating uses a LOT of gas. Accelerating includes not only pulling away from a stop, but any increase in speed - of course. It also includes going uphill. Even a rise so slight as to be unoticeable will drop the MPG quie noticeably - 2-3 or more MPG. I can tell if a stretch of road which seems flat and level is rising or not by watching the MPG readout, as it's quite sensitive. It doesn't matter so much how strongly you're accelerating, just that you are.

At the same time, dumping the gas wide open at any speed seems not to get as much gain in speed as there is drop in MPG. Therefore morderation is key. The ScanGuage has a readout for throttle position and engine load (the amount of power being used compared to what's available at that point). Accelerating at around 65%-75% load seems to be the sweet spot for the Forester, but I'm still playing with that, and haven't done any specific testing. This is what I would call "moderate" acceleration, not "brisk", not "easy". It's less than you'd think. But 75% load at 50 MPH is a lot stronger acceleration than 75% load at 30 MPH - at higher RPMs there's more torque available. The message is don't just slam your foot down, accelerate your acceleration so that pumping loss and so-on are less of an influence. You can start to increase your throttle and rate of acceleration once you get up to about 2000+ RPM while still maintaining the same load percentage and roughly the same MPG.

One thing that was very counter-intuitive was to find out that higher RPMs doesn't necessarily mean lower MPG. When pulling up a pretty significant hill, I was able to improve the MPG 2-3 points by downshifting. This raises the RPM, increasing torque, and so one is able to back off the accelerator quite a bit, and maintain the same speed. Equating uphill to accelerating going down the road, staying in a lower gear longer may improve your azcceleration rate, thus get you into high gear and high MPG sooner.

Someone asked about torque and horsepower. The way I understand it is that torque is what accelerates you, while power has to do with how much ground you cover in a period of time. Like MPG and GPH, they're clearly related in actual practice, particularly when driving an automatic with less control of RPM.

I'm learning how to play the terrain. One feels like maintaining a steady speed is the best way to go, but this means accelerating for every uphill, in effect. You want to arrange things so that you accelerate strongly downhill, where you have gravity's help, and let speed bleed off on the way uphill. This way the acceleration is short, and you spend the least amount of time in lower MPGs, and you're not accelerating uphill just to maintain speed. Avoid acceleration as much as possible, but when you do, use gravity if possible, otherwise get it over with as soon as possible wihtout flooring it or downshifting more than one gear.

The so-called P&G - Pulse and Glide - technique is indeed a wonderful way to improve gas mileage. Here in New England I use the terrain to do this. As a rule, I acelerate up to something a bit over the speed limit at the very start of even a slight downhill, then pop it into neutral, or back way off on the gas, and coast back down to some speed that's the lowest I'll accept given conditions, traffic and how much of a hurry I'm in. Sometimes I'll put the Cruise Control at this min speed, ortherwise I almost never use it any more. Sometimes if I have a lot of speed to make back up, I'll accelerate even up to 99% load (the max readout available - I assume it's really 100%) but for a very short time. The MPG drops like a stone - down to 4-5 MPG - but only for a couple of seconds, and then one can glide in neutral or foot off for a long time with gas mileage in the 70's, 80's and even well over 100 MPG if at highway speed. Where traffic is not in the way, I found I had some mental adjusting to do to allow my speed to vary as much as it needs to.

I turn the engine off if I catch a stoplight that's going to be more than roughly 10 secs. long. Sitting still, your gas mileage is zero - and you can sit there and literally watch your average MPG number tick down. Very discouraging. Other than that I don't ever turn the engine off. Coasting in neutral with the MPG at 70-100+ MPG is good enough, thankyouvery much, and I usually do it as much as possible, even if it's only a few seconds. It's nice to see the ScanGuage reading 0.25 gallons per hour, too. Again, that's good enough for me.

Of course, what we're after is not necessarily lower MPG, it's using the least amount of gas possible to get where we need to go. This is about GPH as much as MPG. However, MPG is definitely time-related, too. And it's not just the number, it's how long you keep it up. They are not independant numbers, but MPG is a lot easier to understand and work with while still at the start of the learning curve. Soon I'll start comparing different driving methods and speeds over the same course to see how or if total gas used varies, and if there's any usable correlation with instant MPG going down the road. For example, I recently found out that going to the grocery store across town via a 4-lane divided highway over a big hill used more gas than going through town with traffic, stop signs and stoplights. Go figure. I think it's because the in-town route offers far more opportunities for P&G driving and being able to stay longer at more efficient speeds, and even though there's a long coast at 130 MPG down the hill on the highway, it doesn't make up for the grind uphill at 50-55MPH at 12-14 MPG and a high GPH figure. This is the opposite result, by the way, from my GMC truck.

I've also been able to corroborate that the most efficient speed for my Forester is about 40-45 MPH. I think of it as a range from about 37 - 47 MPH. Above and below that the MPG drops at least 2-3 points. On a recent 160+ mile round trip on 2-lane country roads and small towns, I ran 31 MPG overall. I think it would have been better if I hadn't felt like I needed to maintain 50-55 MPH or so on the more major roads. I personally feel like it gets very risky the more one deviates from traffic flow, so I feel constrained not to move either too slow or two fast on major highways where one becomes a major hazard if speed departs from general traffic flow by more than a few MPH. An accident would be a far greater disruption to my day than the loss of a potential mile per gallon.

In general I've been getting almost 400 miles per tank - a little less.

Sorry for the lengthy post. I thought it'd be better to have it all in one place than piecemeal in reply to others' points.
Regards -