An Engineer's Tips on Fuel Economy

by Tom Wagner, Jr.


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I have read many articles on "fuel saving" tips. Most are reminders of the same, obvious things. Many typical pointers are not practical with a return to higher (70 mph) freeway speed limits, like driving 55 mph. Having an automotive engineering background and a love for driving, I now offer a few tips of my own. I don't claim to be an expert, nor do I seek the "ideal", perfect fuel economy. I am satisfied with meeting or exceeding the EPA CAFÉ highway mpg estimates with cars well above 150,000 miles. I typically drive 20K-35K miles per year on my commuting, and another 15K-25K miles per year on the family car. This is mostly highway miles, as the EPA defines them.

On typical, 2-lanes (per side) freeways, your realistic (courteous driver) choices are often limited to:

  1. Driving 80 mph in the fast lane or else tailgating traffic backs up behind you - Or

  2. Driving in the right lane, which allows the cars to pass in the proper lane; However, you will inherently get stuck behind trucks, and be in the way of vehicles entering the freeway, thus constantly needing to vary your speed (between 50-70 mph). You will often be unable to get out from behind a slow car or truck, as a wall of cars, 1/2 mile long, whiz by you in the fast lane, filling every possible spot. Both of the above situations waste fuel. Driving slower than 65 on a freeway causes me to be the roadblock, which I see as being rude, so I don't propose that here. For the "fast lane", I find 75 mph to be workable, in most cases, for cruising, but of course moving over, as needed, to allow faster drivers through. I am sharing a few tips to minimize fuel use under those conditions.

Tip #1. Know your vehicle. From sub-compact up through mid-sized cars, many 3-speed automatic transmissions (and 5-speed manuals) typically involve engine revolutions of 3,000 RPM @ 70 mph. Even without a tachometer, I can tell by the whining above 75 mph in my 1993 Dodge Spirit (3-speed auto-trans with 2.5L 4-cylinder engine). The EPA CAFÉ mpg estimates = 23 City /28 highway. I can usually achieve 28-30 mpg if I stay below 78mph. I haven't been able to drive less than 70 and maintain my desired road space or avoid a lineup of tailgaters behind me. When properly tuned up (spark plugs, distributor cap & rotor, etc), this car achieves this fuel efficiency even while currently at 173,000 miles.

For most V-6 and V-8 engines, a 4-speed automatic transmission (or 5-6 speed manual) has a cruising, overdrive gear. (Example: My 1994 Dodge Intrepid, which revs 2,200 rpm @ 75mph). Shifting in and out of overdrive is more fuel consuming than cruising at 80 mph. This was true for a Dodge Ram Van I had, and also for the new family Durango. The "ideal" cruising speeds for those vehicles are between 60 and 77 mph (not counting wind effects). See Tip #2 for more about wind effects.

Tip #1A. Know your vehicle's fuel brand preferences: For my Dodge Fleet (Spirit, Intrepid, and Durango), this is Marathon, Mobil, and Amoco. BP seems to work fine as well. I have not had good results with Shell gas with Chryslers. I stay away from cheaper generics as well.

Tip #2. Pay attention to the wind velocity (direction and speed). Think simple physics here: Driving 65 in a 20mph headwind = the car fighting an 85mph effective headwind. In a tail-wind, the opposite is true. 80 mph with 20 mph tailwind = 60 mph effective headwind. The one difference I am eager to quantify is the effective difference in engine rpm at 85 mph vs. 60 mph. But, since vehicle frontal area is by far the biggest factor for power consumed at highway speeds, I'll ignore the other factors. Tail winds are definitely fuel savers. You don't need fancy wind measuring devices; again, use common sense. Your tachometer (if you have one), ears, and feel of the accelerator will tell you if your car is straining or comfortably cruising at a given speed. Above all, try not to drive any faster than needed while bucking a headwind. This wind effect is even more noticeable when towing a full height, travel trailer. Every car has its' own "sweet" cruising speed, often close to the maximum engine output torque.

Tip #3. Wash/wax your car before a long trip. Even the $5 wash/wax special at your local neighborhood auto-wash can do wonders for several hundred miles of highway cruising. For a '96 Dodge Ram Van on a 1,600 mile trip, we noticed an increase from 15 to 16 mpg (7%) by doing this. Using Rain-X windshield washer solvent may help as well. I like it for its water beading performance.

Tip #4. Use cruise control only on relatively flat areas. It is wonderful for freeway cruising sometimes, (and fine for most rural, freeway overpasses), but in hilly areas, I do not use it. It tends to push a car full throttle uphill, which sucks in extra gas trying to maintain the set speed, and will often cause a downshifting out of overdrive. Instead, I ease off the gas just before an abrupt hill crest. This also helps me be prepared for anything unexpected that I can't see until I reach the top.

If traffic is clear around you, it is fun to coast or accelerate slightly downhill, through the valley, and see how far the coasting momentum carries you up the next hill. In a car with 4th gear overdrive, it is best for the transmission not to be shifting back and forth, in and out of overdrive anyway. Premature wear, transmission repairs are costly, needless to say. If I am turning off a road at the bottom of a hill, I turn off overdrive before descending to save brake wear.

Tip #5. Know the roads and traffic lights on your normal commuting paths. Yes, I know this is basic. Roads with predictably timed lights are easier to manage. Some roads with close packed traffic lights (every 1/4 mile, etc.) can take 5 minutes or more per mile if you miss a particular one. Approaching a yellow light while driving over 50 mph can be dangerous and wear-out brakes. It also seems that stopping at an "almost red" yellow light causes many surprises, rear-ending collisions, as most people here in Michigan, fly through them. Including at least one after the red light is on. Noting timing of particular traffic lights, times of day, etc. can help minimize those urges to "blow ahead of the pack" and get caught by the next red light. I actually count blinks of certain "don't walk" signs when I see the transition from "walk", which helps me anticipate the "yellow light" timing of those intersections. I really like to avoid sailing through intersections during that yellow to red light transition, especially fast.

Back to freeways: After sitting in bumper to bumper traffic, a sudden clear zone seems to mysteriously propel cars (on its own) up to 70 mph, only to bring them to a screeching halt 1/2 mile later. Looking and thinking ahead can help steady the traffic flow and avoid this panic, chain-reaction. On the other hand, driving too slow can cause others to impatiently dart around you, which is again dangerous. In traffic jams (10 mph and slower), I try to minimize the constant "stop and go" (like a slinky) pattern by driving a steady, intermediate pace that will help develop steady traffic flow behind me and minimize brake use for all of us. I also move over to the right edge occasionally to let drivers behind me see the traffic jam ahead, proving that I'm not the cause.

Tip #6. The jury is out on oil additives and engine treatments, but I have had good results with Slick-50 engine-treatment. They recommend adding a 32-oz bottle in place of one quart of oil every 50,000 miles. Instead, I add 8oz every 12,000 miles (once every three oil changes). It's simpler and a keeps more steady amount, I think.

For new cars, I usually don't start doing this until at least 20,000 miles. I like to get a good mpg baseline first. Slick-50 doesn't seem to hurt, and where it is really designed to help is during cold engine starts (before the oil is pumped at full pressure to the engine). For motor oil, I typically use Castrol, Pennzoil, or Valvoline 10w/30. I have used Slick-50 and an equivalent (Protec) in the 1990's on a Chevy Cavalier, with similar mileage. It still achieved the EPA rated 32 mpg at 172,000 miles.

A good way to determine if such an additive (or any engine related part replacement) helps, is to collect mpg data over a period of time (say, a season of work commuting under similar circumstances), then plot it as a Bell curve- Normal Distribution. Actually, just knowing the average and standard deviation will do. A plot is not needed. I use +/- 2 Standard Deviations (Sigma), not 3, since +/-2 covers 95% of data. For example, my Dodge Spirit (last summer for three months of work commuting) achieved 28.4 average mpg with a standard deviation of 0.86.

With a good numerical mpg baseline, change the part in question (add the oil additive, etc) and collect more mpg data points. If the numbers don't show any difference under similar driving conditions, sometimes just a smoother sounding engine is convincing enough of success.

Best wishes to all, and please read all the more standard pointers. Also, proper tire inflation is actually very important. Happy trails and remember that a safe arrival is still the most important goal of driving. Strategic positioning in traffic (and on freeways) can help increase reaction time, which can help avoid chain-reaction type accidents.


You can contact Tom Wagner, Jr. at wagnert@tacom.army.mil or tgwagnerjr@hotmail.com. He is interested in hearing any special tips to maximize mpg fuel efficiency for towing a 24-foot, 4,000 lb. travel trailer with a van or mid-sized SUV. 10 mpg seems to be the theoretical limit, with 7-8 being more typical.

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