For my vehicles, a 2006 Ram QC SB w/Hemi 545RFE and a 2010 Journey SXT 3.5L 62TE, 60 mph seems to be the sweet spot. 60 mph allows the transmissions to get into top gear. The Hemi turns about 1700-1800 rpms while the 3.5L in the Journey runs at 1600 rpms. If I go any slower in the Ram, it'll drop into 4th but it's only a marginal difference. 5th has a ration of .67 while 4th is .75.
Our old '00 T&C Ltd 3.8L/41TE had optimal fuel mileage at around the same speed. The Ram has averaged 17.5 mpg in 177K miles with a personal best of 21.02 mpg - conditions for that run were cruising at 65 mph, no AC and a very light load. Best in the Journey was 26.9 mpg at 60-65 mph.
Every vehicle is going to be different, so you have to experiment. In general, the lowest speed that keeps you in top gear without lugging or downshifting, and high vacuum, is your optimal speed for gas mileage. But more than worrying about the best speed, the greatest effect you can have on improving gas mileage is from:
1) Minimizing idling - pick a low-traffic route with fewest traffic lights and stop signs.
2) Accelerate and decelerate slowly, coasting a lot and anticipating speed changes.
3) Keep the vehicle in maximum tune with tires properly inflated and aligned.
You can go a couple of psi higher in the front tires where the weight bias is, leave the rears at spec.
The lock-up converter applies in OD around 47 mph with the 41TE. 50-55 would be my guess. It would vary between cars, equipment and final drive ratios.
Rolling the windows up and using fresh air or A/C is more efficient for air drag at expressway speeds than with the windows down.
Don't become a traffic hazard at lowered speeds.
Min speed on the interstate is 40 but I always stay above that value & in the designated slow lane (right). If people rear-end me on the 10-lane-wide road, then they will be held at fault by the police & insurance company.
I disagree with point 2. My experience with my Honda is that staying in gears 1-4 makes its MPG meter drop. It's better to get to the most-efficient gear as soon as possible with strong acceleration (but not WOT).
Well, I don't mean lag forever in each gear. But "strong acceleration" is a sure way to use more gas. You will find it in every publication and recommendation in print, and 35 years of driving experience has proven it to me, too. Not many people have gotten 22 mpg highway out of a 66 Belvedere wagon with slant-6, or 24 mpg highway out of a 72 Dart with slant-6, or 40 mpg highway out of a 92 Daytona on a long trip. YMMV.
The truest measure of fuel economy is not mpg, but rather pounds of fuel per hour burned.
People used to drive with a vacuum gauge to train themselves to drive for best economy driving habits. Now we have ECO 'fuelminder' lights and fairly accurate digital average (cumulative) MPG readouts.
I think that the instant MPG readouts were discontinued because people were obsessing too much on them? Sitting at a stoplight was 0 mpg and coasting down a hill was 99 mpg.
Cruise control is nice for flat country, but it will downshift on hills too easily in order to maintain speed. I wish that it would allow a speed drop up hills and maybe a speed gain down hills. It can't anticipate hills. On hills, I take over the throttle manually.
Reasonable forward acceleration is best. Fuel is converted into work and it takes more work to hustle up to speed quickly than to get there with reason (one would think). It mainly depends upon how much mass you are accelerating with what displacement. Here are some results: http://www.crxmpg.com/acceleration-and-fuel-economy-tested/
My manal shift Beetle uses a cruise control that does not shift down on hills. Yesterday I drove 50mph (1750 rpm) on a three hundred+ mile trip and scored 37mpg. That's almost 10 more than what this car has averaged since I bought it.
Yes, obviously a car with a manual transmission will not downshift on hills.
Generally, cars before model-year 2008 do best at around 55 mph because that was the CAFE test spec, as I vaguely recall. That's how GM got all those "30 mpg highway" ratings... until 2008. Then they bumped it up to, as I recall, 65 mph for most of the time and sometimes higher -- ? we can look it up, of course.
For some cars I'd now expect 65 to be nearly as good as 55 -- but not for a ’97 Avenger automatic. I will have to experiment with our 300C. The Dart 1.4 did insanely well at 60 mph but dropped quickly above 65. The Fiat 500 was similar in my (limited) experience. Our minivan certainly does better as you go slower but it's an old four-speed and with that huge frontal area, you'd expect to be brutally punished for high speed driving.
One reason I get lousy mileage from my '74 Valiant slant six is I don't drive it in its ideal range, which I think was 35-40 mph! Definitely poor mileage as you go up to 70! Amazed I ever got 18 mpg from my old 318.
The sweet spot on my Stratus was 65. Faster than that and the MPG suffered.
Also worth noting that on my Chicago-to-Carlisle trips, I got better MPG heading east than I did on the return trip going west. Best was 34MPG - no A/C used because we drove overnight in the cooler temps.
The Hemi in my Ram does well up to about ~70 mph. Granted it does not do as well as it does at 60-65, but it's acceptable. Above about 72 mph and the MDS really doesn't activate so fuel economy starts to plummet. At 60-65 it averages about 18-20 mpg depending on load, terrain and whether the AC is on or not. At 75 mph it struggles to get 18 - probably 17.5 if I'm lucky. At 75 mph I'm just getting to the Hemi's true power band.........at 70 it's turning 2,000 rpms and at 75 it's around 2200 rpm with the real power band beginning at 2500 rpm. She'll run there all day if needed.
Of course , if I have a full load in the bed or am towing, fuel economy goes out the window and quickly drops to 15 mpg or so - especially in Tow/Haul mode. MDS does not activate in Tow/Haul or No OD.
I drove over the I-5 grapevine (mountains north of L.A.) but I think the penalty of climbing was canceled by the free energy I got going down the other side. In neutral. At 85mph. The rest of the trip through the valley was flat.
My return trip was on the Pacific Coast Highway which is a mountainous mess with lots of 20mph turns and frequent braking-and-accelerating. Still managed to get 32mpg.
With a low mileage Spirit in good shape the difference between 50 -60 mph economy seems to be insignificant. I am getting 28 mpg on flat ground with no A/C/ Tire pressure seems to have an amazing influence on economy. One tire, deflated to 22 rather than 35 lbs, drags economy down to around 23 mpg. A dirty air filter also shows up rather dramatic decreases in mileage. City driving, the real McCoy stop and go 30-35- mph has a great effect 18 mpg. The A/C seems to lower ultimate mileage from 28 down to 25 mpg with the 3.0 V6.
But this cranky @#$%^&! Mitsubishi engine DEMANDS fuel with urea additive in it like Chevron or Shell. Or periodic adding of Chevron Techron. How bad does it get? AWFUL! Mileage drops to 18 mpg with stalling. Tossing in a bottle of Techron gradually raises mileage back up to 28. This effect has been noted repeatedly and with ghastly predictable loss in fuel economy. Down here I am using PEMEX PREMIUM which is SUPER SHELL with nitorgen (urea) additive. I am not even going to try and guess WHY this engine is so temperamental and how a blasted urea additive can can such a dratic effect on it; It does not make sense.
I hit a speed bump too had a few years ago, and the muffler and catalytic converter fell to the gound. "AHA!" I thought "Now were going to see some fuel mileage figures to knock your hat off!"
Wrong. Two thousand miles of driving the same route with the same weather and load and...zip. Nada. 28 mpg. You could have knocked me over with a feather.
Probably a dollar short & a day late, as well as irrelevant with my opinion, but seems like every car I've ever owned, the sweet spot was right around 60 - All my friends driving their econo-boxes refused to believe the mileage I used to get! My '89 Ford Country Squire would kick out 25 easy on the highway if I kept it right around there.....once you took it up a notch & blasted down the turnpike at 70-75, it drank fuel like a fiend and I'd only get about 16.......I always figured it was worth the slowing down, because even going from 70 to 60 would only add like what, a half an hour driving from Long Island down to Baltimore?