How do I maximize MPG? from miles per hour

[mikeleblanc413]

I've been so very lucky to have this incredible teaching opportunity come my way...I'll be teaching the children who travel with the Carson and Barnes circus...shortly, I'll be leaving Texas for Wyoming to meet with the circus...in your experience, what miles per hour has maximized miles per gallon. I have a 2000 Winnebago Minnie Winnie, V-10, total weight is 1500 below maximum...all thoughts appreciated.

[pianotuna]

Hi,

I do see 29 mpg on my scangauge when coasting, and sometimes on hills it goes to 0 (i.e infinitely high gas mileage)

[tpi]

Not necessarily. My view is similar to Tatest. The best fuel economy will be in the highest gear at the lowest RPM. Extending to way slower than you want to go.

So if you can get it in top gear at 1600 RPM, go for it. Of course the automatic will downshift if it has to. But the closest the automatic will allow to lugging the engine the better for fuel economy. Anytime you downshift and raise RPM, the reason shouldn't be fuel economy.

I have a new Camry w/ 6 speed automatic and four cylinder. It is operating the engine in areas I used to consider lugging. It will do acceleration in the 1300-2000 RPM range under light to medium throttle. This increases cylinder pressures for more efficient operation.

The other big lesson of the Scangauge is to back off when you see a stop ahead and let it roll down as long as practical. The instantaneous fuel mileage under this circumstance can be over 40 MPG. On some cars it is close to 100 MPG.

So essentially trust the transmission programming. Accelerate in a way that it upshifts as soon as possible. Only manually downshift for reasons other than fuel economy. Like Tatest said, the mileage comes best at lowest practical speeds.

[pianotuna]

Hi tpi,

So if I'm going up a hill I want to stay close to 2750 rpm? For max torque? I don't really understand all this stuff.

[tpi]

pianotuna wrote:
Hi,

Not quite flat for the 6.8 liter v-10?

I'd say thats pretty flat 🙂 Note the scale amplifies small difference in torque. It's not linear to zero.

[pianotuna]

Hi,

Not quite flat for the 6.8 liter v-10?

[bmoon1952]

tatest:
I second what Ron Dittmer said. You explained the V-10's performance envelope better than any one else I have read. I watch my Scangauge II's MPG, Throttle position, Transmission temp and RPM. I could not imagine traveling without it. I had no idea the torque curve of the V-10 was that flat. I copied your last post and will make myself a bullet point guide posted on my dash to run my V-10. Keeping it in the highest gear you can for climbing makes sense. I am always going into my personally assigned headwind in my 31C 2005 Minnie. My best MPG average has been 8.0 MPG running from the swamps of Southeast Texas to Disney World and back. I use my Scangauge for MPG averaging since filling my motorhome up is a hit and miss proposition.
Thanks Again..

[tatest]

The 6.8 V-10 has an almost flat torque curve, 400-410 lb-ft from 1900 to 4000 RPM, in the tune used for the E-450. The 420 "peak" is just a little bump where exhaust tuning and the tuning of the high-rpm section of the intake manifold reinforce each other. Maximum engine efficiency (lowest specific fuel consumption) will be when producing maximum torque anywhere in this range, so it is not like you have to find a RPM sweet spot with the V-10, you just have to find a balance between using power to overcome drag, and covering ground. That's why MPH for best MPG might be very low, as there is no engine efficiency penalty for running the engine way below the torque "peak."

If you need to get up a hill, the 6.8 is almost as efficient producing 300 HP at 4000 as it is producing 250 at 2200, but if you let it run on up to 5000, it won't give you any more power and will use more fuel trying.

The E-450 tune for the 5.4 has a definite sweet spot 2100-2800 RPM, but torque doesn't fall off seriously until after 4000, where you have 95% of peak HP, and again you have another 1000 RPM with full power but less economy. The 5.4 has some of the same engine tuning tricks, and can be driven about the same as the 6.8.

I've not seen a torque curve for truck tunings of the 5.7, or even the 6.0 for the Express, but without variable valve timing or variable intake manifold lengths, you usually get a smooth round curve, where a relatively flat torque peak identifies the center of a range of maximum efficiency, and you have about 800 RPM range around that without much difference in torque. Ford's 4.6 and Chevy's 4.8 look like this. You can get quite a bit more power (up to 80% of peak) with a 800 RPM increase over peak torque RPM with little hit on efficiency, but torque will fall quite a bit, and specific fuel consumption go up, before you get to the RPM for peak HP, which might be close to 5000 for these smaller V-8s.

[Gene_in_NE]

mikeleblanc413 - Someone already mentioned proper tire air pressure. I will expand on that thought. Know the axle weights of your RV when in travel mode. Check the manufacturers "tire inflation chart" for that weight. Since economy is your goal, increase the tire pressure by about 5 psi (do not exceed the maximum allowable for the tire). The rounder the tire the less rolling resistance you have.

I will 2nd "ron.dittmer" comments about "tatest" article above - well written and understandable.

I also have a Scan Gauge II and maybe I'm not guessing correctly, but it seems like the tachometer (rpm of the engine) is a better indicator of miles per gallon. In my 5.7L Chevy, 2,450 rpm seemed to yield the best mpg.

I have often wondered if the horsepower and torgue curve for my engine might provide some information. Maybe the engine rpm somewhere between the peak horsepower (4,000) and the peak torque (2,400) would be where the engine is the happiest (sweet spot). I would need an engine design engineer opinion to discuss that.

Of course wind would change the answer.

[dbbls]

Have you ever driven a car made in the 40's or early 50 and remember the vacuum windshield wipers? Imagine doing that and trying to keep the wipers going. This will improve your fuel mileage.

[ron_dittmer]

Nice write-up tatest. It makes sense too.

[tatest]

My experience with my V10, 12,500 pound C motorhome, the best MPG speed will be dependent on wind conditions (but not nearly so much as with an airplane), and it will be slower than you want to travel, likely slower than minimum legal speeds on rural limited access highways.

A device like a Scan Gauge can help you tune this, because it calculates an almost instantaneous (really short term average) MPG. You may not like what it tells you.

For my motorhome, in still air and level ground, best MPG is right where it shifts into overdrive in normal acceleration, and can hold the speed at light throttle. This is usually around 35-40 mph. MPG can be well over 10 MPG, I've seen 12-13 for long periods on the gauge, but it fluctuates a lot with slightest difference in grade, at this low a speed.

With a 10-15 MPH tailwind, I get the same MPG going 10-15 MPH faster. Seldom am I so lucky the wind is almost always against me.

Going into headwinds, more throttle is needed. At too slow a speed, this means a downshift, so to stay in overdrive, the speed might need to be raised to 50-55 mph. This might be a 9-10 mpg speed range in no wind, but might drop to 7-8 in a 10-15 MPH headwind, as if I were trying to drive 65-70.

Running the generator (1/2 gph no matter what speed) or dash A/C (5-10 HP extra load) raises the optimum MPG speed, because you need to cover more miles even though that also costs some fuel. The engine cooling fan is a huge consumer of power, no matter what speed, but fortunately it is usually declutched at highway speeds.

In general, on high speed rural highways, the best speed for MPG you can realistically get will be the slowest speed you can comfortably and safely travel in the traffic environment. This is the reason DOT picks 55 MPH as an economy driving speed, not that 55 is a magic number that works the same for all vehicles.

I've done some of the same tests on my Honda Fit, the optimum speeds are quite a bit higher, probably because the cross-over point for power to overcome drag vs power to run accessories. 5-10 HP to run a small A/C vs 16-20 HP to cruise 55, is a bigger relative factor than 10-15 for A/C against 120-150 HP pushing that big box against the wind.

[tatest]

pianotuna wrote:
Hi Dakzuki,

I thought it was the cube of the velocity?

Power required increases with the cube, drag with the square. For fuel consumption purposes, it is the power required that is important.

[pianotuna]

Hi Dakzuki,

I thought it was the cube of the velocity?

[Dakzuki]

Drag increases with the square of velocity. Twice the speed means four times the drag. Easy math to do. Slower will yield better fuel economy.

[pianotuna]

Hi Ron,

Very nice install on the scangauge.