Improving fuel efficiency with heavy vehicles

[Redsky]

I receive a publication for the heavy trucking industry's fleet operators and the current issue had an article on 121 ways to save money on fuel. Many were applicable to RV users, some are obvious and others are not.

Some pertinent ones for people driving or towing RV's were these:

Buy higher MPG vehicles

Get vehicle with automatic transmission

Right size the engine - more power than is needed wastes fuel

Gear the truck for how and where it will be used

Match truck and trailer size including using a fairing to match
height and width of trailer being towed - can save 10%

Roll on aluminum wheels

Buy fuel efficient tires

Using high capacity singles instead of dauls

Have tires aligned on the trailer

Find ideal tire pressure - too high can lead to tire failure. Too low can hurt fuel economy with a 20% increase in rolling resistance with a tire under-inflated by 10%

Use tire pressure monitoring system

Don't replace tires too early - Tires at the latter stages are more fuel efficient.

Keep tires balanced. Testing has shown up to a 2% improvement from this alone.

Buy only as much fuel as needed for the trip. Avoid hauling extra weight with the additional fuel and keeps fuel in the tank fresher.

Buy fuel close to where the trip ends. This helps with fuel formulated for colder or warmer areas at the start of the trip from where it will end.

Optimize your routes to minimize miles driven and to avoid changes in elevation.

Don't let the engine idle while stopped or us it to warm up or cool down the cab. Turn off the engine whenever possible.

Reduce AC use.

Complete DPF regenerations when they first appear. It is always more efficient to all a diesel particulate filter regeneration to finish completely as opposed to waiting until the next opportunity.

Keep warm-ups brief.

Observe the sweet spot while driving. Operate at the maximum torque or power RPM.

Upshift early and often. This involves barely running the engine past idle using only as much horsepower as needed an gradually going into higher gears.

Let momentum do the work - use the rig's weight to help push it uphill. Take your foot off the accelerator as you enter a town and at least 50 feet before you top a hill.

Let it lug (applies to diesel engines)

Skip gears on downgrades.

Stay on the interstate if you can. Easier to be able to cruise at a steady speed and route is usually flatter.

Slow down -nothing gained by arriving early and each 5 MPH over 55 MPH reduces fuel economy by 0.5 MPG for a heavy vehicle.

Don't tailgate which makes for a lot more slowing down or stopping and then having to regain speed again.

Avoid rush hours.

Use cruise control wisely. Good for the open highway but in town it will result in more braking.

[pnichols]

What if the torque curve is pretty flat ... then what RPM do you drive at?

[Gjac]

royl wrote:

Gjac wrote:

Cummins12V98 wrote:
"Let it lug (applies to diesel engines) "

I don't think so! That's NOT the sweet spot! Maybe BIG rigs but not pickups!

I have always wondered about this. Can someone more knowledgeable than myself elaborate on this. I always thought that if you were in the highest gear possible at the lowest rpm while maintaining the speed you want to travel at would be your best MPG. Is this not right? Or does this hurt the eng?

No you want to operate at your maximum torque rpm. If you look at the specs on your vehicle it will list the rpm where you are at max torque. Forget horsepower, the torque is what matters. Mine is at 62 mph and that is where I stay, if I try to run 55 my transmission is constantly shifting up and down wasting fuel.

My peak torque was at 1800 rpm stock(before headers} this is somewhere around 55 mph, mine will not downshift until 45 mph on a flat. So you are saying that I should get better fuel economy at 55 mph(max torque) than at 45 mph which to me is lugging the eng?

[pnichols]

Allworth wrote:
The dually guys are like that.. Sometimes seems like more tires than good sense.

Good sense is not the issue.

What's actually going on with the relevant physics is the only thing that counts. Good physics is sometimes difficult to explain clearly enough for folks to understand. Emotion and preference are easy to talk about, understand, and identify with ... and the marketing guys for RVs and everything else know this. 😉

[Airstreamer67]

tegu69 wrote:

travelnutz wrote:
So true that slowing down is the biggest fuel saver of all. More savings than all the other's mentioned combined!

The rolling resistance for a given vehicle setup is constant whether at 5 MPH or 80 MPH. However, the air/wind resistance multiplies as speed increases. Like about 4 times the air/wind resistance going from say 40 MPH to 80 MPH and it takes a lot more fuel accordingly to overcome that greatly increased resistance while only gaining a mere 40 MPH. Also saves on brake, tire, and engine wear and vastly increases safety when emergency braking and accident avoidance actions are taken. It's WIN WIN WIN!

While I would agree that slowing down(to a sweet spot, not 40mph) is a big fuel saver, I don't think its the biggest. What about wind. Recently, we were traveling for several days into a 30-35 mph wind. I was getting about 6 1/2 to 7 mpg. Later on return trip we had a stiff tail wind and I got almost 12mpg. You might say that there is nothing you can do about that. Actually there is. I got on the road earlier with the headwind and then stopped before the wind really got cranking in the afternoon.

Well, it's true wind has a lot of influence on MPGs, but the relative speed you travel remains just as important. To Wit: Driving into a 20 MPH headwind at 55 MPH equates to using fuel to overcome a 75 MPH combined windload. Traveling into the 20 MPH headwind at 75 MPH equates to overcoming a 95 MPH windload. No?

[tegu69]

travelnutz wrote:
So true that slowing down is the biggest fuel saver of all. More savings than all the other's mentioned combined!

The rolling resistance for a given vehicle setup is constant whether at 5 MPH or 80 MPH. However, the air/wind resistance multiplies as speed increases. Like about 4 times the air/wind resistance going from say 40 MPH to 80 MPH and it takes a lot more fuel accordingly to overcome that greatly increased resistance while only gaining a mere 40 MPH. Also saves on brake, tire, and engine wear and vastly increases safety when emergency braking and accident avoidance actions are taken. It's WIN WIN WIN!

While I would agree that slowing down(to a sweet spot, not 40mph) is a big fuel saver, I don't think its the biggest. What about wind. Recently, we were traveling for several days into a 30-35 mph wind. I was getting about 6 1/2 to 7 mpg. Later on return trip we had a stiff tail wind and I got almost 12mpg. You might say that there is nothing you can do about that. Actually there is. I got on the road earlier with the headwind and then stopped before the wind really got cranking in the afternoon.

[Allworth]

The dually guys are like that.. Sometimes seems like more tires than good sense.

[pnichols]

artguys wrote:
pnichols...reduced mass and friction.

Mass...less overall wheel/tire weight, or unsprung weight. Higher weights require more energy to turn.

Friction...normally a larger tire foot print as the new singles have increase friction, but because you've got 2 side walls as opposed to 4, rolling resistance is greatly reduced and that negates the foot print issue.

Thanks much for getting back to me on this.

Why isn't the unsprung weight of a huge single about the same as the unsprung weight of two smaller tires in a dual set (assuming the rims are steel in both cases) - no fair comparing some light-weight alloy to steel ... or at least close enough so as to be insignificant? However if the single does indeed weigh enough less to be significant, then the more energy required to spin the dual set is only required when coming up to speed - once rolling the energy to keep them rolling will be the same.

Also, why aren't the flexing sidewalls of two huge singles - carrying the same overall weight as the flexing sidewalls of four smaller tires from dual sets (with each carrying one-half the weight of the large single) - creating about same total rolling friction?

For kindof a counter-example related to rolling friction: On our motorhome I recently changed the stock Michelin tires to new Michelin tires of the same load class but of a taller sidewall and narrower tread width design. Michelin rates this new taller/narrower tire as a Green X tire with reduced rolling friction over their wider/lower profile old design that was on our motorhome.

I'm wondering if the new low profile truck tires aren't all about the fact that they "wallow" less under side loads because of less sidewall height (the same as in race cars) ... and the fact that they indeed do weigh a lot less than dual sets because their rims look to me to be made from a lightweight alloy (aluminum or magnesium). If so on this last rim material issue, then why couldn't the dual set rims also be made of the same lighter weight material so as to reduce their unsprung weight (which reduces the energy required to overcome their moment of inertia when accelerating)?

[artguys]

For those considering the super singles stay away from aggressive and deep tread designs.

[artguys]

pnichols...reduced mass and friction.

Mass...less overall wheel/tire weight, or unsprung weight. Higher weights require more energy to turn.

Friction...normally a larger tire foot print as the new singles have increase friction, but because youv??...smart phone typo, couldn't extricate Mr yellow man...continuing, because you've got 2 side walls as opposed to 4, rolling resistance is greatly reduced and that negates the foot print issue.

[royl]

Gjac wrote:

Cummins12V98 wrote:
"Let it lug (applies to diesel engines) "

I don't think so! That's NOT the sweet spot! Maybe BIG rigs but not pickups!

I have always wondered about this. Can someone more knowledgeable than myself elaborate on this. I always thought that if you were in the highest gear possible at the lowest rpm while maintaining the speed you want to travel at would be your best MPG. Is this not right? Or does this hurt the eng?

No you want to operate at your maximum torque rpm. If you look at the specs on your vehicle it will list the rpm where you are at max torque. Forget horsepower, the torque is what matters. Mine is at 62 mph and that is where I stay, if I try to run 55 my transmission is constantly shifting up and down wasting fuel.

[Gjac]

Cummins12V98 wrote:
"Let it lug (applies to diesel engines) "

I don't think so! That's NOT the sweet spot! Maybe BIG rigs but not pickups!

I have always wondered about this. Can someone more knowledgeable than myself elaborate on this. I always thought that if you were in the highest gear possible at the lowest rpm while maintaining the speed you want to travel at would be your best MPG. Is this not right? Or does this hurt the eng?

[Cummins12V98]

"Let it lug (applies to diesel engines) "

I don't think so! That's NOT the sweet spot! Maybe BIG rigs but not pickups!

[2oldman]

I can't recall any truck going under 65mph. We've all gotten used to high fuel prices, but at first it was a shock and 'everyone' was driving slower. Not any more.

[travelnutz]

So true that slowing down is the biggest fuel saver of all. More savings than all the other's mentioned combined!

The rolling resistance for a given vehicle setup is constant whether at 5 MPH or 80 MPH. However, the air/wind resistance multiplies as speed increases. Like about 4 times the air/wind resistance going from say 40 MPH to 80 MPH and it takes a lot more fuel accordingly to overcome that greatly increased resistance while only gaining a mere 40 MPH. Also saves on brake, tire, and engine wear and vastly increases safety when emergency braking and accident avoidance actions are taken. It's WIN WIN WIN!