Is it more fuel efficient to drive in 5th gear at 50-55mph considering higher rev. per mile) or 6th gear at 60-65mph (considering higher wind resistance)?

I'd say lower RPM's will get you not only better miles per gallon, but will be less wear and tear on your transmission over time. I have a gas rig, but I've found 60-62mph to be the ideal cruising speed.

Either way you slice it, mpg's are horrible on big RV's and the difference is probably minuscule anyway.

quote:

Is it more fuel efficient to drive in 5th gear at 50-55mph considering higher rev. per mile) or 6th gear at 60-65mph (considering higher wind resistance)?

It would be bad physics to think there is less wind resistance as it is wind resistance per mile that matters. The wind resistance, while less at lower speeds, will still add up to the same amount when the extra miles are made up.

Although gearing is variable throughout vehicles, it is fair to assume the factory match up of rear end gearing - transmission gearing and engine peak power/efficiency are ideal in top gear at highway speeds. Therefore, I will bet you would get horrible mileage running in 5th versus 6th.

It would be hard to make a one size fits all statement on this, as there are variables in the engine's fuel programming.

The only sure thing is that drag increases with the square of the velocity. In this instance, that means that the coach has over 41% more drag at 62.5 mph than at 52.5 mph.

The engine would have to be pretty inefficient at 52.5 mph and pretty efficient at 62.5 mph to make fuel burn equal. Some of this would vary with engine size, too. Smaller engines burn less fuel when buzzed than larger ones. Computerized engines would have different results than mechanical.

Overall, different coaches with different engines would probably have different results. The stuff I have glanced at is all over the map. A couple of cars I had with digital readouts had results that were not parallel, either.

Some of the diesel forums have guys with access to a lot of good factory information, and might have detailed figures.

If I ever get on a level road, I would like to try to get the answer for our coach, too. A marine flowmeter would tell the tale. I have one somewhere. I was going to put it in while I had the tank down last month, but ran out of time. (Got the urge to take a trip.)

Ummm, hate to disagree but respectfully, drag increases by the cube of the velocity.

On my Breakaway, I have to go 58 MPH before is will shift into 6th, and have found 60 will yield about the best MPG. I have had a lot of instances where 60 MPH into a head wind will give me 9.5 MPG but 75 with a tail wind has yielded 11.7 MPG

Alot has to do with the final drive gearing also, I know of some 32 foot Breakaways that have the same trans and diff that I do (4.1:1) but has 22.5" wheels and these coaches don't shift into 6th until at least 60-65, maybe even higher.


Many years ago, it was said that 59 +/- was the most optimum speed for the best MPG. Today, cars with the super deep overdrive final gearing (to meet CAFE), not sure if that is the case any longer.

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Originally posted by MWrench:
Ummm, hate to disagree but respectfully, drag increases by the cube of the velocity.

Could you mean the power required to overcome drag increases with the cube of velocity?

From: The Physics Hypertextbook

"Does drag increase as the square of speed? The square root of speed? The cube of speed … ? According to our model, it should be the first of these. Drag should be proportional to the square of speed. R ∝ v2"

He goes on to get complicated, but avers that as long as there are not large changes, the squared expression works.

Wikipedia has "fluid drag increases with the square of velocity"
buried in a page full of stuff I forgot from classes in maybe 1964 or so. Actually, I don't remember much of the sixties, at all.

Either way, speed sucks fuel.

Best engine economy is at just on the peak of the torque curve; all factors considered, higher speeds result in greater friction drag.

On mine, going into OD (4th) doesn't result in better economy at the usual speeds I run (~60).

quote:

Originally posted by Rusty:
Best engine economy is at just on the peak of the torque curve; all factors considered, higher speeds result in greater friction drag.

On mine, going into OD (4th) doesn't result in better economy at the usual speeds I run (~60).

In the test cell, we found recip engines would show the best fuel consumption around the torque peak, but only at 85% to 100% throttle. At lower throttle settings, the economy went away, and loads and rpm had to be juggled for max efficiency.

That rule works quiet well on constant speed and constant load engines sized for just one load.
A good flight engineer on a DC6 or 7 could do wonders by doing their own thing instead of following charts and manuals. In WWII, Lindbergh flew to the Pacific to show pilots how to get way better range out of their P-38s. He did this by the seat of his pants, with none of the instrumentation that the later airliners had. His manipulation of rpm and prop pitch ran counter to what the engineers and power curve charts said, but 400 extra miles of range was no fluke.

Back to motor homes, as an example of the variables, my gasser runs around 60 mph at its torque peak of 2800. Mileage at that speed improves when below the torque peak in overdrive if not towing or climbing, but I don't have good figures on that, as we usually have a heavy toad or boat behind us.

My 454 truck seems similar. When I bought it, I chose a remainder '95 over the '96s just out because I didn't like the higher RPM torque peak of the '96. It turns out that the '96s have so much other different things going on that they get better mileage well below their torque peak, but don't have the throttle response with a load that mine does.

Lots of variables, so results all over the map. I have discussed this at length with a number very knowledgeable and experienced people, and the consensus is "It Depends". That should sound familiar here.

I find that driving a gasser by the vacuum gauge is a good way to save gas. I had a '76 Olds with a factory vacuum gauge in the instrument cluster. Whatever the speed, I select gears and throttle setting to try to keep the vacuum above the power piston lift of 8-10 inches.

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Originally posted by bill h:

I find that driving a gasser by the vacuum gauge is a good way to save gas. I had a '76 Olds with a factory vacuum gauge in the instrument cluster. Whatever the speed, I select gears and throttle setting to try to keep the vacuum above the power piston lift of 8-10 inches.

On my Regal the PO also said that in the 12 years that he had the coach he got the best mileage when he could keep the vacuum above 10 inches .. a nice gauge to have on a gasser.

quote:

a nice gauge to have on a gasser.

Ditto that.....It's been a cost-effective add-on for me. Except when pulling a hard grade, I try to
drive according to the vacuum gauge. That keeps the Q-jet's secondaries closed, giving me much better
mpg with little difference in elapsed trip times....The GPS is my primary speedometer, and 61 mph seems
to be the sweet spot, just before the bow wave starts to really diminish mileage.

Based on needle movements & trends, it also gives you advance notice of some pending mechanical issues.
Highly recommend a vacuum gauge for gassers - less than 50 bucks for a reliable one.

FMI: http://findarticles.com/p/arti..._200108/ai_n8978956/

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Could you mean the power required to overcome drag increases with the cube of velocity?

DUH--(to me) Yup That is what I was referring to!

I find if I can keep the boost below 12 PSI, and the EGT below 1050, I get the best mileage. I drove all over Big Bend Country of Texas last month and never got into 5th, still averaged mid 10s. Just can't be in a hurry!

Ok, I will be watching the mail for an EGT and a Boost gauge (pre-installed, of course).

Seriously, I have no idea how I could route gauge cabling from engine compartment to cockpit! Over roof?

I routed my cable from the EGT sensor along the frame left rail with all the other stuff (trans cable etc.)that goes from rear to front and up thru the generator compartment to the dash. I routed the Boost pressure tubing along the right frame rail, again with the other wires and cable from rear to front thru the generator compartment and again to the dash. nor hard just a lot of laying on the back fishing the tubing/cable thru.

Same here, although I routed both down the left fram rail.

Gee, you guys are fantastic! Here, I always thought that you should drive as fast as you could so that you would get to where you are going quicker, therefore turning the engine off and saving lots of fuel
Jim