Fuel efficiency vs speed - What is optimal speed to drive - It depends ....

[Mac99]

Cummins says the optimum rpm for most efficient operation
is 75% of max rpm. generally max rpm on the ISC is 2200,
thus 1650 rpm or so. Should give you a speed of 63-64mph.

[Old-Biscuit]

I get in......drive at the Speed I am comfortable with based on road condition/traffic and am not concerned with MPG
It's an RV ..........hobbies come with costs

Sweet spot is roughly 1700 RPM/63MPH

At 3/8th tank I begin looking for fueling station

[Kid Gloves]

Doesn’t it require the same amount of energy for a 40,000lb vehicle to accelerate from 0-60mph, regardless of the amount of time it takes to reach that speed?

If you goose it to 60mph in 30 seconds you’re using more fuel for a shorter length of time. However, if you nurse it to 60mph in 90 seconds, you’re using less fuel for a greater length of time. Some smart guy came up with a formula to figure this stuff out.

If it’s a “one time” acceleration to get up to speed, it really doesn’t matter. If you’re going full throttle and then full brakes between red lights, you’re wasting energy.

[Crasher]

Keep in mind that these boxes on wheels were born to get lousy fuel economy. The economy numbers of my combined 47,000# box is not something to dwell on. I drive 62-63 mph just because its my most relaxing speed. Some days I get really great mileage at that speed with a strong tail wind and some days it really sucks (literally sucks the fuel) when the wind Gods are mad at me. Fuel efficiency and motorhome in the same sentence is an Oxymoron. My comfort speed keeps me happy and let the mpg be what it is.

[Skip426]

Both Cat and Cummins published their findings .

As you drive look at the hotel/motel rate signs along the highway and the restaurants, and remember you have your room and lunch counter with you, you can afford to spend a few more $$ on fuel for your coach than you would in your car.

[pasdad1]

The most efficient speed is the point where you get into the highest transmission gear. On a DP with Allison 6 speed in economy mode, about 55-60 mph gets the fastest speed with lowest RPM, AND the least wind resistance.

This assumes flat terrain and negligible wind.

[Kid Gloves]

Originally Posted by Crasher

Fuel efficiency and motorhome in the same sentence is an Oxymoron. My comfort speed keeps me happy and let the mpg be what it is.

Fuel efficiency is relevant whether it’s 6mpg or 50mpg. An increase of .8mpg on a vehicle that normally gets 8mpg is a 10% increase. That’s not insignificant, even for an owner who doesn’t give a hoot about fuel efficiency.

Besides that, the conversation is about fuel efficiency, right?

[Gordon Dewald]

Originally Posted by Mac99

Cummins says the optimum rpm for most efficient operation
is 75% of max rpm. generally max rpm on the ISC is 2200,
thus 1650 rpm or so. Should give you a speed of 63-64mph.

I spoke to a Cummins representative about best mileage. She asked for my serial number and came back with the 1650 rpm for optimum mileage.

[Rob_M]

Originally Posted by pasdad1

The most efficient speed is the point where you get into the highest transmission gear. On a DP with Allison 6 speed in economy mode, about 55-60 mph gets the fastest speed with lowest RPM, AND the least wind resistance.

This assumes flat terrain and negligible wind.

Theoretically yes, though at 60mph while towing my 7,000# Excursion, my DP gets exactly the same fuel mileage either in 5th or 6th gear. If this has something to do with the Banks programmer I do not know. I have also found that putting the Allison in economy mode makes no difference in fuel economy on mine.
I enjoy trying different things when traversing miles and miles of Texas.

‘02 Tradition 40’
8.3 350hp Cummins with Banks kit.
430hp 1260tq

[wolfe10]

Experience with three diesel pushers and a quarter million miles:


Part of the equation is rear axle ratio.


Some DP's have a rear axle ratio such that there is really no "best MPG speed" in 6th gear as the speed to stay in 6th gear is high enough that aerodynamic drag is significant.


Totally agree, if you can get into 6th gear, economy mode and 200 or so RPM above peak torque RPM, THAT will be your best MPG, assuming this is at or under 63 MPH. Faster than that, the aerodynamic drag of that huge box will eat fuel.

[L2D2]

Our sweet spot seems to be around 1900 rpm which typically has us around 63-65mph. If I have the opportunity to draft a convoy of semi's I take it. We also see an increase in mpg with Lukens as an additive but it's expensive and we only use it on every 4th or 5th fill-up. There is not a cheap additive that makes it worth using all the time that I am aware of.

[bpu699]

Originally Posted by Kid Gloves

Caterpillar published a study, Understanding Coach/RV Performance. Here’s one page. It shows HP demand for various weight coaches at various speeds. One conclusion they reached was that, all else being equal, an increase in speed from 60mph to 70mph, increased fuel consumption by .8mpg.

So, at a cost of 0.8 gallons I can travel 15% faster (70 vs 60).

So, for $2.50 an hour my RV is 15% faster in getting me where I’m going…

Worthy trade off…

[bmcgc]

Last summer I tried tooling along at 65 mph on I-10 and I-20 to maximize mileage.

All I accomplished was slowing traffic down and keeping a lot more cars near me.

It was most comfortable to drive 70-75 mph and blend better with traffic.

It probably cost me an extra $100 in fuel, but I figure that is a small price to pay.

[Unplanned Tourist]

Looks like 55 is around best speed according to this graph of fuel efficiency at different car speeds.......

https://mpgforspeed.com/fegov_graph.gif

Your results my vary.