MPG lower while using cruise control

[StevenB]

So I commute 115 miles one way to work and have been testing my MPG between using cruise control versus keeping my foot steady on the throttle, I'm wondering if anyone else has noticed what I noticed.

When NOT using CC I can get an average from my driveway to work 26-28 MPG at a steady 70-75mph (with some normal traffic here and there where I was on and off the throttle)

This past weekend I made the same trip on an early Sunday morning (no traffic at all) meaning I was set on CC for 100 of the 115 miles (freeway) at 73mph and I only averaged 21 MPG.

This is the first vehicle I've owned where I get an instant MPG reading but with every other vehicle I have owned CC gets me better MPG (by resetting a trip odo at a full tank and doing the math at the next refill). What gives?

 

[John]

That's interesting. I seem to get better mileage using cruise control. It's hard to do an apples to apples comparison since there are so many variables such as temperature, wind, etc.

 

[murrrphh]

Were both trips with the new shoes? Would it be possible cruise is having a harder time finding the sweet spot gear with the larger tires and wheels?

 

[StevenB]

Were both trips with the new shoes? Would it be possible cruise is having a harder time finding the sweet spot gear with the larger tires and wheels?

Yes, it was with the new wheel / tire setup and I thought about that but I had my first hunch about this issue when I was still running the stock wheel / tire setup. Just never tested it like I did this past weekend. It'll do the same when im towing, get better MPG on the throttle then CC and that was with the stock wheels and tires.

 

[cevans]

Not unusual for high efficiency engines. The TDI, CRD and CDI guys have known about this for a while. Hyper-milers will use the accelerate-and-coast driving method for max MPGs.

It is counter-intuitive that accelerating and coasting uses less fuel until you realize that this engine uses zero fuel when coasting. Whenever you inject fuel you aren't getting 100% efficiency from that fuel, so there is a built-in efficiency loss. By using zero fuel, you are eliminating that loss. The more time you spend coasting the less fuel you waste, and that translates to a small bump in MPG over a long trip.

When you are driving with traffic you are often accelerating and coasting with traffic vs. cruise control for constant load. So, if you want max MPG, drive with traffic and limit brake usage as much as possible.

 

[StevenB]

Not unusual for high efficiency engines. The TDI, CRD and CDI guys have known about this for a while. Hyper-milers will use the accelerate-and-coast driving method for max MPGs.

It is counter-intuitive that accelerating and coasting uses less fuel until you realize that this engine uses zero fuel when coasting. Whenever you inject fuel you aren't getting 100% efficiency from that fuel, so there is a built-in efficiency loss. By using zero fuel, you are eliminating that loss. The more time you spend coasting the less fuel you waste, and that translates to a small bump in MPG over a long trip.

When you are driving with traffic you are often accelerating and coasting with traffic vs. cruise control for constant load. So, if you want max MPG, drive with traffic and limit brake usage as much as possible.

Makes sense...

 

[flying7]

is the MPG value coming from the dash you checking with?

 

[StevenB]

is the MPG value coming from the dash you checking with?

Yes just the dash reading

 

[MBDon]

There's been a lot of research done on this topic. Obviously, how heavy your accelerator foot is will have a direct bearing on non-cruise mpg. However, all things being equal, it seems that non-cruise control driving will typically yield slightly better mpg than using cruise.

 

[Stephen Lemmon]

I've noticed large variations in my mileage for no apparent reason. The answer lies somewhere among outdoor temperature, humidity and regeneration cycles. Best mileage I ever got was 32 avg on I 40 in the mountains around Asheville (only once). Mountains at 70 best?? Sometimes it drops to as low as 24.2 in the flat lands of Florida.

 

[gawoods]

What is the terrain like on your commute? If it is hilly you will get better mpg's using the steady foot method. The vehicle speed will decrease using the steady foot method when climbing hills but with the CC it will maintain the set speed which uses more fuel.

 

[BoostN]

What is the terrain like on your commute? If it is hilly you will get better mpg's using the steady foot method. The vehicle speed will decrease using the steady foot method when climbing hills but with the CC it will maintain the set speed which uses more fuel.

Welcome to the forum!

 

[gawoods]

Welcome to the forum!

Thank you.

 

[EOC]

I've noticed this as well. Specifically this summer on a trip from central Ky to he very northern part of Michigan. At 70 mph it was getting worse mpg using cruise, versus foot usage. We were getting around 27 mph with cruise and around half way changed to foot use, it bumped to 31 mph. Thought maybe it was terrain so changed it up on the way back and it did the same thing. So we don't use cruise unless we are stretching legs on long trips.

 

[Green Diesel]

The cruise control uses a PID control loop to govern the set point speed. The stock tuning is very aggressive and holds speed within 0.5mph of the set point. This leads to more fuel burn during cruise. The GDE tune relaxes the governors a bit and the speed will fluctuate +/-1.5mph around the set point when traversing hills. This change alone gains roughly 1.5mpg while cruise is engaged vs. the stock tuning. Driving with the foot and anticipating hills, etc. will still give the best fuel economy return.

 

[bobcat67]

I noticed a 40 mph cross wind and 80 mph speed on I-90 through south dakota yield about 15 mpg regardless of foot or cruise control use

 

[fhedrickjr]

I have heard this before also. My Son also swears by it. Also heard it several years on the original Top Gear. They were testing a diesel sedan and the "presenter" said he did much better with foot driving. I use cruise control because I don't want to get pulled over for excessive speed. I have a tendency to speed up after a while on the road. Being used to the speed, it feels like I have slowed down so I accelerate. Caught myself doing 80 before. Also very annoying when you are on cruise and are passing someone and they start speeding up due to foot driving. Also annoying when you have to pass someone, then they pass you back and lift and you either have to brake or pass them yet again!! I couldn't be bothered!! I LOVE cruise control.

 

[Reimaster]

We have a 2016 Ram 1500 Limited Crew Cab 4X4 Ecodiesel with 3.92 gears in the rear end and a little over 7700 miles on the odometer. The drive train is stock because it's the wife's"Mall-Crawler." On a trip from Van Nuys to Los Alamitos via, 405 north, 118 east, 210 east, 605 south to Katella east to Los Al (it's a long route but avoids the LAX traffic) I averaged 30 mpg round trip. This routing is mostly level ground. On a recent trip from Van Nuys to Bakersfield, via I-5, up and over the Grapevine and back, we averaged 28 mpg. This route goes from about sea level to about 4000 feet. Both trips using cruise control. For my truck, I get better mileage when using cruise control. Can't explain why. I've also noticed that the amount of pedal pressure is greater when I control the speed with my foot. When switching to cruise control, as I ease off the pedal the truck will hold the set speed or rather appears to at less pressure. This may just be the fact that two separate mechanisms are responsible for speed control. I've also noticed that if driving conditions permit and I can use cruise control in city traffic, again I get better mileage but this is nerve wracking because I'm increasing and decreasing the speed with the cruise control increase/decrease buttons. City mileage usually ranges from 18 to 20 mpg. If I use the accelerate to 40 mph and then coast or ease up on the accelerator as the speed picks up, I can squeeze 20 mpg or a little better out of the truck fairly consistently even in city traffic,. I have noticed that sometimes, when on the highway, going downhill, the cruise control will get worse mileage so I disengage it and just coast.

 

[BJE]

I get better gas mileage by foot rather than cruise. Seems I can get the the transmission to downshift by foot at the same speed the cruise wants to keep the RPMs up.
I also do too much around town driving causing the need for regeneration. Notice a significant drop in mpg just before a regen occurs. So it seems to me that if you are starting off a trip with an exhaust filter that is dirtier than a previous trip, this could account for variations in MPG.

 

[BoostN]

I get better gas mileage by foot rather than cruise. Seems I can get the the transmission to downshift by foot at the same speed the cruise wants to keep the RPMs up.
I also do too much around town driving causing the need for regeneration. Notice a significant drop in mpg just before a regen occurs. So it seems to me that if you are starting off a trip with an exhaust filter that is dirtier than a previous trip, this could account for variations in MPG.

Welcome!
Yes, starting a trip in regen mode would affect your fuel mileage readings.