35d - Better mileage in sport mode?

[Johnhanson]

Hey guys,
Just finished a 2000km trip in our '15 35d. We had the roof rack and box on and was averaging about 125kph. As per my usual luck I had varying degrees of headwinds BOTH ways. This lead to a bit of a heavier load on the engine and poorer fuel mileage. Normally I'll get 8L/100km highway but this trip was around 10 due to the box and winds.

I often enjoy driving in sport mode on the highway to firm up the ride and steering and decrease some of the floatiness I find in most SUVs. I noticed while leaving the transmission in sport it keeps the car in 7th gear cruising (2400rpm at 130 vs 1900rpm in 8th). I played around various stretches cruising in 7th and 8th gear and found that while the mileage wasn't worse - it actually seemed to be about 3% better.

I also noticed while cruising in 7th under the extra load from the roof box and wind there was less transmitted vibration from the engine.

I'm assuming the only explanation is with a heavier engine load the turbos are providing a bit of boost and possibly not running as efficiently as when in the lower gear (higher revs and running easier).

As an aside I remember a friend of mine had an early 80's diesel pickup and would always get better mileage at higher speeds - possibly by finding an efficiency sweet spot in the rev range. I know in my NA V8 RS5 (and any other vehicle I've owned) there is NO paradoxical mileage improvement with higher revs.

Anybody else find this? With no decrease in mileage, better power availability with higher revs, less engine vibration, and no noticeable extra cabin volume, I can't really see any reason to not have the transmission in sport mode always.

[MattBianco]

A lot of things go into fuel efficiency, but the biggest culprit for inefficiency is acceleration. For instance, unless you are running the A/C Eco mode engaged at highway cruising speed will not do anything for the car as it takes a certain level of power to maintain the speed. I suspect there is more at work than being in 7th gear, including variables you have mentioned already (headwind, tailwind...). One or two wide-open-throttle accelerations from a dead stop will make a huge difference in L/100km for shorter trips. For your 2000km trip, that should not be the case. The car, and all others, are engineered to leave very little free lunch, other than some chips which increase mpg or power, but at the exposes of emissions. 42pilot is in my view is most knowledgable on this subject and I hope he chimes in. My gut tells me you cannot replicate the results.

Also, be mindful of turbo"s". It is just a single turbo with a nice marketing veneer of "TwinPower".

I have a whole thread with MPG in my footnote and so far no one has had a similar experience, but heck, we can learn something from your experience if more people chime in.

[Johnhanson]

Originally Posted by MattBianco

A lot of things go into fuel efficiency, but the biggest culprit for inefficiency is acceleration. For instance, unless you are running the A/C Eco mode engaged at highway cruising speed will not do anything for the car as it takes a certain level of power to maintain the speed. I suspect there is more at work than being in 7th gear, including variables you have mentioned already (headwind, tailwind...). One or two wide-open-throttle accelerations from a dead stop will make a huge difference in L/100km for shorter trips. For your 2000km trip, that should not be the case. The car, and all others, are engineered to leave very little free lunch, other than some chips which increase mpg or power, but at the exposes of emissions. 42pilot is in my view is most knowledgable on this subject and I hope he chimes in. My gut tells me you cannot replicate the results.

Also, be mindful of turbo"s". It is just a single turbo with a nice marketing veneer of "TwinPower".

I have a whole thread with MPG in my footnote and so far no one has had a similar experience, but heck, we can learn something from your experience if more people chime in.

Yeah this was constant cruising on flat land for repeated periods of 30 minutes on double lane highway with no changes in speed. The only difference was stretches in sport and comfort (7th and 8th gear) as well as variations in head-wind speed - though relatively constant through the day. The car will go 800-1100km on a tank, so same fuel all day with only two stops for food and peeing. There aren't many other variables. This is not one of those "wow I drove 400km on one tank and 800km on another what gives?" threads.

The only other uncontrolled variable is the accuracy of the onboard mileage computer - which over the last 30,000km in this vehicle I have found to be the most accurate of any I've driven when compared with manual calculations.

As for replication I alternated between sport and comfort around 10 or 12 times - I don't want to sound like a science nerd but I am well acquainted with scientific method and the importance of reproducibility and elimination of bias and confounding.

[42pilot]

Originally Posted by Johnhanson

Yeah this was constant cruising on flat land for repeated periods of 30 minutes on double lane highway with no changes in speed. The only difference was stretches in sport and comfort (7th and 8th gear) as well as variations in head-wind speed - though relatively constant through the day. The car will go 800-1100km on a tank, so same fuel all day with only two stops for food and peeing. There aren't many other variables. This is not one of those "wow I drove 400km on one tank and 800km on another what gives?" threads.

The only other uncontrolled variable is the accuracy of the onboard mileage computer - which over the last 30,000km in this vehicle I have found to be the most accurate of any I've driven when compared with manual calculations.

As for replication I alternated between sport and comfort around 10 or 12 times - I don't want to sound like a science nerd but I am well acquainted with scientific method and the importance of reproducibility and elimination of bias and confounding.

Sport and Comfort are two different maps within the ECU. I've explained the differences in another thread but for this discussion, the most important points are:

1. the torque converter locks up much sooner in sport - probably by 500 - 750 rpm which is more efficient under heavier load (aerodynamic drag is a load too).

2. the Sport map locks out 8th gear which is like a super overdrive gear (approx 1:0.667). 7th is technically an overdrive gear too since its ratio is under 1:1 (approx 1:0.839), but it CAN be more efficient under certain conditions. Not until you get to 6th gear do you get to 1:1 ratio.

So, with the combination of speed, aerodynamic drag (parasite drag for us pilots), relative winds and changing between Sport and Comfort (locking the TC and changing gearing), you found a way to gain 3% economy.

There is a magic RPM where best fuel consumption, ideal torque, ideal HP, and ideal EGT come together on an engine dyno. But it's like chasing the wind. Put that engine in a car, hook up a drivetrain, inflate the tires, add temp, humidity, altitude and a myriad of other variables, and you'll never find it again.

But that's why our cars are more computers than ever before - always chasing that ideal combination through the 170+ sensors and electronic transmissions.