is it better to go with the flow on the hwy?

[MetroMPG]

The question stands: Is less fuel consumed overall if you speed to keep up with traffic instead of driving the speed limit when everyone around you is speeding? Need someone with statistics on that one.

it's an interesting question actually.

the stretch of 4-lane freeway that passes thru my neck of the woods has a limit of 100km/h. there is almost always a service road or 2-lane hwy that parallels it with a limit of 80k.

if i'm just tooling around i normally opt to drive on the 80k road at or slightly above the limit, rather than on the freeway where my chosen speed is usually below the average of traffic.

but having said that, i have noticed once or twice that after merging from the 80kph hwy onto the 100kph freeway into a steady flow of traffic, my average fuel consumption reading has started to climb.

i haven't tested this - it's just an observation i've made maybe 2 or 3 times. it suggests aerodynamic factors at play, and maybe in a certain density of traffic, you're better off going with the flow to take advantage of this effect.

the questions are what speed? what density? i don't know. hard to test in controlled conditions, too.

and i should point out that i don't tailgate - at hwy speeds i'm usually 3 seconds or more from the car ahead.

if you doubt the possibility of a stream of traffic significantly altering the flow of air on (or near) a roadway, you just have to stand beside one for a few seconds.

...or look at this college project to generate electricity by putting wind turbines next to a busy stretch of toronto highway: Tapping the Wind Force of Traffic

[SVOboy]

Well, if you look at that graph I posted in the other thread and figure it peaks at 100kmh, you've gotta think that's for the average cars and some cars (like yours and the crx hf) have really long trannies that could put the peak a bit higher than the average. Can you get MAP readings on the scangauge?

[MetroMPG]

my car's "sweet spot" for mpg vs speed is significantly less than 100 km/h. in fact i think it's closer to 70 km/h, though I haven't plotted it out yet. (it's on the list.)

that was one of the first things i thought when i saw my mpg climbing as i joined the flow of traffic on the freeway: "hmm, maybe my car gets better mileage at higher speeds" but i've since learned it's just not the case.

that's why i'm thinking it may be air flow generated by the traffic.

(i can get MAP readings on the SG. why do you ask?)

[kickflipjr]

I go sightly slower then the flow on the hiways. I just look for a truck going a slow pace (55-65) and stay behind him (2-3 seconds). I think there is some drafting benifit because there is less road noise.

[MetroMPG]

i actually do the opposite. i don't like following trucks for visibility reasons (want to see ahead as far as possible), so when i find one going the speed i want to travel at, i usually get out ahead of him by 5 or 6 seconds.

that way the truck acts as a "blocker" for faster cars coming up from behind.

my rationale: people barreling down the highway while not paying attention are much less likely to run into the back of a giant truck than into the back of a little car. i'm thinking that a big truck enters a driver's awareness sooner than a tiny car, so traffic flows more safely if it "breaks" around him rather than me.

and i don't impede the truck - i keep an eye on the mirror and if he speeds up (e.g. approaching hills), so do i.

you're probably right about getting higher mpg behind the truck though. at approximately 2 seconds and closer, the scangauge shows the benefit. you can usually tell you're getting there by a change in the sound of wind buffeting the car.

[Capcom]

Originally Posted by MetroMPG

my car's "sweet spot" for mpg vs speed is significantly less than 100 km/h. in fact i think it's closer to 70 km/h, though I haven't plotted it out yet. (it's on the list.)

In a modern fuel injected car, best mpg is at the lowest possible speed at the highest possible gear.

For a level road: My car has the best mpg between 50-60 km/h (31-37mph) at the highest gear. Between 50-60km/h speed if i am not not accelerating anymore my car's automatic transmission shifts to the highest gear and instant consumption reading from the trip computer is between 3-4 liters/100km
But of course when going uphill best mpg speed at the highest gear must be "increased" to 70-80 km/h (43-50mph) or even more due to the angle of the incline. If not: car stalls and downshifts to the 3rd gear resulting more fuel consumption.

By the way i guess air flow generated by the traffic helps mpg but of course we shouldn't get too close behind other vehicles for safety issues.

[CosmicMC]

Originally Posted by Capcom

In a modern fuel injected car, best mpg is at the lowest possible speed at the highest possible gear.

In theory, perhaps that's true, but I don't know if that's necessarily true in practice. It would have to be plotted on a graph with a scan tool. It's possible that on some cars the engine would have to work so hard at that low RPM to overcome wind resistance that it would actually be more efficient at a higher RPM. It would also depend on when the lockup torque converter kicks in. I don't know for sure, I'm just proposing the idea for testing. My theory is that it depends on the car. If the torque output of the engine is low enough at say 1000 RPM, it's possible higher RPMs would actually increase the fuel efficiency. Say the car hits top gear at 38 but the torque converter doesn't lock up until 40. You'll get better mileage by accelerating slightly until the TC locks up.

On my car, the sweet spot is between 45 and 50. The lockup torque converter kicks in at 42 MPH under low acceleration, so the car is in highest gear and the torque converter locks up at 42, but the mileage peak isn't until about 47. Admittedly, that's rough estimation based on the car's "Instant MPG" dash readout, but I'm reasonably sure the sweet spot is not at lowest RPMs in high gear.

[Matt Timion]

I havn't read all of the thread yet, but I wanted to point out something rather obvious (i hope).

As it is aerodynamic drag increases exponentially as you go faster. It requires exponetentially more power to overcome aerodynamic drag as you go above ~60mph. While your car may be have a "sweet spot" above this speed, the word required to overcome drag at this speed makes this sweet spot irrelevant.

I believe the 60mph figure comes directly from an aerodynamic standpoint. If you are travelling at high speeds often your real concern should be making your car more aerodynamic.

[Capcom]

Originally Posted by CosmicMC

In theory, perhaps that's true, but I don't know if that's necessarily true in practice. It would have to be plotted on a graph with a scan tool. It's possible that on some cars the engine would have to work so hard at that low RPM to overcome wind resistance that it would actually be more efficient at a higher RPM. It would also depend on when the lockup torque converter kicks in. I don't know for sure, I'm just proposing the idea for testing. My theory is that it depends on the car. If the torque output of the engine is low enough at say 1000 RPM, it's possible higher RPMs would actually increase the fuel efficiency. Say the car hits top gear at 38 but the torque converter doesn't lock up until 40. You'll get better mileage by accelerating slightly until the TC locks up.

On my car, the sweet spot is between 45 and 50. The lockup torque converter kicks in at 42 MPH under low acceleration, so the car is in highest gear and the torque converter locks up at 42, but the mileage peak isn't until about 47. Admittedly, that's rough estimation based on the car's "Instant MPG" dash readout, but I'm reasonably sure the sweet spot is not at lowest RPMs in high gear.

I understand. My english is not good so i didnt mention the torque converter issue.

In fact we must say: In a modern fuel injected car, best mpg is at the lowest possible speed at the lowest possible RPM at the highest possible gear.

If i am not further accelerating on a level road my car shifts to highest gear and locksup at 50-60kmh . It is an Opel Vectra Automatic and we also tested this in same Vectra model with a stick shift. And also in a stick shifted "Renault Megane 1.6" ..Results are same.. Besides Vectra's have GM engines.. It would be very unusual and very rare for fuel injected cars that provide better mpg at highest RPM's.
But of course theres nothing wrong testing this again on other cars and in fact you are right also it depends on car...But a well calibrated fuel injected engine should never give better mpg at higher RPMs..
Anyway always 5-10mph speed difference fluctuates for the best mpg/low RPM issue and its rather difficult to test it on the road and i guess at least we agree on the urban myth: "Best MPG is at 55mph speed limit" is not true.

[rh77]

First, I agree with Matt -- simple physics dictate that the energy to move the "wall of air" in front of a vehicle quickly rises above 60mph, so torque-converter, gearing, or whatever is quickly nulled-out by the energy required to move the air. There's a thread on a Yahoo group that's been belaboring this, but it's simple physics! If you've ever had to pull-over to the side of the Interstate (to change a tire, etc) -- you can feel that wall of air hit your vehicle as the high-speed vehicle passes by.

On the issue of 2-lane or multi-lane: getting around is similar for me -- I can either take the 6-lane Interstate at 65+ or the 2-lane road where the limit it 55, more enforced and passing is rare due to hills, so I stay at 55-60. I'm assuming that I get much better mileage if I take that road than if I take the 2-lane (the wind resistance hits hard over 60-65 on the Interstate). The temptation to keep up with the traffic on the Interstate is too high around here -- I get passed by 80% of traffic if I do 5-over, and I inadvertently speed up to "go with the flow". I really need to get a real-time scangauge setup or similar to find out the most efficient speed, but I'm guessing that it's similar to the EPA estimate of 60. Since my torque-converter engages at around 45 mph to yield an RPM of around 2000, that would produce the best mileage -- but that would almost be impossible at sustained distances around here -- you would be a hazard.

Before I came to this site, I would cruise at 80+. But I've since slowed down -- in addition, I've taken a more laid-back driving attitude around town. I never let the car next to me accelerate faster at a light, but now, what's the hurry? It's a paradigm shift that I really needed.

RH77