Higher speed equals lower mileage? Not always...

[glenhead]

It'll be interesting to see what discussion this spawns...

I make a trip between Austin and Waco (around 100 miles each way) every other week to take my daughter to spend the weekend with my ex. About once every six weeks or so, we go on the rest of the way to the Ft Worth area, where my parents, my wife's parents, and my ex live (around 200 miles each way). We make the trip in our 2005 1/2-ton Suburban 4x4, 5.3l V-8. Fully loaded with family, dogs, and assorted junk, we tip the scales at a tad over 6200 pounds.

I'm a statistical analysis engineer by education and experience. In my little world, there's no such thing as too much data. I've been tracking this for over a year, and have left out anomalous trips (heavy headwinds or tailwinds, or towing a trailer). The results are for both directions - the trip is pretty flat, elevation differential of less than 200 ft between here and there. 90% of the trips are made in the late afternoon. When I make a driving trip, the only time we leave the highway is for gasoline or nature breaks.

When I set the cruise control on 74 and leave it there, we can average just over 70mph for the trip. We get passed by about half the vehicles on I-35, but we're not impeding traffic - any slower is, frankly, unsafe. At that speed, I get an average of 17.6 mpg, +/- 0.3 mpg.

When I go with the faster traffic, running 85-90mph, we can average about 82mph for the trip. Running at the higher speed, there's more variability in speed, and lots more acceleration/deceleration. At that speed, I get an average of 19.2 mpg, +/- 0.1 mpg - it's pretty much rock steady. With a tailwind, I've gotten 20.2 mpg over the 200-mile trip at an average of 83mph. Into a 40+mph headwind, it only dropped it to 18.6 mpg. I left out the three tailwind-enhanced trips in the overall average - the 19.2 mpg is a true average under non-enhanced conditions.

These results carry over to other routes, as well. On a trip to north-central Kansas last summer, I ran the higher speed northbound, into a headwind about 80% of the way, with an overall rise in elevation of right around 3000 feet. At an average speed of 81mph, we got 19.1 mpg. On the way home, I ran at the slower speed, averaging 72mph with a quartering wind, dropping back down the 3000 ft, and got 17.7 mpg.

I can only guess it's a sweet spot in the drivetrain, but it flies in the face of what "everyone knows". Don't believe conventional knowledge - find out what your particular vehicle does!

[jbarnhart]

I think your data and experience tell an interesting story.

Perhaps one reason the Prius seems more "monotonic" in it's speed/fuel consumption curve is because of its unique ECVT driveline. It eliminiates the fixed ratios which are probably the root cause of the "sweet spot" in your truck.

[Cheap!]

I agree that all vehicles have a mechanical sweet spot where they run at there optimum efficiency. Anyone know where it is for the 2006 Prius? Not talking about P&G but cruise control. What is the most efficient speed on flat terrain?

[nerfer]

<div class='quotetop'>QUOTE(Cheap! @ Jul 6 2006, 04:26 PM) [snapback]282222[/snapback]</div>

I agree that all vehicles have a mechanical sweet spot where they run at there optimum efficiency. Anyone know where it is for the 2006 Prius? Not talking about P&G but cruise control. What is the most efficient speed on flat terrain?
[/b]

There's been a couple places where this is gathered:
http://privatenrg.com/#Bill_Moores - watts/mile from 40mph to 80mph. Also nice discussion on wind effects

Priuschat had a good thread recently (look in the fuel economy forum!), I like this chart:
http://priuschat.com/index.php?showtopic=1...id=210092&st=26
(I had to find this using the advanced features of google - set it to return results from only priuschat.com, since I haven't had much luck with the local search feature when using multiple words).

Basically there's a cutoff point above 40mph, and maximum mpg is about 33 mph.
The folks that got 109 mpg in their Prius used the pulse-and-glide technique between 40 mph and 30 mph continuously.

For other vehicles (standard transmission), my understanding the sweet spot is right after the transmission is shifted into the highest gear (often in the 40's mph) - best rpm/distance covered ratio and before excess speed causes high wind drag. Not sure why 80mph is better than 70 mph for your SUV, but I bet 55 mph would trump both.

[jburns]

<div class='quotetop'>QUOTE(nerfer @ Jul 6 2006, 06:20 PM) [snapback]282238[/snapback]</div>

Not sure why 80mph is better than 70 mph for your SUV, but I bet 55 mph would trump both.
[/b]

Especially when you factor in getting rammed repeatedly from behind giving you that free turbo boost effect.

[GasGuzzler87]

The answer is obvious. You have an automatic and it its lower revs in a higher gear. Not rocket science. Case solved.

[glenhead]

<div class='quotetop'>QUOTE(GasGuzzler87 @ Jul 9 2006, 09:39 PM) [snapback]283605[/snapback]</div>

The answer is obvious. You have an automatic and it its lower revs in a higher gear. Not rocket science. Case solved.
[/b]

Ummmm... okaaaayyyyyyy.....

Let's see - 75mph is c.2050rpm, in overdrive (or "fourth", as it's called nowadays). 85mph is c.2200rpm, same gear.

Lower revs? Nope. Higher gear? Nope. Rocket science? I wonder. Case solved? You decide...

[Sarge]

[quote=glenhead @ Jul 6 2006, 11:42 AM) [snapback]282025[/snapback]</div>

I can only guess it's a sweet spot in the drivetrain, but it flies in the face of what "everyone knows". Don't believe conventional knowledge - find out what your particular vehicle does!
[/b]

I agree that with a conventional transmission (i.e. Non-Prius), the 'sweet spot' will vary for every vehicle. In my previous 5-speed Integra, I found that I generally hit better efficiency at 80-85MPH (~35MPG), whereas I would get about 30-32MPG] 65-70MPH.

On the other hand, my non-engineer understanding of the Prius is that due to the design of the 'transmission', mechanically speaking, the faster you go, the more gas is burned (assuming all other environmental conditions are equal, which of course they never are). Period.

Theoretically, the best fuel efficiency in a conventional vehicle should be when it is in top gear, but turning at the slowest RPM possible without causing undue stress on the engine. That would have been ~45MPH in my Integra (5th gear @ 2000RPM). But on the other hand, 80MPH also yielded pretty good results, so what do I know... :P

- Kevin

[GasGuzzler87]

<div class='quotetop'>QUOTE(glenhead @ Jul 11 2006, 11:08 AM) [snapback]284451[/snapback]</div>

Ummmm... okaaaayyyyyyy.....

Let's see - 75mph is c.2050rpm, in overdrive (or "fourth", as it's called nowadays). 85mph is c.2200rpm, same gear.

Lower revs? Nope. Higher gear? Nope. Rocket science? I wonder. Case solved? You decide...
[/b]

Well I'm unsure of the way the ECU manages fuel in a piston engine but the rotary ECU cycles through several different fuel delivery modes. I can conly remember the name of two of them (there's about 5) one is "open loop" and the other is "double loop" anyway this system for some reason give you the best mileage at 90 Mp/h in 4th at around 4500 RPM or so, I don't understand why but it doeas. Maybe the Ford has an ECU controlled fuel delivery system that helps it out. Also maybe since the explorer is such a high vehicle, the air is causeing a slight bit of lift that lessens the load on the wheels and drivetrain but a 150 RPM difference would barely effect mileage. There a little more complex but these could possibly be close to your answer.

[jayman]

<div class='quotetop'>QUOTE(glenhead @ Jul 6 2006, 10:42 AM) [snapback]282025[/snapback]</div>

When I set the cruise control on 74 and leave it there, we can average just over 70mph for the trip. We get passed by about half the vehicles on I-35, but we're not impeding traffic - any slower is, frankly, unsafe. At that speed, I get an average of 17.6 mpg, +/- 0.3 mpg.

When I go with the faster traffic, running 85-90mph, we can average about 82mph for the trip. Running at the higher speed, there's more variability in speed, and lots more acceleration/deceleration. At that speed, I get an average of 19.2 mpg, +/- 0.1 mpg - it's pretty much rock steady. [/b]

Do you have the GT4 or GT5 gear option? That is, does your vehicle have 3.73 gearing or 4.10 gearing? If I recall, the build code list is on the inside of the glovebox.

I had a 2000 GMC Sierra that I special ordered to pull a 6,000 lb camper: half ton 4x4, SLT, GT5 (4.10 gearing), Z71 offroad, towing package, etc. With the 4.10 gears, I almost ALWAYS got better fuel economy than comparable Chevy/GMC trucks with 3.73 gears, usually by 2-3 MPG at the normal highway speeds here.

In Manitoba, most of the traffic is moving at around 110 km/h or about 70 MPH. Unless I went faster than 110 - which I rarely did - I got better fuel economy. The difference was even more pronounced at speeds of 60-80 km/h.

It's my opinion the 5.3 Vortec has a sweet spot around 2200-2500 RPM. Any slower than that, you're not really "lugging" the vehicle but you're using more gas. I also carefully tracked the fuel consumption as I'm given an allowance for business mileage.

Another odd thing: Driving around Winnipeg, at speeds up to 80 km/h, I got around 1.5 MPG better by leaving the shift in 3, instead of OD.