[Delta Flyer]

Last week a post was made I'm not so sure of. It stated that crusing at 60 mph will get the best milage. While I have not proven it, 50-55 mph (90-80 kph) is probably the best crusing speed for maximum mpg. It's too slow for most urban driving, so I drive about 60 mph in the mornings.

The conditions I think would get the best mpg:

• Moderate temperatures so A/C is off[/*:m:1miud1e5]
• Just after a heavy rain, but the roads just dried[/*:m:1miud1e5]
• Crusing on a level highway at 50-55 mph[/*:m:1miud1e5]

Feel free to differ. Some of you are getting awsome milage and I suspect it's keeping speed under 55 mph.

 

[Delta Flyer]

I mentioned just after a heavy rain because my mpg improved at least 5% this morning. The humid air may have helped.

 

[mjrogers]

According to Click n Clack ...

According to Click n Clack the way to get optimal gas mileage is to drive steady at the slowest speed your highest gear can maintain (25-30 mpg in 5th gear), dry air, tires inflated to at least factory specs, and minimal braking.

They don't reco drafting behind trucks, or overinflation of tires for safety reasons.

This is way too slow for highway speed. So, pragmatically, the slowest steady speed you can maintain will give you the best gas mileage

 

[Delta Flyer]

My best guess is 45 mph on level land is the slowest maintainable speed in 5th gear - not sure.

This makes me wonder if Honda should have offered a 6th gear optimised for cruising at 65 mph. Yes, I know that wind resistance becomes a very significant factor above 50 mph.

 

[mjrogers]

I've been able to maintain 5th gear at this low speed.

My last 10 minutes of my late night drive home are on a 30 mph road with timed lights. I'm usually the only one on it and have been able to maintain 5th gear at this low speed.

 

[The Citrus Smoothie]

My dim & distant memory is telling me that the 102MPG record set around the coast of Britain was at an average speed of 22mph (presumably mostly at 25mph in 5th).

 

[lethostigma]

Wow 25 mph in 5th gear, I'll have to try that. It almost sounds unsafe because the car would have such a hard time accelerating. I jump out of 5th as soon as I drop below 50 mph.

The condition that gets me most jazzed is smooth pavement. There is this road that I take to work that has been recently redone and my car just floats across it, so sweet.

 

[mjrogers]

point is you would only drive this slow in situations..

The whole point is that you would only drive this slow in situations where it would be unlikely you'd either accelerate or deaccelerate. Of course, @ 25mph you could easily shift directly to 3rd gear if neccessary.

 

[lepier]

In town 30 mph driving

Lately I've found I get 15 to 30 MPG (indicated from 55mpg to 75mpg) increase in 4th gear as opposed to 5th gear on a limited stoplight route in town. This is running at a speed limit of 30 mph. -bob-

 

[b1shmu63]

I believe that the Insight was set up to maximize mileage at 60 MPH in 5th gear. If this is true then it would make some sense that at lower speeds a lower gear would be more efficient. 3rd, 4th, and 5th are all overdrive gears which means that the engine is running slower than the wheels. This creates some shearing loss in the gears. The engine is also "tuned" to higher RPMs which means that the scavenging effect of the exhaust system and the ram effect of the intake system are reduced at lower RPMs. Frictional losses in the con rod bearings are greater too when the engine is running at lower RPM for a given HP. There are many other factors as well. It is not clear cut that the best choice for efficiency will always be a higher gear. If someone could run a dyno test to prove this one way or the other it would be a significant contribution to our knowledge base.

Kip

 

[Delta Flyer]

I'm concerned about "lugging". Damaging the engine is not worth higher mpg. During my CRX HF days, I never went in 5th under 45 mph unless I was coasting to a stop. Maybe it's possible to go slower in 5th safely.

 

[b1shmu63]

Christian was only surmising that the trip around England was done in 5th. The 102 MPG was in Imperial gallons and corresponds to 85 MPG US. Some of the American Insighters have done routinely better than this. See: http://www.insightcentral.net/forum/vie ... 36&start=0 where Billy achieved 111 MPG Imperial at between 58 and 68 miles per hour on a 2126 mile round trip. :shock:

Trying to get any significant HP out of the ICE at 1,000 RPM is just rude. Of course, if you are seeing 150 MPG on your instantaneous MPG meter then you are really NOT getting significant power out of the engine and are just running a fast idle with almost no load..... Still, 4th might be a better choice for mileage at that speed.



Kip

 

[ncnightowl]

I believe that the Insight was set up to maximize mileage at 60 MPH in 5th gear. If this is true then it would make some sense that at lower speeds a lower gear would be more efficient. 3rd, 4th, and 5th are all overdrive gears which means that the engine is running slower than the wheels. This creates some shearing loss in the gears. The engine is also "tuned" to higher RPMs which means that the scavenging effect of the exhaust system and the ram effect of the intake system are reduced at lower RPMs. Frictional losses in the con rod bearings are greater too when the engine is running at lower RPM for a given HP. There are many other factors as well. It is not clear cut that the best choice for efficiency will always be a higher gear. If someone could run a dyno test to prove this one way or the other it would be a significant contribution to our knowledge base.

Kip

I have driven route 12 on the Outer Banks - this is a good place to use 5th gear at low speed. Highway is perfectly flat. I usually get 100-120 mpg at 35 mph using 5th. RPM is approximately 1200.. car has no difficulty doing this.

 

[b1shmu63]

Thanks Mark, Around here there is simply no place to test this as everywhere is hilly and curving or has traffic at 55+. If you tried the same route in 4th it could make a case for the 4th versus 5th discussion.

Kip

 

[ncnightowl]

Thanks Mark, Around here there is simply no place to test this as everywhere is hilly and curving or has traffic at 55+. If you tried the same route in 4th it could make a case for the 4th versus 5th discussion.

Kip

I'll try this sometime.. I don't get down there too often, though.

 

[AZCivic]

My dim & distant memory is telling me that the 102MPG record set around the coast of Britain was at an average speed of 22mph (presumably mostly at 25mph in 5th).

Not sure what record you're referring to, although it reminded me that VW recently was in the news for some record gas mileage of their Lupo TDI. Here's a short article about it where it lists the total distance as 4683 kilometers, fuel economy of 2.78l/100km, and average speed of 81km/h.

For those of us used to standard units, that works out to 2903 miles, 84 US miles per gallon, and 50 miles per hour. That's actually pretty dang impressive for a non-hybrid vehicle. It should certainly be possible to achieve a similar feat in an Insight, but 84mpg is quite high, especially for an average speed of 50mph, assuming they mean that in the sense that it includes speed during stop and go traffic, not just cruising on the highway at 50mph.

Anyway, on a similar note, I've found several resources which indicate that the power required to overcome aerodynamic drag increases at the cube of velocity. Now what's the top speed of an Insight? 112mph or so? And by the time you reach that speed, you're probably operating almost entirely on motor power, not so much battery assist, so we'll say that it takes roughly 70hp to get an Insight to do 112mph. Applied to lower speeds, it would thus take the following power levels to maintain speed:

85mph: 31
80mph: 25
75mph: 21
70mph: 17
65mph: 13.7
60mph: 10.8
55mph: 8.3
50mph: 6.2
45mph: 4.5
30mph: 1.3

So you can see that it takes virtually no power to overcome aero drag at 30mph, and only a small amount to overcome drag up to around 60mph or so. From there the penalty increases pretty rapidly. This of course typically begs the question as to why you don't get 300 miles to the gallon if you cruise at 30mph. The answer is that motors are not very efficient at extremely light throttle applications and the pumping losses, internal friction, and power to overcome the friction of the transmission and tires is greather than the power needed to overcome aero drag at such low speeds.

 

[Rick]

Those numbers seem too high. The Insight is merely limited to 113 mph, which happens to be red line in 3rd and the speed rating for the tires. I think as it sits less the govenor the Insight could do about 120. But back in my High School days we figured it only took roughly 6 hp to keep our 3000 pound electric Fiero moving at 50mph. I would imagine it only takes a few HP to move an Insight along at highway speeds. Now we're talking continuous HP here not peak, these are essentially two totally different ratings here.

 

[b1shmu63]

The record is in the Guiness records for the circumnavigation of Britain using a standard route and verified check points. I believe the previous record was 87 MPG and was set by a diesel car.

The VW Lupo can be ordered with an aluminum/magnesium body and thinner glass for an extra 5,000 dollars in Europe. That is the one they use for record setting.

112 MPH is not the absolute maximum speed of the Insight. All except the 2000 model are electronically regulated to that speed because of the speed rating of the LRR tires.

I believe that the energy required to overcome air resistance increases as a squared ratio and that the cubed ratio is for liquids, however, air begins to take on the characteristics of a fluid above 40 MPH.. Rolling resistance increases linearly. You would have to figure out its percentage at the absolute* top speed. Furthermore, it is unlikely that the engine produces its absolute rated power at absolute* maximum speed. You would have to get lucky with the gear ratio.

You are absolutely right about about the pumping losses, internal friction, etc. That is one of the reasons that hybrids work. The smaller the gas engine, the better the efficiency so Toyota went for a larger electric engine on the new Prius model and kept the gas engine about the same.

 

[AZCivic]

I believe that the energy required to overcome air resistance increases as a squared ratio and that the cubed ratio is for liquids, however, air begins to take on the characteristics of a fluid above 40 MPH.. Rolling resistance increases linearly. You would have to figure out its percentage at the absolute* top speed. Furthermore, it is unlikely that the engine produces its absolute rated power at absolute* maximum speed. You would have to get lucky with the gear ratio.

From all of my reading (and I'm no physics expert) drag increases at the square of velocity, but the power required to overcome it increases at the cube of velocity. This comes mainly from web sites of guys with 200+ mph Bonneville Salt Flats racers.

As for calculating the power at non-limited top speed, all you would need to know is the two-way average top speed, and what engine RPM it was turning at that speed. Strap the car on to a dynojet and see what power it makes at that RPM. Some simple math should then tell you the power requirements.

As for the part about the power required being too high, the 6hp figure likely does not account for drivetrain loss, or perhaps not even for tire loss. I find it unlikely that my 5.5hp lawnmower motor would have any hopes of sustaining 50mph in a 3000 pound vehicle, even if it were accellerated to that speed by a tow vehicle and then left to run under the power of the mighty B&S single cylinder.

 

[b1shmu63]

At 200 MPH air would act as a fluid :shock:

There has been lots of speculation regarding the Insight's top speed but no one has found a way to defeat the governor, at least that they are admitting. There are safety, warrantee, and liability issues to consider. It does appear that the Insight is still accelerating at 2 MPH per second as it reaches 111 MPH.

Some years back it was GM that suggested that a full sized sedan could travel at 60 on about 7 or 8 HP. I would love to know what the Insight needs. Perhaps someone already has that info.