Re-Institute National 55mph?

[Big Dave]

Aero drag HP goes up or down with the cube of vehicle speed. No way 55 is more fuel efficient than 35.

But time is also worth consideration.

[NeilBlanchard]

Hi Dave,

It is the square of the speed, IIANM.

Depending on the aero drag, and the engine and the gearing, it is possible that 55 is more MPG than 35.

[Bror Jace]

I think of all the ways we could save fuel, a 10mph reduction on the open road speed (assuming of course we have compliance ... and we won't a lot of the time) is about the least effective.

I'm still convinced that this will amount to just a few percent ... while several times that amount is spent idling in the driveway, supermarket parking lots and stuck in traffic.

I like the idea that a gauge that tells you your real-time fuel economy become mandatory ... which reduces people's ignorance ... and we can't have too much of that. After all, waste is (mostly) caused by ignorance at many levels. I liked jamesqf's comment about the reason people buy SUVs is following a current fad. Far too many people I know buy them and when you ask them, most shrug their shoulders and say "I dunno."

We deal effectively with those blighters and we're more than halfway there!

Cars WILL get more aerodynamic in the coming decade and with Cds of .28 and less, the real-world differences between 55mph and 65mph will all but disappear ... and it will take 2 more decades before the politicians, paid for by the insurance companies (who will be lobbying for a 45mph national limit by then), will even consider raising it back to 65mph so we can get across the state line in less than a 3-day weekend.

But, if the speed limit is reduced to 50-55mph, the benefits of aero are reduced ... and the incentive for the manufacturers to pursue them is also reduced. Hey! I want MORE aero, not less.

A lot said in this thread (plenty by me) is hunch and/or opinion. I'd still love some real-world analysis of the differences in a relatively modern car between 55mph and 65mph. In my '06 Civic, I bet it's 1-1.5 mpg based on trips I've taken. But that's just an educated guess.

[cfg83]

Bror Jace -

Originally Posted by Bror Jace

...

I like the idea that a gauge that tells you your real-time fuel economy become mandatory ... which reduces people's ignorance ... and we can't have too much of that. After all, waste is (mostly) caused by ignorance at many levels. I liked jamesqf's comment about the reason people buy SUVs is following a current fad. Far too many people I know buy them and when you ask them, most shrug their shoulders and say "I dunno."

We deal effectively with those blighters and we're more than halfway there!

Cars WILL get more aerodynamic in the coming decade and with Cds of .28 and less, the real-world differences between 55mph and 65mph will all but disappear ... and it will take 2 more decades before the politicians, paid for by the insurance companies (who will be lobbying for a 45mph national limit by then), will even consider raising it back to 65mph so we can get across the state line in less than a 3-day weekend.

...

I agree with the economy gauge because the information is "already there" in the ECU/PCM, and it's already featured in some cars. If the car comes with any kind of ECU/PCM controlled LCD, the implementation is a no-brainer.

I am not sure about the aero difference, though. Would the real-world difference all but disappear, or would the increased aero all 55 MPH to maintain a meaningful lead? I don't know the answer. I am guessing it would depend on the weight of the car, i.e. f=ma, where the higher mass would bias the car to maintain lower top speeds for better MPG.

CarloSW2

[Big Dave]

NeilBlanchard posted:

“It is the square of the speed, IIANM.”


Dave says:

Aero drag FORCE is proportional to the square of the speed. Power is force times speed, therefore aero drag HP is proportional to the cube of speed.

Normally people begin to consider aero drag HP at the point it generally begins to exceed rolling resistance HP – for most vehicles around 40 MPH – but it is always there, even a 5 MPH. It is just not as important at very low speed.

Fuel burn is a function of power required.

Because of all this when someone tells me that fuel burn rate is lower at 35 than 55, my skepticism meter goes into the red.

[jamesqf]

OK, do you believe that ScanGauge fuel burn rates (GPH) are at least semi-accurate? Because I can be going up a long slope in 5th at about 45 mph, and the ScanGauge will show about 1.0 GPH fuel consumption. Downshift to 3rd, maintaining the same 45 mph speed - and thus the same power output to the wheels - and it will show that I'm burning 1.5 gph.

[MazdaMatt]

possibly the power differnce required to suck in all that extra air (66% more in my car in 3rd vs 5th) past the less opened throttle plate?

[NeilBlanchard]

Hiya,

Originally Posted by jamesqf

OK, do you believe that ScanGauge fuel burn rates (GPH) are at least semi-accurate? Because I can be going up a long slope in 5th at about 45 mph, and the ScanGauge will show about 1.0 GPH fuel consumption. Downshift to 3rd, maintaining the same 45 mph speed - and thus the same power output to the wheels - and it will show that I'm burning 1.5 gph.

For my most recent tank (after two calibration changes), it read 10.0 gallons for the fillup, and I had pumped 9.977 gallons. So, yes it can be quite accurate -- the MPG reading was very close, too.

[Figjam74]

Also, what's the change in RPM?

[jamesqf]

Originally Posted by Figjam74

Also, what's the change in RPM?

From less than 2K to something over 3K - which is the clue to two possible reasons for the increase. One of course is simply the increased friction from the pistons and so on moving faster. Second is that the VTEC shifts to its power mode at about 3K rpm.

In any case, it's not the specific numbers that matter, but the general principle. IC engines do not have a linear fuel in -> power out function, so it's quite possible to have situations where you do in fact get better fuel economy by going faster. With my Insight, one place this is easy to spot is on upgrades, where it's better to go faster, if that means I can stay in 5th instead of downshifting. But as always, your mileage may vary :-)