What's the best speed/rpm combo for best MPG?

[97DevilleBeige]

Hey guys,
For those of you out there with the northstar, what combination of miles per hour and RPMs would yeild the best miles per gallon?

Obviously the lower the rpms, the better, but I've noticed the instant mpg gauge hit nice numbers when you are going 86MPH, so I'm guessing that the engine is using X amount of energy to get to a certain speed, and if you are going the right speed, it only uses a little bit more energy to get to that speed causing a good MPG readout.


Let me know if I'm just completely insane.

[krimson_cardnal]

Can I have some?

[hazcaddy]

There's a few things like what model Caddie (wind resistance) and what type of tires (friction loss) to consider, but on a dead flat road, my '03 DHS's ideal speed for maximum mileage is 71 mph, which gives me 29 mpg on the DIC.

The advantage is that our cops won't pull you over at that speed, whereas 86 mph would not get you anywhere fast seeing as how you'd be talking to law enforcement types all the time...

[krimson_cardnal]

My DeVille likes 70-75 range on an open cruise: 70mph/26mpg - 75mph/24mpg

65 - 70 is okay a lot of the time under 75mph and you're okay - even when 70mph is posted

find the groove K_C

[JimHare]

Well, to answer your question explicitly, the best velocity/rpm for gas mileage is 0/0 - that's the best you'll ever get..

Taking into consideration that you probably mean "while moving", you'll have to experiment a bit. Road conditions, weather, terrain, tire pressure, state of tune, gasoline, all have a say in the argument.

One thing that also adds to the equation is "for how long" - for example, let's say you need to drive 500 miles. At 55, everything else being equal, your mileage will be X mpg.. You will be on the road a little over nine hours (9.09, to be exact). At 75, your mileage will be <X, but you'll be on the road only 6.6 which is a 30% reduction. If you mileage does not SUFFER an equal amount, you might be better off driving faster to burn less fuel overall...

[texascycle]

There are many factors and the number can change daily. On the same road with a tailwind you can go faster with same fuel consumption ... etc ...

As for 0/0 ... miles per gallon requires that you are moving, me thinks ... idling is the worse mpg you will ever get.

[krimson_cardnal]

I vote for driving faster and using less fuel over-all. K_C

[Caddyshack100]

Jim you hit the nail on the head with your numbers, its real nice to see the 'whole' argument played out and not just what the enviro-mentals want people to believe.

[JimHare]

Originally Posted by Caddyshack100

Jim you hit the nail on the head with your numbers, its real nice to see the 'whole' argument played out and not just what the enviro-mentals want people to believe.

I've often wondered how informative a "gallons per hour" gauge would be on a car - I suppose most people don't think about it, but "X" hours at "Y" rpm vs "Less X hours" at "More Y RPM" has gotta have a crossover point at which it uses LESS OVERALL FUEL to driver faster and turn the engine off sooner... one would think...

We probably need Koz to clue us in on this one.

[Ranger]

Sounds like a logical assumption Jim. A trip computer could easily calculate optimum speed given a distance and a known RPM per MPH algorithm stored in the computer.

[JimHare]

Ok, I did a quick little thingy here. I'm not sure how accurate the MPG assumptions are, but I think for illustrative purposes they're probably sufficent. Assming a trip of 200 miles, and speeds of 50, 60 and 70, at 25, 22 and 19 mpg respectivelly, how much could you save OVERALL by maintaining a faster speed? This assumes that TIME has a value, which would be hard to quantify, for one thing. Basically, I calculated the Gallons per MINUTE for the trip, and compared them at the varying LENGTH of TIME that the trip would take at higher speeds. It shows that, as long as our TIME (GPM) savings is greater than the MPG Penalty, percentage wise, it can, under some circumstances, be LESS expensive (in terms of overall amount of gasoline used during the time it takes to make the trip) to go 60 rather than 70, but at 70, you burn more gas in relation to the less time spent operating. In other words, at 60, you pay a 14% MPG penalty, but you're running for 17% less time, so you have a 3% gain:

Math wizzes are welcome to correct my (highly likely) incorrect assumptions and/or calculations:

[Caddyshack100]

Good points, and if you take it to its logical conclusion, the longer the trip the more time saved. That is very measurable, its time that you are on the road less, and doing other things like maybe making money, or not being on the road, means less chance of being hit or being in a accident. If we use the airliner equation. A 747 can fly at 300mph quite well, but at 600mph, it will arrive at its destination twice as fast. I wonder if passengers would stand for that. I don't know the figures, but would a 747 be able to stay in the air for 14hrs? As far as safety, you can be just as dead at 70mph as you can at 60 or 55mph, Speed does not kill...Stopping suddenly does.

[krimson_cardnal]

I like it Jim. Something that I've had rolling around for a while. Need to apply real word dollars to it. Time is the toughest, but as a percentage of the investment [total spent on fuel or total operating exspence] saved, it takes on a "real" value. Cool K_C

[JimHare]

Originally Posted by krimson_cardnal

Time is the toughest, but as a percentage of the investment [total spent on fuel or total operating exspence] saved, it takes on a "real" value. Cool K_C

What I meant by "Time Saving" is time NOT SPENT OPERATING. In other words, the 60MPH trip saved 40 MINUTES over the 50 MPH trip. So 40 minutes, at the 50 MPH rate - you balance the 40 less minutes of engine time vs the extra gas used.

And yes, the longer the trip, the greater the potential savings, until your MPG really starts to drop off percentage wise.

[Ranger]

but would a 747 be able to stay in the air for 14hrs?

Yes.
Flight time from SFO - SYD is about 14 hrs. If you add in reserve fuel it could stay aloft for approx. 16 hrs with a full load, even longer if empty.