MoapaPk wrote:In Albuquerque at ~5200' above sea level, it was fairly imperative to adjust the fuel system for the altitude... bet that may have been mainly to reduce emissions.

Was that a carbureted engine (not manufactured since . . . forever ago)? I am surprised that a modern fuel-injected engine would not automatically adjust for the air pressure.

Good point; the first had a carburetor; after 1988 they were fuel-injected.

butitsadryheat wrote:But what is Bob doing with the extra $150?

JasonH wrote:I think the answer is obvious. Bob is depressed being back on the east coast, so he fillS the back of truck with more booze.

We have a winner!

MoapaPk wrote:Did you rentals out west have cruise control? Did the miles reported on the odometer match what you inferred from maps and road signs?

The more I think about it, the more I realize that my rentals, except for that Outback, weren't all that exceptional. But the answer to both questions is "yes."

The big differences have been when driving my own car.

Not so, Day Hiker.

Most modern cars (anything after 1970) have devices to compenstate for high-altitude operation. The correct amount of air is taken in to mix with the correct amount of fuel.

Don't expect fuel effieciengy to change much at high-altitude.

billisfree wrote:Not so, Day Hiker.

Most modern cars (anything after 1970) have devices to compenstate for high-altitude operation. The correct amount of air is taken in to mix with the correct amount of fuel.

Don't expect fuel effieciengy to change much at high-altitude.

There are two different issues here, and I think you're mixing them. There is the efficiency of the engine, in terms of output power per fuel consumed, and there is "fuel efficiency" of the vehicle, in terms of distance driven per fuel consumed.

I wrote that I don't see how the altitude would affect a modern engine's efficiency, so we agree on that.

For the fuel used per distance driven, that's a different story. At higher altitudes, the air is less dense and there IS less wind drag. Wind drag, of course, is the largest factor at freeway speeds. At higher elevations, less force is required to move the vehicle, so fuel consumption is lower.

The engine still uses the same amount of fuel per work output. It's just that the work output is less at elevation.

Day Hiker wrote:
You already know how how to calculate power versus grade, I believe. The power equals the change in potential energy divided by the time. So at a given horizontal speed, the power (work per time) is proportional to grade. At normal road grades, the road speed is roughly equal to horizontal speed, so we could say that work per time is essentially proportional to grade at a given road speed as well.

To get the fuel usage, here is a bit of info: At a given engine speed, the graph of fuel usage (y-axis) versus power output (x-axis) is roughly linear, as in y=mx+b. The y-intercept is the fuel usage at no load; it takes fuel to spin the engine, of course. The slope will tell you how much additional fuel is required to handle additional load.

The actual values depend on the engine, and I don't have a bunch of example values to offer, unfortunately. The value for b would typically be smaller for a smaller engine, since it takes less power to spin a smaller engine.

I think we can keep things simple by talking about moderate road grades, not ones on which we need to downshift to maintain a safe, time-efficient 90mph. So the comparison between level and different uphill grades can be made simpler by assuming a constant speed (so the same wind drag) and a contant, top gear (so the same engine speed).

Grades of zero to a few percent do not require downshifting (unless the car is junk), and the roads in the western Great Plains would generally not exceed this range. But the 6-percent uphills from Denver to Eisenhower Tunnel do require some downshifting, except maybe for a decent sport bike.

yeah I get that the external power required to overcome grade will be proportional to grade, but didn't know if engine efficiency is also affected by grade, which would make the internal energy (fuel) relationship to grade nonlinear.

this site looks pretty interesting to fool around with various parameters (like air density) and looking at fuel economy. it doesn't account for grade, but I guess you could use the engine efficiency parameter as the slope for fuel (input) / power (output) relationship?

actually, they show the total hp requirement with drag and rolling breakdowns. Using (mass * gravity * vertical velocity) one could create a new column showing the additional horsepower needed when adding a certain grade.

edit: google spreadsheet to mess with

Ze wrote:yeah I get that the external power required to overcome grade will be proportional to grade, but didn't know if engine efficiency is also affected by grade, which would make the internal energy (fuel) relationship to grade nonlinear.

On a grade, the power-to-fuel efficiency would actually be higher, assuming speed and gear ratio (thus engine speed) remains constant. This is because of the relationship between fuel and power (roughly y=mx+b) that I mentioned before, where y is the fuel usage and x is the engine's load, at a given rpm. It's not proportional (b>0), but it is roughly linear (x^1) for a typical engine. So at a given engine speed, at higher power output, the engine is more efficient, basically because it takes a certain amount of power to just spin the engine, under no load. So the higher the load, the higher the proportion of fuel energy is going to the load.

So compare level ground to a grade, given the same vehicle speed on both. If the grade is such that you can remain in the same gear, the comparison can be made. Otherwise, it's more complicated because the engine speed will be different between the two cases.

On level ground, the vehicle is using fuel at a certain rate. Part of that fuel energy goes to power at the drive wheels, and part goes to losses within the drivetrain. On an uphill grade, at the same vehicle speed and same engine speed, the drivetrain losses don't change much, but there is a lot more power going to the wheels, specifically the amount of power required to increase the car's potential energy at a given rate.

On an uphill grade, the total power-to-fuel efficiency will be higher. The vehicle will obviously be using more fuel on the grade, but the efficiency is higher because a smaller proportion is going to drivetrain losses. And by "efficiency," I don't mean fuel per distance; I mean fuel per work output, which includes the increase of potential energy of the car as it goes up the grade.

Interesting comments, Day Hiker.

I do know that in those contests for very high mpg winners... they used the "run and coast" method. Engine runs for a few of seconds... and coasts for awhile.

I'll give this thought.

MoapaPk wrote:I've never gotten better than 26 mpg in my 2003 outback, which was rated for 28 highway. The old epa ratings were based on a "highway" speed averaging less than 50 mph. Above about 55 mph, the efficiency of the 4-cyl goes down from air drag.

Holy cow, I get about 25 MPG in my 2004 GTO on the highway (350 HP V8 ).

Those TWO overdrive gears help a lot.

billisfree wrote:Interesting comments, Day Hiker.

I do know that in those contests for very high mpg winners... they used the "run and coast" method. Engine runs for a few of seconds... and coasts for awhile.

I'll give this thought.

Yes, run and coast, at low speed, is the way to get those mpg records. They use a very small engine that is either at full throttle or shut off.

redneck wrote:

MoapaPk wrote:I've never gotten better than 26 mpg in my 2003 outback, which was rated for 28 highway. The old epa ratings were based on a "highway" speed averaging less than 50 mph. Above about 55 mph, the efficiency of the 4-cyl goes down from air drag.

Holy cow, I get about 25 MPG in my 2004 GTO on the highway (350 HP V8 ).

Those TWO overdrive gears help a lot.

I used to get upper 20s with my 1998 Z28 (305 hp V8 ). I think the transmission might be the same as yours. Fourth gear was 1:1, and sixth gear was 2x fourth gear. At 6000 rpm redline, sixth gear would put you at 250 mph (if the hp were available). So 3000 rpm was 125 mph, which was easily and quickly attained.

It was turning only 2000 rpm at 82 mph. Partly because of that gear ratio, I got better highway mileage with that car than I do with my current one, even though the engine was bigger.

I got over 30 mpg several times, including over 32 mpg on a 65-mph, 2-lane drive from Flagstaff to Monticello. (On an Interstate Highway, I would not have been going 65 mph in that thing, unless I had at least 3 flat tires, or it was snowing.)

Edit: Looks like the transmissions are the same make, but slightly different ratios:
http://en.wikipedia.org/wiki/Borg-Warner_T-56_transmission#Identification

They are both 1:1 in fourth gear, though, making fifth and sixth gears "overdrives." I hope it was obvious when I wrote "2x" that I was comparing the output speed of sixth versus fourth. The sixth-gear ratio was .5:1.

My biggest complaint about our Subaru is it's lack of a 5th gear. The 4 speed automatic is simply inadequate at fast highways speeds. Our Subaru gas mileage falls dramatically above 65 mph and is horrible when driving even faster which is common on the mountain west interstates. We give ourselves a double whammy by sometimes traveling with 4 bikes on the roof...that absolutely kills efficiency.

With all that said, we got our best mileage ever in the Subaru for one week last year in the NE Yellowstone Region -- we averaged 34 mpg for the week. All of the speed limits were generally below 45 mph. I've never done that well with gas mileage in the East.

FortMental wrote:There's a lot that goes into getting mileage out of a tank:

Obviously, ethanol content is a big one. Ethanol has less BTUs than gasoline....
Altitude is also a big one; there's enough of a performance/emissions difference that cars sold in high altitude markets (ie Denver) must be tuned accordingly as they'll run richer. I know that in the Pikes Peak race, a racecar will lose 65 hp between the base and summit from the lack of O2.

Then there's tire inflation, road temperature, air temperature, oil viscosity, vehicle weight, average speed, air filter blockages, driver habits. It's actually pretty hard to compare apples to apples when calculating MPGs, given all of the variables.

FortMental's post is a good one- points out the right variables and the real issue is that there are just too many and there is not enough quantitative data on Bob's driving habits and the condition of his rental cars to make a convincing argument on which is the dominant factor.

Big ones could be speed and driver habits. Drag is linearly proportional to air density but proportional to the square of the speed. Speed is a much bigger factor.

Octane is not a factor as long as the octane used meets the vehicle's design criteria.

Much of the west to my knowledge is at 10% ethanol, most of the time at least. Not sure about the east but my guess would be that it is not higher.

Day Hiker wrote:Edit: Looks like the transmissions are the same make, but slightly different ratios:
http://en.wikipedia.org/wiki/Borg-Warner_T-56_transmission#Identification

They are both 1:1 in fourth gear, though, making fifth and sixth gears "overdrives." I hope it was obvious when I wrote "2x" that I was comparing the output speed of sixth versus fourth. The sixth-gear ratio was .5:1.

Rear end gearing has a lot to do with results, too. I believe my GTO is 3.11:1, which is relatively high. That helps the highway mileage.

My El Camino's rear end is 3.73:1, with a 700R4 four speed automatic with a single overdrive gear. While the GTO cruises at 70MPH at about 1,800 RPM, the El Camino cruises at about 2,500 RPM.

That's a big difference in fuel mileage.

Both cars average about 12 MPG around town.