Hills - Which gear?

[JRMichler]

I ran some quick tests to find which gear was best to climb a local hill. This testing does not come close to meeting the standards in http://ecomodder.com/forum/showthrea...ery-11445.html, but I thought the results might be of interest.

I regularly climb a certain hill. The engine is always fully warmed up, and I normally drive it at 55 MPH in 4th gear. For the test, I also tried 3rd and 5th gears while eyeball averaging the Scangauge MAP and instantaneous MPG displays. The results:

Gear RPM MAP MPG OAT
3rd 3380 10.0 11.7 43 deg F
4th 2470 11.7 13.5 37
4th 2470 12.0 13.2 35
5th 1800 13.9 15.0 27 Unable to maintain speed.

[wobombat]

This sounds like a really steep hill if you can't maintain 55 MPH in 5th with a 175 hp engine. Do you happen to know the grade of the hill?

It just goes to show that staying in the highest gear possible really makes a difference.

[JRMichler]

It's about a 6% grade. I was able to maintain 55 MPH in 5th gear once with a strong tailwind.

The engine is 175 hp, but is only 2.8 liter. Peak torque is at 2800 RPM, so 1800 RPM in 5th is well below peak torque.

[serialk11r]

With a rather heavy vehicle and only 2.8L on board spinning at 1800rpm (god I am SO jealous of that rpm) it's not that much of a surprise 55mph on a slight hill can't be held. That is some very very efficient gearing though :O

[wungun]

I'd tackle the hill in 5th initially, then quickly downshift to 4th when you know the engine is working too hard and your speed is bleeding off too much.

[jakobnev]

How long is this hill?

If it's short enough, then the "semi"-approach of gaining a bit of extra speed before the hill and losing some up the slope and to gain it back after the hill, might be best. (all in fifth)

For a longer hill, alternating between 4th and 5th might be meaningfull.

An interesting experiment might be to pulse in 3rd and lose speed in 5th. (allthough you might feel you are going too far with the hyper-miling)

[JRMichler]

The speed limit changes from 25 to 55 MPH at the bottom of the hill. Total hill height is 140 feet, so no chance for a running start and DWL techniques.

I guesstimated the grade at 6%. It's steep and high enough that the DOT put a steep hill sign at the top.

Just for grins and giggles, I plugged a 6% grade into the drag equation from my coastdown test. The theoretical coasting speed in neutral on a hill of that slope long enough to reach full speed, at the same temperature as the coastdown test, is 84 MPH. That speed would be faster on a warm day.

When I drive that hill in the other direction (downhill), I try to EOC so as to top the hill at 25 MPH. I'm up to 55 MPH when I apply brakes for the speed limit change.

[wmjinman]

I too, was curious about the best way to deal with a hill. Living in Carson City, NV and having frequent (at least a couple times a month) business in Sacramento, CA has me going over the Sierras quite a few times a year, and that is a pretty serious hill!!!

So today I decided to go out and try a ScanGauge test in my Jimmy. This hill is on a section of the "old highway" between Reno and Carson City, and the posted speed limit is 50 mph. I usually used to drive the speed limit, and in "cruise control". This particular hill was on the borderline between being able to go up it in overdrive (4th) or having it downshift to 3rd. It seems wind and load were usually the determining factors... no load and/or tailwind and it'll stay in top gear. A second person (or more) and/or headwind ensures a downshift.

What I did was the "cruise control-ScanGauge trip reset" trick, but only one-way - - UP the hill - - for each test hitting the "reset" at the same spot on the bottom each time & reading the "current avg" at the same spot on the top each time. The hill is probably about a mile long, maybe 6% grade or so. Coasting down it the other direction in neutral will get up above 60 - sometimes above 70, which is pretty scary in a "50 zone":

speed - mpg - 2nd test
60 - - - 13.3 - - - 13.2
55 - - - 13.7 - - - 13.8
50 - - - 13.8
45 - - - 13.8
40 - - - 13.8
35 - - - 13.2

Unfortunately, this seemed to confirm my previous thought that "no matter what I did, it wouldn't matter". Of course, going 60 clearly hurts it, and slowing to 35 does too. Actually, that was a bit of a surprise because I thought going up it in low gear, but SLOW might be the best.

Seems I read something on here about the most efficient "throttle position", or something. Should I be looking at that instead of speed? Any of the wiser people here have any "obvious tips" for the best mpg going up a hill such as this?

Thanks,
Bill

[redpoint5]

In my first post to this forum, I postulated that in the absence of aerodynamic drag, climbing a hill as fast as possible is most efficient.

The reasoning goes like this:

It takes energy just to maintain a stationary position on a hill (when the brakes aren't applied).

The longer a vehicle is stationary on a hill and expending energy to maintain position, the more fuel is consumed.

To make forward progress, an amount of energy greater than the amount required just to maintain position on the hill is needed.

If you are creeping along at a snails pace up a hill, the majority of the fuel consumption is being used just to maintain position on the hill, and a very small fraction of that is making forward progress.

The conclusion is that climbing the hill at ever greater speeds results in a larger portion of the fuel being consumed in forward progress when compared to maintaining position on the hill. Reaching the top quickly means less fuel being consumed to merely maintain position.

That said, aerodynamic drag does exist and is a significant factor considering it is an exponential function. Not only that, but engine operating efficiency must also be factored in.

The conclusion is that the most efficient speed to climb a hill is certainly faster than as slow as possible, and likely slower than as fast as possible. The practical way to find out is to perform a test just like wmjinman and JRMichler.

[redpoint5]

Originally Posted by wmjinman

Seems I read something on here about the most efficient "throttle position", or something. Should I be looking at that instead of speed? Any of the wiser people here have any "obvious tips" for the best mpg going up a hill such as this?

The answer to my question that seemed most reasonable is this;

Originally Posted by Old Tele man

...keep the ENGINE operating within its best BSFC curve area...in the highest gear possible, and "...speed be damned."

...that's where the vacuum gauge and SG really help!