Hills - Which gear?

[wmjinman]

OK, I went out and played on the hill again (burning a bunch of gas to get better gas mileage - LOL), and I think I've got a problem: My car's an AUTOMATIC transmission, right? So when I went out and tried to do the 70 to 80% throttle position (as well as LOD on the ScanGauge), the thing shifted down almost immediately, and then proceeded to accelerate like all get-out. I had to drop way back on the throttle position (50% or so) to keep from shooting right up past 60 mph. And my gas mileage on that run was really "sucky" - like 12.1 or something, as opposed to the 13.8 I was getting yesterday by using cruise control and going up it at anything between 40 mph and 55 mph.

So just for fun, I tried cruise control at 30 mph, but it was real sucky, too - 12.7 mpg. Then I did 35, and it was 13.1. So I did 40, and it was 13.7, like yesterday, and then 45 was 14.0, Whoa, I thought, something's happening here!! So I tried 45, and it showed 14.0 too, as did 50. I did 50 again, and it was 13.9, so at that point, I decided it had to be due to different conditions (maybe lighter because of less gas in the tank than yesterday - maybe a bit warmer, etc.)

So I don't know what to do, guys. I'm not aware of any way to keep my car from downshifting when I give it a certain amount of gas (that seems to be well less than that 75% "sweet spot). I can manually put it in third & try going up the hill at the max BSFC rpm, but the LOD or throttle position will be WAY below that 75% region. Suggestions?

[serialk11r]

Originally Posted by redpoint5

Your basic physics are incorrect. Without a force counteracting gravity, a vehicle will roll downhill. That force is provided by the engine and can be tested by stopping on a steep hill in an automatic transmission car. It takes some amount of energy to prevent the car from rolling downhill when the brakes are released. It takes yet more energy to produce forward movement.

No, it takes no energy to keep a car on a hill. Energy is force * displacement or distance (well, more precisely it's the integral of the cross product of the force and displacement vectors). Hint, brakes don't use energy.

[arcosine]

Its probably best to climb the hill in the highest bear possible. When accelerating in my car (stick shift) I cant get it to go under 29 mpg lugging in fifth gear, and is usually over 30 mpg when floored, but in 4th it's only 22 mpg. I don't know how automatics down shift nowdays, but there use to be a vacuum modulator and a throttle linkage that could be adjusted in the olden days.

[PaleMelanesian]

Ah, with an automatic things are different. As you found, avoiding a downshift is the first priority. Once you've satisfied that, then look at the highest engine load you can sustain. It won't be as high as you can do on a manual, the car won't let you do that.

[wmjinman]

Originally Posted by PaleMelanesian

Ah, with an automatic things are different. As you found, avoiding a downshift is the first priority. Once you've satisfied that, then look at the highest engine load you can sustain. It won't be as high as you can do on a manual, the car won't let you do that.

OK, then I guess my previous strategy is probably the best then; approach the hill at "best mpg speed", then when it downshifts, adjust RPM to best BSFC speed & hold until it can handle top gear again. Guess I need to study the MAP so I will know when that is.... I guess MAP would be my best indicator, huh? I imagine it somehow still uses vacuum as at least one parameter for the "shift decision".

I wonder if it would be of any use to fool around doing "A-B-A" testing to see where my best MPG speed is in THIRD gear, then use that when climbing a hill too steep for 4th (OD)? Maybe do those test both on the flat AND on the hill???

[redpoint5]

Originally Posted by wmjinman

OK, I went out and played on the hill again (burning a bunch of gas to get better gas mileage - LOL), and I think I've got a problem: My car's an AUTOMATIC transmission, right? So when I went out and tried to do the 70 to 80% throttle position (as well as LOD on the ScanGauge), the thing shifted down almost immediately, and then proceeded to accelerate like all get-out.

This is just 1 of the many reasons a manual is superior to an automatic. The only time an automatic has the advantage is when eating a breakfast burrito while shaving in stop and go traffic.

Originally Posted by wmjinman

OK, then I guess my previous strategy is probably the best then; approach the hill at "best mpg speed", then when it downshifts, adjust RPM to best BSFC speed & hold until it can handle top gear again.

I wonder if it would be of any use to fool around doing "A-B-A" testing to see where my best MPG speed is in THIRD gear, then use that when climbing a hill too steep for 4th (OD)? Maybe do those test both on the flat AND on the hill???

Being unfamiliar with your driving conditions, I will suggest that it may be more efficient to speed up before you get to the hill so that you can carry enough momentum to remain in top gear during the climb. Learn exactly how far you can press the accelerator pedal before it downshifts, and then always try to remain in top gear.

ABA testing is always a good idea. You can even test the method of speeding up before the hill to see if it is more efficient.

[rmay635703]

Originally Posted by JRMichler

5th 1800 13.9 15.0 27 Unable to maintain speed.

I do that one, and I bleed speed as much as is possible without having to stop.

[wmjinman]

Thanks, redpoint5,
A lot of the hills around here are too steep & long to be able to do the "run-up" -at least without getting crazy. When heading towards Reno, there's a hill just out of Carson that could *maybe* be taken in top gear if I built up enough speed first, but I think we're probably talking 75mph+, which is 10 mph over the "limit" and would (I would think) cost me so much gas to do, that it would end up being a net loss. ..... maybe not, though. Hmmmmmm.....

On the way home tonight, I tried it on the back side of that hill, where there's a MUCH smaller climb, and I think it worked. I was cruising along at best mpg speed (40mph), but right before the base of the hill, kicked off the cruise control with my brake & then took it up to 45 mph without causing a downshift. Then I kept the accelerator pedal steady, and the speed bled off to about 35 by the time I reached the top. The worst part of that whole deal though, is the top also begins an "on-ramp" type merge with 65 mph (if they're going the limit) traffic. So that gets a little hairy!!

[PaleMelanesian]

That's pretty much what I do in the family vehicle - Honda Odyssey automatic. Gain some speed to I can lose it on the climb while still staying in top gear.

I tested my hill in my manual car again this morning, taking the low-rpm concept to its absurd limit. I started at 18 mph in 5th gear (800 rpm). I was (barely) able to gain speed at first, but started gaining more farther up as rpm increased. I topped out at 40 mph instead of 45. Result: only lost 2 mpg, compared to 3 mpg yesterday starting at 25 mph.

Conclusion: with a manual, lowest rpm is best, down to where you can only maintain speed. (currently at 2500@60mph)

Now, where can I get a transmission to cruise at 1000 rpm on the highway?

[YeahPete]

I approach hills alot different than you guys. What I do is "coast down the hill" really slow and "POWER" over the next hill. I just floor it up the hills about 80% throttle and when I get to about 10mph over the speed limit I pop into nuetral and coast down and as far out until im going about 5 mph under the speed limit. Slower if no ones behind me.