Going up steep hills- downshift or floor it?

[Ecobalt]

When I downshift from 5th to 4th on a hill in my Cobalt, the driver information center "instantaneous mileage" display shows the same mpg before and after the shift. So, I have a bias toward downshifting so as to not "lug" the engine, or even come close to lugging.

BTW, my Cobalt has a tall fifth gear, which is one of the features that enables it to have the highest highway fuel economy of any car that is not a diesel or a hybrid.

[evilskillit]

Originally Posted by Ecobalt

When I downshift from 5th to 4th on a hill in my Cobalt, the driver information center "instantaneous mileage" display shows the same mpg before and after the shift. So, I have a bias toward downshifting so as to not "lug" the engine, or even come close to lugging.

BTW, my Cobalt has a tall fifth gear, which is one of the features that enables it to have the highest highway fuel economy of any car that is not a diesel or a hybrid.

Neat on the tall 5th gear thing. Your 5th gear is probably taller than my 6th. *jealous*

I wonder about factory instantaneous and average mpg. I guess technology has gotten a lot better but in the 90s I never trusted those things, and they never seemed to jive with the simple math of "fill up, divide gallons by miles".

[comptiger5000]

The factory mpg meter on my Jeep is usually off by about 1 - 1.5 mpg in either direction at the end of a tank, so I don't trust it. However, the one in my Mom's 99 Grand Cherokee is within 0.5 mpg (usually closer) every time (computer avg vs hand calc avg).

[99LeCouch]

Going up one hill I'd try keeping the Buick in OD for as long as I could. One day it slowed enough to shift to 3rd, and my mileage went up. The RPM's went from ~1100 to ~1700, and the engine timing started advancing again. I'm guessing the engine was not making enough torque that low to propel the car up the hill.

[bartine]

Sounds like I need to get a scanguage to really see where my car performs most economically. Too many opinions on both sides of the discussion.

[2007 ion2]

In my car, mpg (according to my scangauge) is very similar in 5th verses 4th up a hill, until throttle position is not too high, my car has torque, so unless throttle position is an issue, I stay in the high gear. This is also more efficient up hill because shifting reduces speed, and burns more fuel.

[Mulder25]

If you switch over to gallons per hour(GPH) and MPG, if you have the cheap version, add throttle position and engine load if you have the better one - I dont :-(. Anyways, the more you press the pedal, the higher your GPH will go, regardless of engine RPM, it increases the amount of fuel injected into the cylinder. At no point, can 80% of your max fuel rate be better than 20%. Tomorrow i will be going up and down a particular hill that is perfect for testing, and will post my results, but i just cannot imagine that 80% throttle in high gear would be better than 20% throttle in a lower gear. If someone could elaborate for me, I may be able to avoid the 30C in gas it costs me to run the tests... I get 55MPG on my 2001 corolla 1.8L, so im not exactly in need of making changes, but it adds some spice to driving, I set a goal, and try to beat it for the day.
Also, whoever says that a radio will not affect gas mileage needs to go back to elementary school. I installed an amp, and now run my music louder, since i can without a loss of quality, and I can see and feel a drop in fuel economy when that amp turns on.

[Mulder25]

Originally Posted by 2007 ion2

In my car, mpg (according to my scangauge) is very similar in 5th verses 4th up a hill, until throttle position is not too high, my car has torque, so unless throttle position is an issue, I stay in the high gear. This is also more efficient up hill because shifting reduces speed, and burns more fuel.

You can feel the torque in the seat, but you also need to monitor the manifold vacuum. This can feel similar to torque, but you feel it in the pedal. I dont have the fortune of a vacuum gauge - Am actively trying to find a way to install one without it looking ghetto. But i have learned to feel the vacuum through the pedal. The engine is burning fuel the most efficient at its highest vacuum. When the vacuum drops, the vibration through the pedal drops, you want strong vibrations, assuming they arent strong because of a problem, but in normal conditions, stronger ones are better. This will not apply if you have a car with electrically controlled throttle. Instead, you can try to feel it through the floor with your left foot, but i would recommend removing shoes to do it properly, but this can pose a safety issue.

[serialk11r]

Mulder25 are you talking about literal throttle position? Electronic throttle cars are probably doing worse at 20% than 80%, although that's 20% dictated load vs. 80% dictated load. 80% literal throttle position or 80% on a cable throttle would be almost 100% load at low rpm, and 20% throttle position would probably be landing you much higher in terms of load too.

[jtbo]

I usually try to get enough speed so that I can stay on low load low rpm when climbing the hill. Always that is not possible, so I try to stay on my very small rpm range where engine pulls ok, but does not go to rpm range where boost starts to come on (turbo diesel, it builds boost even when coasting if enough rpm is given).

If RPM is low enough pushing more throttle is not really helping anything, engine needs certain amount of rpm to be able to move car's mass and to overcome resistances and under certain RPM it might be able to do it at full throttle, but then again to move at same speed or bit slower on lower gear much less throttle is needed.

From many graphs I have seen that increase in rpm does not increase fuel consumption a lot, so using lower gear can be useful up to a point, it is not something you can always trust to be magical saviour however, sometimes I believe it is better to build up speed before the hill and let speed decrease while staying on low load and low rpm on top gear, this works best when you have many hills to climb, let downhill build speed, help with motor, then let speed go down at uphill. Problem is that sometimes hill is so big that speed decreases too much, then shifting down is best again.

I have only around 400rpm range where engine pulls fine and there is not much of any boost, higher rpm would work just fine, there would be power, but because of turbo I get more power than I really need, turbo boost triggers also boost enrichment which in mechanical diesel is set to safe side so it does enrichment plenty.

Limiting boost for additional 200-300rpm would give me better economy, but it is not quite simple thing to do.

Without turbo, I certainly would use lower gear with higher rpm so car would pull up hills more easily. As it is with diesels, they run at lean mixture, engine is getting all the time full amount of air, just amount of injected fuel is changed, so in that sense it is bit like lean burn, maybe also does get impact on fuel economy similar way, so that higher load is punishing more.
I have got very low consumption with way I drive, so there must be at least something that I do right, but as always is, there probably always is something more to be gained.