fuel effeciency

[illusion88]

So I was having this debate with a friend of mine.

I was driving up a hill in third gear, my RPMs were about 3500 so I shifted up to fourth and dropped my RPMs to about 2500. To maintain speed I had to press the pedal mostly to the floor. I argue that this is more fuel efficient than staying in third and hardly touching the gas. If the RPMs are lower I'm using less fuel per second. I give up power and gain fuel economy.

He argued that by depressing the gas pedal less I was using less gas because the throttle wasn't opened as much. I can have both the power and fuel economy? (not really becaues I drive a civic).

Who is correct? My obviously.

 

[punjabiplaya]

doesn't just depend on rpm. Depends on load and how much fuel is being pumped in. You could be coasting at like 60 in 3rd gear and being doing 5000 or whatever rpm and using no fuel.

rpm != amount of fuel being consumed

 

[exdeath]

^ This. Get a vacuum gauge. While it's true a gas engine loses much efficiency through the restriction of the throttle plate, running wide open at 0" vacuum and a highly enriched mixure lugging the engine under load for extended periods is only going to use more fuel, not less.

If you are a talking about accelerating, the most efficient method is to accelerate at just under wide open to your desired speed where the engine is running it's most efficient, then cruise in the highest gear that allows you to maintain the lowest vacuum/load.

 

[LTC8K6]

Get a scangauge2 and see...

http://www.scangauge.com/products/

 

[NAC]

I would think that WOT at 2500 would always always be more effecient than low throttle at 3500 rpm. A trip computer with instant fuel economy could tell you.

 

[Eli]

Yep, you would need a Scangauge or something similar to know for sure. It really could go either way.

While it's true that an engine is more efficient the more open the throttle butterfly is, that doesn't mean you're burning less fuel.

Since I have an extremely accurate instantaneous MPG gauge in my car, I can see what is best in any given situation(though I do have a scangauge now too). In my experience though, you were probably right. Opening the throttle more at a lower RPM with such a high overall load(climbing a hill) was most likely better.

With no load.. on flat ground, I could get 80MPG at 4,000RPM in 2nd gear fairly easily. But I would lose speed or my MPG would plummet on even the slightest hill.