I've seen discussions here and elsewhere that imply that staying in the lowest gear isn't always the most efficient way to accelerate.

Not wanting to start an argument (I don't know enough for that anyway, lol) can someone explain why my Jazz which has a CVT (and therefore is a good indicator on the subject) always tries to use low revs except when I kick it down?

Between 20 and 40mph it sits around 2000rpm under normal acceleration in 'd'. In 's' it sits around 3000rpm.

From everything I've seen it done (not that I watch it like a hawk - watching the road is more useful ) it seems to follow two simple rules:

1.Try and keep to 2000rpm.
2.Keep the revs as low as possible unless kicked down.

Surely if higher revs meant higher efficiency it wouldn't prefer to stay at 2000rpm while I accelerate?

Or is this 'efficiency' being suggested actually 'the best acceleration for the least fuel'? IOW you make better use of the fuel you burn but you burn more of it?

 

For the same driving operation -- e.g. accelerating at 2mph per second -- lower revs = higher load = greater fuel efficiency.

The complication is that some operations -- say, accelerating at 5mph per second -- can't be done at low revs, because the engine doesn't have enough guts. Some argue that it is more efficient to accelerate quickly (necessarily at higher revs) and then cruise, than to accelerate slowly at low revs.

 

It certainly isn't the most efficient way to accelerate..you accelerate best when the engine makes the most power. However, it generally is the most efficient way to conserve fuel, which is what I'm sure your implying. Catch the distinction?

Best acceleration and 'least fuel' are tradeoffs.

As the engine revs higher, it pumps more air, consumes more fuel, and does more work. But there is a difference between tip-in throttle accel and 3/4 throttle accel. And there is a difference between high load and no load. There isn't a best way...it depends on your preferences. The fit allows decent acceleration by the fact that the engine is fairly small, and even driving it aggresively still gets better gas mileage than, say, a V6 or V8.

 

Yah, that's what I was wondering.

Cheers.

 

It's better to downshift to get acceleration. Lets say a gear rtio is 2. It means torque is multplied by two. IF gear ratio is 0.5, it means torque is multiplied by 1/2. This is OUTPUT torque. Input torque depends on ur engine. Input torque determines how much fuel you burn.

If you want to accelerate, output torque (torque on the wheels) is what matters. Theres several ways to demand torque. ONe is step on the pedal harder, to open the throttle. The second is if you lie on the sweet spot of the tach, that's when the engine is strongest. The third is change gear ratio (downshift). Downshifting is best for fuel economy b/c you increase output torque w/o demanding more from the engine. The first two methods, since they are engine torque, demand more fuel.

So if you're going uphill on 5th and feel the only way to accelerate is to stomp on the pedal, dont'. downshift to 4th and light foot it. Or third.

edit: actually i'm kinda wrong. it's not input torque that determines fuel consumption, it's input horsepower. So there are cases low torque high RPM can burn more fuel than high torque low RPM. HP = enginetorque * RPM / 5500

 

Yes, if your torque is too low then low RPMs won't save you gas because you're not moving the car efficiently. So where exactly is the sweet spot where you're accelerating slowly, minimizing rpm's, but also not damaging/lugging the engine parts? I've yet to see a clear answer on this basic question.

 

Can you truly find a sweet spot? I'm not an expert, but it seems that every scenario is unique, and the "sweet spot" will change every time. If you start on an incline, or decline or if there is headwind, tailwind or crosswind. There are too many variables in reality to create an accurate test.

It seems to me that to maximize fuel economy in any scenario is to keep a well maintained vehicle. You will never have static results in fuel economy, so control what you can and don't worry too much about the rest.

 

just drive the damn car and maintain it. its made to be driven at the most efficient way possible from the factory. Manufactures already factor in the "different drivers, different driving style screnarios".

 

Sure. But--within what the manufacturer builds in--different driving styles affect different trade-offs of mileage, power, durability. The manual describes suggested shift points (15,27, 39, 53mph) as a "balance" between mileage and power. I wonder where the suggested shift points are to maximize mpg (without undue strain on engine parts) and to maximize "power". I presume the shift points in the manual represent an average between those two.

 

Revving too low can be a bad thing. When you rev low, the gear is too high, and if you need to speed up (or climb a hill) you press on the gas pedal harder.

If you want to climb a hill or accelerate slowly, you're better off downshifting and feather footing, than keeping it in a high gear and lead footing it.

 

^yup. there are SO MANY factors that contribute to gas efficiency. throttle position and revs within a gear do not determine how efficient a vehicle is.

maintainence items (tire psi, oil, trans fluid, braking, etc) all affect efficiency. even uncontrollable elements affect it such as weather, gas quality, etc.

therefore, this is a very vague subject with no real answer.