V6 economy

[db]

apologies if discussed before, couldn't see anything similar.
i drive a 3.0 V6, mostly short journeys and tend to drive 1st - 3rd - 5th and use the torque to keep me moving. Rarely use little more than 2,000 rpm.

How would this compare to using gears sequentially, does it balence out on fuel consumption, ie rev slightly less in 5 gears or rev (and labour) slightly more on 3?
Usual commute is all flat roads with slow but flowing traffic, apart from 4 sets of traffic lights.

Just curious, mostly started driving this way cos synchro on 2nd is bad until fully warmed up.

[AnotherGareth]

Don't have an answer for your questions - it just struck me it sound like you have a 75.

[Ross1988]

I'm not sure either, but I would imagine it puts unnecessary strain on the engine?

[james_gt3rs]

If you are accelerating with the engine below 1500rpm then you are labouring the engine (generally) and therefore using more fuel.

[SVS]

Hi there Doug,

I've a lot of experience driving 3.0 V6s, yet I've never had to resort to 1-3-5 as you described. I tend to drive normally, using whatever gear is most appropriate for the circumstances. I find this gives the best economy such as it is for a 3.0 V6).

Do you 'block change'?

[RobM77]

Obviously two things affect the fuel going into an engine - the revs (more sucking in of fuel per second) and the throttle position (more fuel getting sucked in each time). Driving to the best economy is a balance of these. I would have thought instinctively that to drive to good economy you would need to use as many gears as you can to keep the engine in its sweet spot, regardless of how much torque the engine has.

A good example of this is when I borrowed my Dad's car for the weekend last month. There's no question at all that I drive faster than him on the road, but I love changing gear and he seems to try and avoid it where possible. I got noticeably more miles to the gallon.

[Z.B]

If you never go past 2000rpm you would do better to get a smaller engine and drive it normally.

[waremark]

Z.B said:

If you never go past 2000rpm you would do better to get a smaller engine and drive it normally.

I am not convinced that is correct; do you have specific evidence? The journalists have sometimes suggested that a larger engined version driven gently can give better gas mileage than a smaller engined version driven harder. (Not suggesting that my M3 will ever give better mileage than any of the less powerful threes, sadly!).

[db]

SVS said:

Do you 'block change'?

I'm not sure what you mean by "block change"

[driverrob]

Piston Heads - speed fuel economy matters.
Not

[RobM77]

db said:

SVS said:

Do you 'block change'?

I'm not sure what you mean by "block change"

Block changing just refers to not using the gears sequentially; so rather than 1,2,3,4,5 you might go 1,3,5.

waremark said:

Z.B said:

If you never go past 2000rpm you would do better to get a smaller engine and drive it normally.

I am not convinced that is correct; do you have specific evidence? The journalists have sometimes suggested that a larger engined version driven gently can give better gas mileage than a smaller engined version driven harder. (Not suggesting that my M3 will ever give better mileage than any of the less powerful threes, sadly!).

There's a nice graph somewhere that I can't find sadly, but the answer is that you're both right. Economy is a balance between how much you open the throttle and how fast the engine is turning. If you use a low rpm, such as 2,000, then you'll need to use larger throttle openings to achieve the same performance that you'd get at 3,000rpm. It would depend on a number of factors as to which provided the best economy, but I'd have thought that for most cars, for the sort of accelerations used in everyday driving, I'm not sure that limiting yourself to 2,000rpm would be a wise move. 2,000rpm is an extreme example, and I think the evidence Mark is referring to is for a driver driving fairly briskly, that driver is better off tickling the throttle in an M3 at 3,000rpm than he is revving the nuts off a Fiesta at 6,500rpm with wide open throttle. Clarkson did a good demo of this by getting an M3 to follow a Prius around their test track - the Prius was being caned and the M3 was just following - unsurprisingly the M3 gave better economy.

I've often played around with mpg in various cars, and acceleration lasts a proportionately small time for any given journey. What makes the most difference is how one drives when up to speed. Predicting traffic slowing up ahead allows one the hypermiler's wet dream of being off the throttle completely whilst slowing (where a modern ECU will cut the fuel flow completely), and also flowing with the road's ups and downs and being aware of throttle opening when driving at a constant speed can avoid putting more fuel into the engine than is necessary. My example above of using the gears is also relevant; using 6th gear on the flat is great, but if there's a hill or you wish to increase speed, changing down to do it is usually better for fuel economy. Finally, on the motorway, there's the fact that drag increases with the square of speed, whereas journey time is linearly proportional. So, if you are willing to accept a slightly longer journey time you can save more fuel than you'd immediately think (a 10% longer journey time means a lot more than 10% more fuel costs). It's being aware of these things that seems to make the most difference with my miles to the gallon.

driverrob said:

Piston Heads - speed fuel economy matters.
Not

The way I look at it is if I drive economically on boring roads such as motorways or crowded A roads, it allows me to drive more enthusiastically on interesting roads, such as empty B roads

Edited by RobM77 on Monday 1st November 09:50

[badyaker]

RobM77 said:

The way I look at it is if I drive economically on boring roads such as motorways or crowded A roads, it allows me to drive more enthusiastically on interesting roads, such as empty B roads

This is exactly my philosophy

Along with the usual, accelerate as hard as is sensible to gain desired speed, then shift to the highest gear possible to comfortably sustain a cruise at that speed.

[oldcynic]

This thread intrigues me - back in the days before children I drove an Audi S2 which generally averaged 17-19mpg; however I set myself the challenge of maximising my economy on the boring but necessary 55 mile commute to work. My peak was 35mpg, achieved by always changing gear before 1500rpm and never using the brakes or slowing down for corners, and I would consistently achieve over 30mpg from a car that just wasn't meant to be so green. Trouble was the adaptive engine management thought I'd turned into a grandad and took a while to catch up at the weekends!
As for changing 1st-3rd-5th, you need to learn to double-declutch. Synchromesh is for girls. You've got no hope of maximum economy if you don't even use all available gears.

[waremark]

RobM77 said:

So, if you are willing to accept a slightly longer journey time you can save more fuel than you'd immediately think (a 10% longer journey time means a lot more than 10% more fuel costs).

Or rather a 10% longer journey time means a lot more than 10% less fuel costs!

Sorry to go for pedant points on what was a good and interesting post.

[RobM77]

waremark said:

RobM77 said:

So, if you are willing to accept a slightly longer journey time you can save more fuel than you'd immediately think (a 10% longer journey time means a lot more than 10% more fuel costs).

Or rather a 10% longer journey time means a lot more than 10% less fuel costs!

Sorry to go for pedant points on what was a good and interesting post.

Fair cop! And to think I write for a living eh?

[waremark]

RobM77 said:

waremark said:

RobM77 said:

So, if you are willing to accept a slightly longer journey time you can save more fuel than you'd immediately think (a 10% longer journey time means a lot more than 10% more fuel costs).

Or rather a 10% longer journey time means a lot more than 10% less fuel costs!

Sorry to go for pedant points on what was a good and interesting post.

Fair cop! And to think I write for a living eh?

Just proving that we read your words of wisdom Rob!

[Z.B]

waremark said:

Z.B said:

If you never go past 2000rpm you would do better to get a smaller engine and drive it normally.

I am not convinced that is correct; do you have specific evidence? The journalists have sometimes suggested that a larger engined version driven gently can give better gas mileage than a smaller engined version driven harder. (Not suggesting that my M3 will ever give better mileage than any of the less powerful threes, sadly!).

Well it depends what you are comparing. A 1.8 might be more efficient than a 1.6 - especially if the gearing is taller.

But the OP was talking about a V6. I don't know what car he/she has, but I also have a V6 in a car which more often comes with 4 cylinder or TD engines. Official combined consumption for mine is about 30mpg, whereas if I'd had the 1.8 it would be 37.2mpg - a 7.2mpg penalty.

The difference in consumption between eco-style and flat-out is rarely more than that on the road, and usually less than 5mpg. So even if the difference is somewhat less than the official figures suggest, I reckon the smaller engine will always be more economical - that's the effect of extra cylinders and weight.

My empirical experience is that large capacity or high performance engines don't respond well to being molly-coddled. In town they will be barely more than ticking over so a high proportion of the energy goes on lugging extra weight and compression, whereas the efficiency is higher when under moderate use.

Refuse collection lorries and local buses have puny engines. Why? Because they don't need to do high speeds, but they do need to be efficient. The moral: only buy what you intend to use. I do make full use of the power and flexibility available from a larger engine so I put up with the slightly heavy consumption (25-30mpg cross country, at best). But it would be pointless if I was happy to tootle - a smaller engine would do the job.