I have recently set myself a weekly competition to fill my car each Friday evening and try to make it a lower price each week.

Last week was £56... I just filled up now for £47 and I'm already confident of beating that next week.

I drive 66 miles a day.


Anyway I was wondering something about how fuel economy works, ie when the engine uses petrol (I'm a relatively new driver).

When you are going down hill and not accelerating, what determines the petrol consumption? Will engine-breaking in 3rd (high revs) use more petrol than milder engine-breaking in 6th?

Also, when going up hill, when you're in a high gear and accelerate and nothing happens (I believe this is called lugging the engine?) Will it use more petrol with your foot to the floor even though the car is going exactly the same speed as when your foot is only slightly down?

Thank you in anticipation of your tips and advice!

The latest engines use no fuel at all when engine breaking.
Older engines, I guess the standard amount of fuel is being fed to the engine at the same rate regardless of which gear your in.

Yes it will. Your right foot doesn't control the speed of the car, it controls the rate of fuel flow to the engine.

Specifics depend on the engine but IIRC 1500-2000rpm is the most efficient range. Stick the car in the highest gear for that range and you'll find the most efficient speed (typically 56-60mph in 5/6th gear). With respect to ingear acceleration, drop down a cog rather than labour the engine. Overinflating the tyres imprves mpg but reduces grip.

I also recall the egg thing where you imagine a raw egg between your foot and the gas pedal. The aim is to accelerate without breaking the egg. Always keep the car in gear as coasting uses more fuel.

I have a turbocharged engine and as long as I don't engage the turbo, I can get decent mpg. Have managed 40mpg on a run to London. The last few weeks, I've been leaving home 10-15 mins earlier. Coupled with gentle driving, I have found a marked improvement in refuelling. Instead of every 10 days, I've managed to eek it out to every 14 days.

I also calculated fuel consumption along various routes from work to home. Basically found that although Inhad higher mpg along the motorway route (stayed at 60mph), the length meant I still used more fuel than if I had gone via the busiest but shortest route. Consequently I went from filling every 7 days to every 10 days.

As I understand it, if you back off the accelerator (in gear), regardless of whether you are going down hill or not, the engine management system will cut off the fuel to the engine (because the motion of the vehicle is essentially turning the engine over) so you will be using nothing. If your car has an instantaneous fuel consumption readout you can see this as it will jump up to 99.9 mpg or go blank. The gear you are will dictate how quickly you slow down.
If you put the clutch in or put the gearbox in neutral the motion of the car is no longer turning the engine so it will have to consume some fuel to keep going.

So, in answer to your first question, you will use no fuel while you have your foot off the accelerator (or it is positioned at a point that's below the current engine speed) however, in 3rd gear you will slow down a lot faster than in 6th gear. If you have a clear view of the road ahead it's generally more fuel efficient to control your speed using the gears and the accelerator rather than breaking and having to build up speed again afterwards.

Having been watching the instantaneous consumption readout I believe the answer to the second question is yes as the engine will be trying to accelerate the vehicle by using more fuel. If you put your foot down and nothing happens it's usually a good indication that you need to drop a gear.

It all assumes that he has a modern car.


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He talks about 6th gear, to the assumption has basis.

Mark

Aha, I hadn't realised that, but of course! I always try to engine break as much as possible in heavy traffic. I like the raw egg analogy, I have been doing that on the motorway but I've just been trying to put as least weight on the accelerator as possible while maintaining 60mph or so. I went from being the fastest onthe motorway on the way home to being probably about 5th slowest, and it's saved me a tenner in a week, not bad! Not to mention the added years on my life for being less stressed and chilling listening to the radio at a constant speed all the way home, so relaxing!

Car isn't THAT new, 2003 Toyota Corolla, the 6th gear is my favourite, it's a fantastic cruiser and will do until I can afford a TDI Passat or something.

It's a Diesel then? I have a 2004 petrol corolla and it only has five gears.

I can't afford a new one, so I'm glad it's still going strong despite the 134k miles on the clock.

Most cars for the last dozen or more years have employed fuel cut-off on the overrun to improve economy. It isn't that new.

Keep an eye on tyre pressures too. If I start to notice my fuel economy dropping off, I check my tyres.

Ah yes, it's 2001... petrol.

If your car has fuel injection it will shut off when you decelerate, if your car has a carburettor it was new in the mid eighties at the latest.

It has electronic fuel injection because Google says so. my car, courtesy of wikipedia, it's a european sporty version with 6 gears apparently, base for the WRC rally car http://en.wikipedia.org/wiki/File:Toyot ... chback.jpg

So I have a newer version of the same car but with less gears, how the [LIFTED] does that work?
Is yours the sports model? That's the only thing I can think of to account for the extra gear with them both being petrol.
I wish mine had a sixth gear. It drinks petrol if you drift over 70 on the motorway.

Dunno, I didn't think it was a sporty version but wikipedia seems to think it is if it has 6 gears, it's a 1.6 and comfortable keeping up with Audi [LIFTED] in the BMW lane.

Must have been a special edition. The sports version was a 1.8