I heard that running the engine in a gear where its lowest at its RPM before it starts labouring will maximise fuel economy. Supposedly because you're getting the highest speed for the lowest reasonable revs. Might be BS though.

Not true. The idea is to use as little throttle as possible (or safe to do so) and use the gears accordingly. The more you've got your foot down, the more fuel you're using, so if you've got it in 5th at 20mph you may be at low rpm but you'd have to have your foot right down which would use lots of fuel rather than very light throttle in say, 3rd gear.

Avoid slowing down quickly, leave plenty of space in front of you so you can accelerate as much as is needed then use engine braking to slow down.

I don't understand. The engine pulls its fuel in on the induction stroke, and the carb or injection system mixes the fuel and air to the proper ratio for the air mass flowing into the engine. So at low RPM, it is pulling through less fuel/air mixture into the engine, and therefore less fuel. If it sent more fuel into the engine at lower RPM at fully open throttle, surely that would mean it's either running very rich or somehow using forced induction?

couldnt be bothered to read through 3 pages so i dont know if it has been said, but you can always cut a hole in the floor and drive like the flint stones

I don't understand. The engine pulls its fuel in on the induction stroke, and the carb or injection system mixes the fuel and air to the proper ratio for the air mass flowing into the engine. So at low RPM, it is pulling through less fuel/air mixture into the engine, and therefore less fuel. If it sent more fuel into the engine at lower RPM at fully open throttle, surely that would mean it's either running very rich or somehow using forced induction?

it sends more fuel in by way the engine complete more cycles in the same time. if an engine was running at 1000rpm for 1 min and used 10ml of fuel (for arguments sake) it will use less fuel than one running at 5000rpm for one minute because there will be 5 times as much fuel put into the engine, so in the same minute it would inject 50ml. no major changes to the stoichiometric ratio.

The engine always runs under vacuum, even at very low revs if you put your foot down and open the choke you'll put more air into the engine and so more fuel.

The combustion mixture is better mixed on a light throttle when at low revs, so it burns more efficiently.

You have more power for the amount of fuel you're using if you drop down a gear sometimes, it'r not running efficiently if it's struggling (and labouring at very low revs also damages the engine).

it sends more fuel in by way the engine complete more cycles in the same time. if an engine was running at 1000rpm for 1 min and used 10ml of fuel (for arguments sake) it will use less fuel than one running at 5000rpm for one minute because there will be 5 times as much fuel put into the engine, so in the same minute it would inject 50ml. no major changes to the stoichiometric ratio.

Yeah, that's what I was getting at

The engine always runs under vacuum, even at very low revs if you put your foot down and open the choke you'll put more air into the engine and so more fuel.

The combustion mixture is better mixed on a light throttle when at low revs, so it burns more efficiently.

You have more power for the amount of fuel you're using if you drop down a gear sometimes, it'r not running efficiently if it's struggling (and labouring at very low revs also damages the engine).

I see how that works. I used to cruise at 30mph in 4th gear in my Mk2 5 speed, does that sound like a bad idea?

The engine always runs under vacuum, even at very low revs if you put your foot down and open the choke you'll put more air into the engine and so more fuel.

The combustion mixture is better mixed on a light throttle when at low revs, so it burns more efficiently.

You have more power for the amount of fuel you're using if you drop down a gear sometimes, it'r not running efficiently if it's struggling (and labouring at very low revs also damages the engine).

Not true- you have a far greater chance of fuel dropout at low gas speeds especially when running carbs, or a low pressure (port injected) efi system.

What you need is more swirl (and tumble) to achieve a greater percentage of fuel being atomised, and a more even vapourisation in the charge- leads to a more efficient burn, and in turn greater economy.

This is why most OEMs now have tumble flaps fitted- and what EcoTek always claimed to do with their 'bleed' valves (although they could never alter the physical hardware of the port runner.

For fuel efficiency you're best off using the engines torque band, then at peak torque speed, change gear. Regarding booting it at low rpm, not good- engines often have low VE at this speed, as such have poor BSFC, especially if being enriched due to knock limitations or on boost transients, better to pick the correct gear and a more efficient operating condition.

For best fuel economy out of your current car, make sure the engine is in good health with ignition and cam timing optimised, with a regular service, and tyres at the correct pressure.

If you really need economy, get a light diesel. They really are more economic than Petrols from a scientific viewpoint, from a financial one depends on the car and your driving style. My Audi averages 47mpg, even when cruising at speed- current VAG TSFI gasoline engines aren't too far behind this now.

^^ yep. My middle paragraph was in agreement, maybe written badly - if you're driving at very low revs, a near closed throttle will agitate/tumble the air more and improve economy. Opening the throttle when going uphill won't speed the car up because it can't pull it, but it'll use more fuel as the air speed is slower and smoother, so less agitation. Which is why i stated earlier that i drop down, and you're better off doing so, if you need more than 'hold speed' from the car.

Different fuel systems can vary slightly here even on the same fuel, with 38mm bike carbs not being 38mm at all when driving at 1500rpm on a half open throttle - they're not even 10mm.

Cable MPi can govern fuel whilst keeping it a spray, but not air speed. Whilst fly-by-wire can control air speed too (typically).

Fuel drop is a problem limited to old technology really, spi and car carbs having the weakness with low air speed.

Todays governed diesels are getting impressive, but Peugeot's latest 1.6d is a good example and it failed me. To be fair though, the little 207 was actually a huge 1385kgs!

Anyway to the point, you only need two mirrors by law, this includes the rear-view mirror. So take one off, less drag, better fuel economy. You'd be a fool not to!

So turns out I'm wrong the requirement is:

Obligatory mirror options are:

a. an exterior mirror fitted to the offside (right-hand side when seated in the driver’s seat), or

b. an exterior mirror fitted to the nearside (left hand side when seated in the driver’s seat), or

c. an interior mirror.

Goods vehicles are required to have two.

So blindness verses fuel economy.

mopeds can have no mirrors.........

mopeds can also be very cold noisy and uncomfortable and also slow

With taking off mirrors, I don't think it's worth it. Just take shit you dont need out of the boot.

Or just take a shit if you're going to get that anal about weight saving..

mopeds can also be very cold noisy and uncomfortable and also slow

point taken.

my old vespa used to do 120mpg mind.

Or just take a shit if you're going to get that anal about weight saving..

Not for weight saving. But to reduce drag! Stick your hand out the window at 60mph, you know what I'm saying..