what speed waste less gas??

I would hope you drive the streets. Also, I really think you're question has minute consequences that won't matter. You're talking about a 500 RPM difference.

Black Stang 97 said:

ok i drive to work by streets, do i waste more gas driving at 40 in 4th gear at 1.8k rpm or driving 40 in 5th at 1.3k rpm?? also, is it true when engine braking, no gas is being burned above 1.5k rpm??

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rpm is not the whole cause of it, it's to do with throttle position.

If you let the engine labour and end up giving it a fair boot full a lot of fuel will be injected even though due to the rpm range not much speed will be generated.

If the throttle is open at say 8% then yes the engine should be more fuel efficent at lower rpms. But that's not the whole story, as the engine is not a stationary item, so minutes per gallon matters little. What you're after is miles per gallon, so a balance of speed is involved. You aim is not how long the engine will run on a tank of petrol but how far you can travel.

And yes, with the throttle plate closed no fuel should be being injected into the engine, so coasting in gear down hills or up to junctions can save you fuel, although it doesn't quite work the same for autos due to how automatic gearboxes work.

300bhp/ton said:

And yes, with the throttle plate closed no fuel should be being injected into the engine, so coasting in gear down hills or up to junctions can save you fuel, although it doesn't quite work the same for autos due to how automatic gearboxes work.

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Although I'm no genious, I have to disagree. An internal combustion engine takes gasoline to run, and when you engine brake, the engine is still running, regardless of the fact that all it's doing is slowing you down.

If perhaps you're thinking of a deisel engine which uses compression to slow you down, then you'd be absolutely correct.

I wanna hear more about this engine braking related to fuel. Can anyone clarify this?

The way I see it, The more throttle you apply, the more fuel that is consumed, because you're allowing your EFI to draw MORE FUEL.

The lower the RPM's, the less fuel the you are burning. In Neutral if you rev at 2000 RPM's, you will expire your fuel in a shorter time than at idle speed.

To conserve fuel, you would want to have the lowest RPM's at idle, or at rolling speed... PERIOD.

Downshifting/engine braking is only using compression to reduce miles per hour... it's not using fuel, unless your foot is on the accelerator.

To conserve, you should run her up through the gears at the lowest RPM's possible for a shift AND switch gears before RMPS get so high that you're fueling RPM's instead of miles per hour.

It's no fun driving like that and you can under-rev the motor, which can be more harmful than the added cost of fuel.

I would suggest running it up to 1.5k rpm between shifts and pretending that an egg is below the pedal.

It will save fuel, but the ride will suck!

Generally, 50-55mph is usually what will get you the best MPG highway and is what the EPA uses when they give those inflated MPG figures.

300bhp/ton said:

And yes, with the throttle plate closed no fuel should be being injected into the engine, so coasting in gear down hills or up to junctions can save you fuel, although it doesn't quite work the same for autos due to how automatic gearboxes work.

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Huh?
So, when you start the car with the throttle plate closed, it uses no fuel?
OR, when coasting in gear (or out of gear for that matter) downhill, does the engine magically stay running with no fuel?

Didn't we address this whole issue a week or so ago? This is so silly. The answer to the thread is 'the lowest RPMs you can go' will net you the least amount of fuel used. Not rocket science here people. And please save your breath if you want to split some technical hair that may save another 1/100th of a gallon of fuel per million miles...it just doesn't matter. The best mileage, when cruising at highway speed, is achieved in the top gear.

When you idle your engine, will it not run out of gas eventually?

I've always wondered if i used more gas when i used the engine to slow me down compared to putting it in neutral and letting the brakes do the work.. ya'll are confusing sometimes haha

sgarlic said:

Although I'm no genious, I have to disagree. An internal combustion engine takes gasoline to run, and when you engine brake, the engine is still running, regardless of the fact that all it's doing is slowing you down.

If perhaps you're thinking of a deisel engine which uses compression to slow you down, then you'd be absolutely correct.

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they work the same in many respects.

If fuel was still being injected then you would not slow but accelarate.

Think about it, with no fuel an engine is just a giant air pump, sucks in air, compresses it and exhausts it. Hence you have a different exhaust note when throttling down.

twogts4us said:

Huh?
So, when you start the car with the throttle plate closed, it uses no fuel?
OR, when coasting in gear (or out of gear for that matter) downhill, does the engine magically stay running with no fuel?

Didn't we address this whole issue a week or so ago? This is so silly. The answer to the thread is 'the lowest RPMs you can go' will net you the least amount of fuel used. Not rocket science here people. And please save your breath if you want to split some technical hair that may save another 1/100th of a gallon of fuel per million miles...it just doesn't matter. The best mileage, when cruising at highway speed, is achieved in the top gear.

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1st off rocket science is actually very simple, in it's basic concept. Rocket motors where developed YEARS before internal combustion petrol engines, even before steam.

As for the engine running at idle or coasting in N. Yes here it requires fuel, as it is the combustion that foces the cylinders down generating rotary motion.

When coasting in gear, the engine is connected to the wheels via the gearbox, which are in contact with the ground. Gravity, or rather the stored potential energy being turned into kenetic energy as the car coasts turns the engine over, thus there is no need for any fuel. This works on carb engines too.

In my TR7 I've improved the MPG over the same route by 3mpg by imploying this technique over just driving gentley.

And if you read my 1st post, I explained that the lowest rpms may not be the most economical, as in this respect we are interested in how many miles per gallon not how many minutes the engine will run with a gallon of fuel.

If you're not sure take it all the way back to the start.

The engines most economical mode is at idle, it will run longer here in minutes than anywhere else in the rpms. But you MPG is worse than bad, in fact it would be ZERO because the car wouldn't be moving. You could use 12 gallons of fuel (which would take a long time, probably over week constant running at idle) but you would still have 0mpg.

The balance is to drive as quick as you can while maintaining minimal throttle postition, You may notice that once up to speed (55-60mph) in mnay cars you can almost completly lift your foot off the pedal, only the slightest pressure is required to maintain speed.

At 40mph, a similar amount of throttle opening is probably required, so almost the same amount of fuel is being injected into the cylinders, but at 40mph as opposed to 50mph you'll cover less distance for the same fuel.

Anything above 55mph in most modern cars and aerodynamic drag will start to have a bigger impact on MPG.

Usually 50-55mph is considered an optimum speed range as it balances speed with aerodynamic drag. This is why you'll find lots of Police cars traverling at this speed (in the UK anyhow).

wow nice write up and i do agree that having the car be in lower rpm's at certain speeds is not always economical

0000mph would be my guess

All you really need to know is this:

We drive Mustangs. If we were that concerned about wasting gas, we'd be driving Honda Accord's.

Redsnake2 said:

0000mph would be my guess

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True if the engine is not on. If the engine is on then you get 0mpg

300bhp/ton said:

1st off rocket science is actually very simple, in it's basic concept. Rocket motors where developed YEARS before internal combustion petrol engines, even before steam.

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"This isn't rocket science" is a figure of speech, not to be taken literally...

300bhp/ton said:

As for the engine running at idle or coasting in N. Yes here it requires fuel, as it is the combustion that foces the cylinders down generating rotary motion.

When coasting in gear, the engine is connected to the wheels via the gearbox, which are in contact with the ground. Gravity, or rather the stored potential energy being turned into kenetic energy as the car coasts turns the engine over, thus there is no need for any fuel. This works on carb engines too.

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So you are saying if you are in gear and coasting, the engine just free wheels??? And then if I push the clutch in, it's magically starts running (on fuel) again??? Come on...

i find that the most efficient rpm the 4.6 likes to be in is 2-2250 rpm's. and its favorite gear is 4th. on city roads i keep the motor in those rpm ranges when cruising along, and i find it gives me great mileage, cause its the fastest speed with the least throttle applied. 5th gear is way too low and should only be used in 65+mph on the freeway.

i mainly drive backroads, so i do 50 in 4th gear at 2200rpm, and the whole way the needle doesnt move. the second i go into 5th gear and bog the motor, you can tell it starts sucking up the gas, cause to stay at 50mph at 1300rpm in 5th gear it requires more throttle to maintain speed.

twogts4us said:

"This isn't rocket science" is a figure of speech, not to be taken literally...


So you are saying if you are in gear and coasting, the engine just free wheels??? And then if I push the clutch in, it's magically starts running (on fuel) again??? Come on...

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you don't have to believe me, but why not do some research and look it up! This is not a new concept, internal combustion engines have been fairly common place since the early part of last century.

Then come back and call BS if you like?

There is ALWAYS fuel being burned. Until you turn the ignition source off, especially in a carbureted engine.