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#4777375 - 06/04/18 03:51 PM Speed / RPM / Fuel Economy
NICAT Offline

Registered: 04/05/18
Posts: 64
Loc: Azerbaijan
Why driving at higher speed (Even if we drive at the highest gear at low rpm) cause more fuel consumption?
Can you please tell me how to calculate (predict) the most optimum speed & rpm ranges for fuel economy ?

Edited by NICAT (06/04/18 03:52 PM)

#4777380 - 06/04/18 03:54 PM Re: Speed / RPM / Fuel Economy [Re: NICAT]
dbias Online   content

Registered: 12/24/14
Posts: 177
Loc: USA
Aerodynamic drag. In the US chasing the all important epa mpg numbers the sweet spot is around 55 mph as the test drive cycle has a lot of that so engineers are rewarded for optimizing gearing for that speed.

Edited by dbias (06/04/18 04:23 PM)
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#4777384 - 06/04/18 03:56 PM Re: Speed / RPM / Fuel Economy [Re: NICAT]
E365 Offline

Registered: 12/16/07
Posts: 479
Loc: USA
Generally, drag increases at the square of velocity. So, when your speed doubles, the drag your car has to push against goes up by 4x.

I’ve heard it said that best fuel economy will be in top gear, just above the speed where the engine is lugging. For many vehicles this would be very roughly 40-45 MPH (65-73 km/h). ** I have no factual data to show this correct, but it seems accurate from my observations. **

Edited by E365 (06/04/18 04:01 PM)
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#4777404 - 06/04/18 04:09 PM Re: Speed / RPM / Fuel Economy [Re: NICAT]
VeryNoisyPoet Offline

Registered: 12/16/17
Posts: 350
Loc: Massachusetts
As the vehicle moves faster, it must push the air out of the way faster, which causes more drag and takes more engine power. This is not linear, with the equation being 1/2 x air density x drag coefficient x cross sectional area x speed^2. So a small increase in speed can cause a large increase in drag. Tires will also cause more drag with higher speed, but this is linear and quite small compared to air drag.

Inside the engine and transmission, parasitic load from the water pump, oil pump, fan, and internal friction consume more energy at higher RPM. Additionally, an engine has to work harder to overcome its own air pumping losses at higher speeds. But an engine operated too slow and out of its power band will need to work harder and require frequent changing of gears, which reduces economy.

Every engine and vehicle will be different, so often it is best to experiment with different speeds. In general, there will be a sweet spot where the engine has enough power to cruise without bogging down but RPM is not high enough to cause excessive drag. For example, my automatic transmission car has a fuel economy peak between 55 and 65 mph (1980 to 2340 RPM). This is a sweet spot for boost, torque, and efficiency in my case, where the torque converter can remain locked 99% of the time. Below those speeds economy tapers off slowly because of unlocking converter and more frequent gear changes; above those speeds economy drops sharply as both engine drag and air drag increase.

A manual transmission gives much better control and has no torque converter or pumping losses like an automatic, which is why manual transmission versions of the same vehicle will often get significantly higher fuel economy when handled by a skilled driver.
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#4777412 - 06/04/18 04:15 PM Re: Speed / RPM / Fuel Economy [Re: NICAT]
WobblyElvis Offline

Registered: 09/23/14
Posts: 1424
Loc: Toronto Canada
A fun calculator dealing with drag and mileage can be found here...

#4777414 - 06/04/18 04:19 PM Re: Speed / RPM / Fuel Economy [Re: NICAT]
Trav Offline

Registered: 11/20/06
Posts: 19658
Loc: MA,
BMW some years ago said the engines are most efficient at 2,900 RPM. Right or wrong I have no idea.
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#4777425 - 06/04/18 04:27 PM Re: Speed / RPM / Fuel Economy [Re: NICAT]
JAG Offline

Registered: 10/23/05
Posts: 4930
Loc: Fredericksburg, VA
Being in top gear a small amount above the lowest speed that the engine doesn’t feel like it’s lugging is a good rule of thumb for best fuel economy, as E365 said. That will be too slow for some roads/highways. Avoiding brisk or hard acceleration will help because the air-fuel ratio is likely enriched to avoid detonation. Also with automatic transmissions, the shift points will move to higher rpms.

If you want to get more precise than the rule of thumb, you should try to find the brake specific fuel consumption (BSFC, see link below) data for your engine and use that along with some calculations to find out how to drive most efficiently. If you have a real-time readout of your gas mileage, you can experiment with driving on flat ground to see which gear gives best gas mileage at different speeds or different speeds in the same gear.

#4777430 - 06/04/18 04:34 PM Re: Speed / RPM / Fuel Economy [Re: NICAT]
JLTD Offline

Registered: 12/15/17
Posts: 942
Loc: US
Originally Posted By: NICAT
Why driving at higher speed (Even if we drive at the highest gear at low rpm) cause more fuel consumption?
Can you please tell me how to calculate (predict) the most optimum speed & rpm ranges for fuel economy ?

Minimize aerodynamic drag and maximize engine efficiency for that speed. It may not be the highest an example my Toyota gets 30+mpg in 4th gear(it is a 5 speed auto) at 40mph/65kph; that is the highest I have observed.
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#4777463 - 06/04/18 05:04 PM Re: Speed / RPM / Fuel Economy [Re: Trav]
wha232 Offline

Registered: 04/04/10
Posts: 24
Loc: PA
My Honda Fit has about a 3-4 mpg difference between 70 mph and 60 mph

#4777490 - 06/04/18 05:47 PM Re: Speed / RPM / Fuel Economy [Re: NICAT]
ZZman Offline

Registered: 03/17/08
Posts: 5911
Loc: Michigan
If I hold my hand out the window making wavy motions my mpg goes down a little. If I hold my hand out in a open palm position it goes way down.
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#4777497 - 06/04/18 05:59 PM Re: Speed / RPM / Fuel Economy [Re: NICAT]
Uphill_Both_Ways Offline

Registered: 11/06/17
Posts: 75
Loc: Winnipeg
My '08 Impala with GM's 3.5-litre engine returns five to 5.1 litres per 100 kilometres, or 55 to 56 mpg imperial at 70 km/h (42 mph) on billiard-table-flat ground (which we have plenty of) in summer temperatures and no wind. The engine turns over at 1,100 or 1,150 rpm (the tach is too course for an exact reading) in fourth (top) gear.

This drops to 40 to 42 mpg imperial at 100 km/h (62 mph) under the same conditions, then drops precipitously at 110 to 120 km/h. At 120 it returns 26 to 27 mpg imperial, thanks to all that speed-manufactured wind.

I have no feel for U.S. gallons, which Canada didn't use pre-metric. The speedo, odometer and fuel-usage computer can be switched to "English" for travel in the States, but the mileage computation is in U.S. gallons, despite the "English" setting. So I use a laminated cheat sheet.
2008 Impala 3.5

#4777509 - 06/04/18 06:10 PM Re: Speed / RPM / Fuel Economy [Re: ZZman]
bbhero Offline

Registered: 03/20/15
Posts: 4842
Loc: Virginia
Funny post ZZ LOL

But your pain index goes up to 10 if a hornet or wasp happens to hit your open palm going down the road.
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#4777523 - 06/04/18 06:22 PM Re: Speed / RPM / Fuel Economy [Re: NICAT]
fdcg27 Offline

Registered: 09/25/09
Posts: 15919
Loc: OH
It requires more power to go faster regardless of engine revs.
To make more power the engine must burn more fuel.
Total drag is the difference and it increases with speed.
This seems intuitively obvious.
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#4777579 - 06/04/18 07:24 PM Re: Speed / RPM / Fuel Economy [Re: NICAT]
CR94 Offline

Registered: 03/20/16
Posts: 1289
Loc: Western S.C. since 1996
The basics:

Aerodynamic drag is proportional to the square of speed, therefore the energy required to overcome it for a particular distance is too. That also means power required to overcome aero drag is proportionate to the cube of the speed.

At low speeds, aero drag, which is almost independent of speed, is smaller than tire drag. (The speed at which they're equal obviously varies with the vehicle, but is typically around 35-40 mph.)
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#4777591 - 06/04/18 07:39 PM Re: Speed / RPM / Fuel Economy [Re: NICAT]
supton Online   content

Registered: 11/09/08
Posts: 12344
Loc: NH
Measure and repeat.

I have not spent the time to verify but I swear, my truck does better at 45-50mph and in 5th than it does at 6th at 60mph. Wind drag goes up fast, and worse, it won't hold a hill in sixth at only 60mph. And it just can't do 50mph at sixth (though it tries). But in general, slower speed in topmost gear is best. As is only going downhill with a tailwind.

You can get a Scanguage (not sure if the app Torq does likewise) and have instant mpg readout, if your car lacks one. That can give some real time feedback, faster than the standard "do something over three tanks".
2011 Toyota Camry, base, 2.5L/6MT, 163k, hers
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