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Speed / RPM / Fuel Economy #4777375
06/04/18 03:51 PM
06/04/18 03:51 PM
Joined: Apr 2018
Posts: 82
Azerbaijan
NICAT Offline OP
NICAT  Offline OP
Joined: Apr 2018
Posts: 82
Azerbaijan
Hi.
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 ?
Regards

Last edited by NICAT; 06/04/18 03:52 PM.
Re: Speed / RPM / Fuel Economy [Re: NICAT] #4777380
06/04/18 03:54 PM
06/04/18 03:54 PM
Joined: Dec 2014
Posts: 211
USA
dbias Offline
dbias  Offline
Joined: Dec 2014
Posts: 211
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.

Last edited by dbias; 06/04/18 04:23 PM.

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Re: Speed / RPM / Fuel Economy [Re: NICAT] #4777384
06/04/18 03:56 PM
06/04/18 03:56 PM
Joined: Dec 2007
Posts: 504
USA
E365 Offline
E365  Offline
Joined: Dec 2007
Posts: 504
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.

https://www.grc.nasa.gov/www/k-12/rocket/vel.html

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. **

Last edited by E365; 06/04/18 04:01 PM.

Whatever’s on sale...
Re: Speed / RPM / Fuel Economy [Re: NICAT] #4777404
06/04/18 04:09 PM
06/04/18 04:09 PM
Joined: Dec 2017
Posts: 412
Massachusetts
VeryNoisyPoet Online content
VeryNoisyPoet  Online Content
Joined: Dec 2017
Posts: 412
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.


1998 V70 GLT / 194k miles [10w-30 Valvoline Syn w/ Maxlife Tech + Mann W917 filter]

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Re: Speed / RPM / Fuel Economy [Re: NICAT] #4777412
06/04/18 04:15 PM
06/04/18 04:15 PM
Joined: Sep 2014
Posts: 1,459
Toronto Canada
WobblyElvis Offline
WobblyElvis  Offline
Joined: Sep 2014
Posts: 1,459
Toronto Canada
A fun calculator dealing with drag and mileage can be found here...
http://ecomodder.com/forum/tool-aero-rolling-resistance.php

Re: Speed / RPM / Fuel Economy [Re: NICAT] #4777414
06/04/18 04:19 PM
06/04/18 04:19 PM
Joined: Nov 2006
Posts: 20,099
MA, Mittelfranken.de
Trav Offline
Trav  Offline
Joined: Nov 2006
Posts: 20,099
MA, Mittelfranken.de
BMW some years ago said the engines are most efficient at 2,900 RPM. Right or wrong I have no idea.


ASE L1, Master. Deutsch Meisterbrief.
Re: Speed / RPM / Fuel Economy [Re: NICAT] #4777425
06/04/18 04:27 PM
06/04/18 04:27 PM
Joined: Oct 2005
Posts: 5,027
Fredericksburg, VA
JAG Offline
JAG  Offline
Joined: Oct 2005
Posts: 5,027
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.
https://www.eng-tips.com/viewthread.cfm?qid=363722

Re: Speed / RPM / Fuel Economy [Re: NICAT] #4777430
06/04/18 04:34 PM
06/04/18 04:34 PM
Joined: Dec 2017
Posts: 1,140
US
JLTD Online shocked
JLTD  Online Shocked
Joined: Dec 2017
Posts: 1,140
US
Originally Posted By: NICAT
Hi.
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 ?
Regards


Minimize aerodynamic drag and maximize engine efficiency for that speed. It may not be the highest gear....as 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.


I'm a thickie; assimilate
But 20s have their place

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Re: Speed / RPM / Fuel Economy [Re: Trav] #4777463
06/04/18 05:04 PM
06/04/18 05:04 PM
Joined: Apr 2010
Posts: 25
PA
wha232 Offline
wha232  Offline
Joined: Apr 2010
Posts: 25
PA
My Honda Fit has about a 3-4 mpg difference between 70 mph and 60 mph

Re: Speed / RPM / Fuel Economy [Re: NICAT] #4777490
06/04/18 05:47 PM
06/04/18 05:47 PM
Joined: Mar 2008
Posts: 5,988
Michigan
ZZman Offline
ZZman  Offline
Joined: Mar 2008
Posts: 5,988
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.


2003 Mercury Grand Marquis. Rotella T6 Multi-Vehicle oil
2008 Sebring Hardtop convertible. Rotella T6 Multi-Vehicle oil
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Re: Speed / RPM / Fuel Economy [Re: NICAT] #4777497
06/04/18 05:59 PM
06/04/18 05:59 PM
Joined: Nov 2017
Posts: 139
Winnipeg
Uphill_Both_Ways Offline
Uphill_Both_Ways  Offline
Joined: Nov 2017
Posts: 139
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
Re: Speed / RPM / Fuel Economy [Re: ZZman] #4777509
06/04/18 06:10 PM
06/04/18 06:10 PM
Joined: Mar 2015
Posts: 5,092
Virginia
bbhero Offline
bbhero  Offline
Joined: Mar 2015
Posts: 5,092
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.


Nissan Altima 3.5 Coupe
Quaker State High Mileage 5w30 Purolator Boss 14610
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Re: Speed / RPM / Fuel Economy [Re: NICAT] #4777523
06/04/18 06:22 PM
06/04/18 06:22 PM
Joined: Sep 2009
Posts: 16,108
OH
fdcg27 Offline
fdcg27  Offline
Joined: Sep 2009
Posts: 16,108
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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Re: Speed / RPM / Fuel Economy [Re: NICAT] #4777579
06/04/18 07:24 PM
06/04/18 07:24 PM
Joined: Mar 2016
Posts: 1,465
Western S.C. since 1996
CR94 Offline
CR94  Offline
Joined: Mar 2016
Posts: 1,465
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.)


2011 Toyota Prius now at 93K
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Re: Speed / RPM / Fuel Economy [Re: NICAT] #4777591
06/04/18 07:39 PM
06/04/18 07:39 PM
Joined: Nov 2008
Posts: 12,682
NH
supton Offline
supton  Offline
Joined: Nov 2008
Posts: 12,682
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, 169k, hers
2010 Toyota Tundra DC, 4.6L/6AT, 144k, ours
1999 Toyota Camry LE, 2.2L/4AT, 192k, his
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