Optimum Highway Cruising Speed For Fuel Consumption

[sundaypsychos]

Driving from Bangkok to Chiang mai a few days ago It seemed to me I was getting better fuel economy at speeds marginally over 120km/h than if i were to hover about the 115km/h mark, is this possible or have any kind of technical explanation or was I just imagining things?

Truck is a 3litre turbo toyota vigo with no modifications.

What speeds do you guys find you get the best fuel economy at whilst retaining a decent highway pace? (i.e not including those of us who dawdle along at 80km/h.)

[MoonRiverOasis]

There's no one magic speed - different road surfaces and even different ambient temps will change the 'optimum' highway cruising speed for every vehicle. That said, the slower you go with the lowest RPM will always get the best fuel economy result.

[gregb]

That is a very tough question. My Toyota Altis gets the maximum fuel economy on a flat surface between 99 - 103. It is a function of the fact that I have a very low Cd (.29) and pump the tires up to 38 psi for highway driving.

As a general rule of thumb, any car will get the best brake specific fuel consumption at about half its maximum RPM. Since for most cars this is about 6000, you should expect your engine to peak right around 3000. However, as you go faster the drag on the car goes up by the square of the velocity. So at some point, usually before you actually reach 3000 RPM in your top gear, the efficiency lost to atmospheric friction has overwhelmed the tiny increase in brake specific fuel consumption you get from your engine.

In my case, because of the low Cd of my car, that happens around 2600 RPM. At a higher altitude my optimum speed would increase slightly.

As a sanity check, you will never achieve optimal fuel efficiency if you are running above 50% of your maximum RPM (unless you have a very special engine which was optimized to be underpowered for your vehicle and run constantly at high RPM.) I find it hard to believe your fuel economy went up by increasing speed from 115 - 120 since the brake specific fuel consumption curve is usually a monotonically decreasing function after the peak, and that speed is usually well above the peak.

However, it is possible there is some kind of resonance which is created on your vehicle right around 115. If that is the case, you would probably get better efficiency dropping back to 110 vs. increasing to 120. For most older cars (20+ years) with a high Cd, optimum speed is around 80. For more modern, aerodynamic cars, optimum is around 95 - 100.

I would check your figures again, because I don't believe a Vigo should be getting optimum economy over 100. On a vehicle with a high Cd like that your best economy should be somewhere in the low 90's.

[katabeachbum]

There's no one magic speed - different road surfaces and even different ambient temps will change the 'optimum' highway cruising speed for every vehicle. That said, the slower you go with the lowest RPM will always get the best fuel economy result.

Vigo 3,0d4d auto 4 door 4x4, 100 kmh is like 11-12 km/L, while 160kmh is like 8-9km/L at 3000rpm. between 1400 rpm and 3400 rpm is max tourque band, so you get milage according to speed

a lid or vinyl cover for the bed improves milage above 60 kmh

[mdechgan]

The best fuel economy is when the car is in its highest gear but slow enough to be within the optimum torque brand.

I would say around 2,000 rpms in the top gear. Maybe 80kmh which is the actual speed limit in Thailand.

[Loz]

I agree.

I used to thrape my cars in the UK and never gave a thought to fuel consumption. Here I have found I drive at 2500rpm in Top most of the time as I have a highway commute. I drive at 90kph and if I go up to 3000rpm I am doing 106kph. I never go over this. Sometimes if I don't pay attention I find I am doing 2200rpm around 80 kph... I get great milage from my 3.0 Toyota sportrider. When I go back home I will never get a speeding ticket again!

Amazing Thailand!

[katabeachbum]

The best fuel economy is when the car is in its highest gear but slow enough to be within the optimum torque brand.

I would say around 2,000 rpms in the top gear. Maybe 80kmh which is the actual speed limit in Thailand.

different raods, different speedlimits, but max 120 is always accepted on highways.

OPs max tourque starts at 1400 rpm, but thats rather slow

[Hog Head]

Max fuel economy occurs at the engine torque peak - in my case 1500 rpm and most small pickup diesel engines will be similar. If this is too slow for you then a faster road speed will burn more fuel.

[aussiebebe]

There's no one magic speed - different road surfaces and even different ambient temps will change the 'optimum' highway cruising speed for every vehicle. That said, the slower you go with the lowest RPM will always get the best fuel economy result.

I agree, that's the best answer - the most efficient speed changes all the time and is affected by every possible factor. I noticed this when I got a plug-in device called a 'Kiwi MPG' for my older Toyota (unecessary for most modern cars display fuel consumption on the dash as standard). Optimum speed depends on fuel load, passenger load, tire psi, surface, incline, weather, wind, air con/stero useage - everything.

[otherstuff1957]

I always keep track of the fuel usage in my 2.5 liter Isuzu Station Wagon (stick shift): at 90kph - 15kpl, at 100kph - 14kpl, at 110 - 13kpl and the one time I kept it down to about 80 (upcountry roads in Sukothai and Prae) I got 16kpl.

I usually cruise at 90 - 100.

[mdechgan]

I really think the best is 80kph. Any faster and drag and wind resistance comes into the equation.

[katabeachbum]

I really think the best is 80kph. Any faster and drag and wind resistance comes into the equation.

Ultimate milage depends on gear box/ ratio. VIGO 3,0 comes with 3 different ratios for 5 speed manual and one for auto. 1400 rpm/max tourque in top gear provides optimal milage, and this would be 60-75 kmh.

VIGO 3,0 4x4 auto 4 door really amazes, loaded 800kg with 2 Ninja 650R/passangers and gear at 160 kmh at 3000 rpm managing 8-9 km/L. Not many other vehicles similar spec can manage similar milage at this speed

[Big Guns]

I think somewhere between 100-120 kph is best for fuel consumption and making different progress. Unfortunately I go for making decent progress ahead of maximising fuel consumption and cruise at about 140 kph In terms of the best fuel consumption for pickups D-Max rules the roost.

[katabeachbum]

In terms of the best fuel consumption for pickups D-Max rules the roost.

Coconut cruisers, 2door pickups loaded with 1500kg coconuts doing Surat Phuket Surat together once a week, one 2,5 Mazda BT50, one DMax 2,5 and one Vigo 3,0, all low spec 2008, high end ratio. BT 50 rules on milage at 80-100 kmh.

[Big Guns]

That's a strange test you've brought up Kata. I don't know if it will catch on. Maybe JD Power will incorporate the coconut test in their customer satisfaction survey. Apart from your coconut test it seems to be an accepted fact that D-Max has the best fuel consumption.

[katabeachbum]

That's a strange test you've brought up Kata. I don't know if it will catch on. Maybe JD Power will incorporate the coconut test in their customer satisfaction survey. Apart from your coconut test it seems to be an accepted fact that D-Max has the best fuel consumption.

Its not my test, its farmers I have known for 7 years. The Mazda owner has 120 baht more in his pocket on each trip, or save 950 baht a month on fuel, thats 10% higher net income. The simplest test possible, same driving, same load, same dieselpump early morning to avoid hot diesel.

Wonder which Dmax spec you are refering to as accepted fact, or is it only hearsay from internett? Engine, transmission, ratio, wheels, body, load?

Most farmers use 2,5, and all the mentioned are 2008 commonrail. 5 speed manual, highest ratio, stock small steel rims front, reinforced rear springs, wide high loadindex reartyres steel rims. Must be a million of these in LOS

On occations a 2005 DMax and an older Toyota Tiger goes, needs much more fuel than the others. Different engine and ratio.

Notice, as most farmers do, vehicles are overloaded with more than 50%

[Big Guns]

Its really difficult to find actual like for like performance and fuel consumption figures for Thai built pickups. I just did a comparison of top spec 4x4 pickups on 'What Car'. They were for combined mpg/0-60 mph/top speed mph

Hilux Vigo; 34/11.8/109

D-Max; 33.6/10.3/109

Navara; 28.8/11.9/105

Please note these figures are in miles per gallon and miles per hour

[katabeachbum]

Its really difficult to find actual like for like performance and fuel consumption figures for Thai built pickups. I just did a comparison of top spec 4x4 pickups on 'What Car'. They were for combined mpg/0-60 mph/top speed mph

Hilux Vigo; 34/11.8/109

D-Max; 33.6/10.3/109

Navara; 28.8/11.9/105

Please note these figures are in miles per gallon and miles per hour

hmm, not mentioning any specs, not even if its manual or auto, and not if its loaded or not (2000 pund difference), but anyway not same engines as in LOS. UK is presently Euro IV, and VIGO 3,0 comes with 2 different EuroIV outputs depending on which market its sold in, and 2 different autos

In some European countries they should in addition all be restricted to 100mph/160 kmh top speed

For Vigo and Navara 0-60 and top speed is far better in LOS.

Edited July 19, 2010 by katabeachbum

[Big Guns]

In answer to the original posters question check out the following thread

http://auto.howstuffworks.com/fuel-efficiency/fuel-economy/question4771.htm

It seems that the optimum speed to maximise fuel efficiency is somewhere between 40-60 mph. I can't be bothered to do the kph conversion, but I guess 70-100 kph. Hope this helps.

Apparently the D-Max can get 49.7 KM/L, but not if its driven by me.

[katabeachbum]

In answer to the original posters question check out the following thread

http://auto.howstuff...uestion4771.htm

It seems that the optimum speed to maximise fuel efficiency is somewhere between 40-60 mph. I can't be bothered to do the kph conversion, but I guess 70-100 kph. Hope this helps.

Multiply by 1,6 and you get kmh from mph:rolleyes:

Apparently the D-Max can get 49.7 KM/L, but not if its driven by me.

and Vigos have been claimed capable of doing Chiang Mai to Phuket, more than 1500 km, on one tank containing 75 litres. But not if its driven by me.

seems like mileage is the same though

[sundaypsychos]

interesting stuff, i asked purely out of interest after watching the mpg gauge on the truck bouncing around as i got on/off the accelerator. personally i could never cruise at these optimal fuel economy speeds! I think making decent progress has to come into the equation somewhere and thats why i find about 140km/h is usually what i hit.

[katabeachbum]

interesting stuff, i asked purely out of interest after watching the mpg gauge on the truck bouncing around as i got on/off the accelerator. personally i could never cruise at these optimal fuel economy speeds! I think making decent progress has to come into the equation somewhere and thats why i find about 140km/h is usually what i hit.

agree, LOS commonrail pickups run very inexpensively at 140-160kmh. Impressing.