Ecenomical Cruising Speed of a car

thegreatchimp

Does anyone have definitve figures on the ecenomical driving speeds of cars?

I drive a 1.6 litre ptrol mazda 323. I've been trying to save money on petrol by driving at a fuel efficient speed. However I've heard many different figures for this "magic" speed. Originally I was told 70mph (110kmh), but other people swear it is as low as 56mph (90kmh) and even 50mph (80km/h). That's a pretty massive variation.

I realise there's will be a variance based on engine size and power to weight ratio, but I would have thought that calculating this would be fairly straight forward; but amazingly I haven't been able to find anything definitive on the net. Not even road authorities and car manufacturers seem to know, and posts on forums are just as varied as the above figures. Surely there's some mechanic or engine expert out there that can shed light ont this...

Any replied appreciated.Thanks

Sean

johndaman66

In a way I think your sort of answering your own question. It very much depends on the car and the engine, load on the car, gradient of the road etc etc.

A car will have a sweet spot at which economy is maximised. Above this engine speed economy will be compromised but below this engine speed you may be labouring the engine, again compromising economy but also placing addional pressure on engine bearings and possibly other components.

I couldn't tell you the sweet spot of of a 1.6 Mazda 323 but certainly wouldn't think its as high as 70 mph. If its a naturally aspirated engine I'd hazard a guess of between 50 and 60 mph approx.

Mycroft H

55MPH in my Clio. It's been the same by ±5MPH in every other car I've driven. Bearing in mind I've not driven a 6sp yet.

Definitely not 70mph, when your at that speed most of the engines effort goes into defeating air resistance. Doubling speed quadruples drag.

CJhaughey

Aerodynamics plays a huge part at higher speeds, wind resistance increases exponentially.
At twice the speed the power required to overcome the drag is a multiple of 4.

MuppetCheck

I've played around with this to aid my boredon on the motorway. For some reason, over many journeys on my route 130kph is more economical than 120kph. Even at 120kph the engine isn't laboured (just over 2k revs). It is a diesel though.

Nissan doctor

As a VERY basic rule of thumb, its normally AROUND 100kph as this is in and around the national speed limit of most countries so most manufacturers design the gearing and fuel maps around this. Also, above 100kph you start getting into the issues of wind resistance and aerodynamics etc etc.

But really, as stated above, there is no one specific 'speed' that is most economical, it depends on far too many factors. Its your control of the throttle and the application of engine load that will most aid fuel economy, not driving at a certain speed. E.G, driving down a hill at 120kph while barely touching the throttle will be vastly more economical then going up a hill at 80kph with your foot on the floor.

CiniO

Nissan doctor wrote: »

As a very basic rule of thumb, its normally around 100kph as this is in and around the national speed limit of most countries so most manufacturers design the gearing and fuel maps around this. Also, above 100kph you start getting into the issues of wind resistance and aerodynamics etc etc.

I've never had or probably even driven a car that would be more economical during constant run at 100km/h than at 80km/h.

In my Mazda 6 2.0 it seems that best fuel economy is on 5th gear at about 70 to 80km/h. Above that fuel economy goes down a bit, while over 140 km/h it starts literally sucking fuel. AFAIR at 200km/h its consuming about 18 l/100km which is about 15 MPG.
Also at lower speed fuel economy suffers, as below 70 rpm on fifth gear fall too low, and fuel consumption increses again, while obviously if you reduce to 4th, fuel economy is going to suffer as well.

So keeping it between 70 and 80 km/h seems to be the most economical option, and I have very similar experience with most other cars.

Mycroft H

There probably is cars out there Clino, I've not come across them but I'd be thinking along the lines of a 4L TDI A8 with a 6/7sp gearbox.

deckie27

My wife's car only looses about 5mpg between about 110k and 160k

CiniO

deckie27 wrote: »

My wife's car only looses about 5mpg between about 110k and 160k

What car is it?

bladespin

Often got 50mpg ish in the Alfa by keeping it around 50mph on long runs (was torture though).

deckie27

CiniO wrote: »

deckie27 wrote: »

My wife's car only looses about 5mpg between about 110k and 160k

What car is it?

Saab 95, petrol

bladespin

Great bangernomics cars now too

mickdw

Going by the instant fuel useage display giving litres / 100km, my car is happiest at 45 to 50 mph. Thats a 6 speed 1.8T petrol.

Yakuza

My current car (Qashqai+2) has an optimal speed around 100kph (1.6 petrol, 5 gears), the previous one (same make and model, 1.5 daysul and 6 gears) had a sweet spot somewhere between 100 and 110 kph. Both are fairly underpowered. Having driven several hired E92 320d and a 120d, they seem to do well up to 120kph.

CiniO

Yakuza wrote: »

My current car (Qashqai+2) has an optimal speed around 100kph (1.6 petrol, 5 gears), the previous one (same make and model, 1.5 daysul and 6 gears) had a sweet spot somewhere between 100 and 110 kph. Both are fairly underpowered. Having driven several hired E92 320d and a 120d, they seem to do well up to 120kph.

Optimal in what sense?
Are you trying to say, that your Qashqai will use less fuel if you travel steadily at 100km/h than if you were travelling at 80km/h?

vibe666

i would have imagined that (assuming no wind and a flat, straight road) the lowest speed you can maintain in your highest gear without labouring the engine would generally be the most economical?

CJhaughey

I think its more likely to be the point at which the torque curve intersects with the drag curve.
Probably 50mph for most cars and motorcycles.

Sesshoumaru

In the Leaf I can see real time energy usage, 85 to 90 kph is the energy efficient sweet spot for this car. Any higher and air resistance adds a lot to energy consumption. I don't think air resistance will differ much for most cars, so I think 90kph should be your cruising speed if you want to save as much fuel as possible.

unkel

vibe666 wrote: »

i would have imagined that (assuming no wind and a flat, straight road) the lowest speed you can maintain in your highest gear without labouring the engine would generally be the most economical?

The lowest speed in the highest gear would use less fuel than any speed higher than that. Per hour. Not per mile

anthony4335

deckie27 wrote: »

Saab 95, petrol

From 28mpg to 23mpg!

Yakuza

CiniO wrote: »

Optimal in what sense?
Are you trying to say, that your Qashqai will use less fuel if you travel steadily at 100km/h than if you were travelling at 80km/h?

Not at all. When I say optimal I mean speed (and inversely, journey time) versus fuel consumed. Basically, after roughly 100kph in my current car, my consumption starts to increase at a greater percentage than increase in speed (I say roughly, depending on whether it's just me in the car, or the whole family / luggage etc). Yes, at 80kph consumption is better than at 100kph but I don't want to spend all day driving!.

If a 10% increase in speed results in a 15% increase in consumption (going on the instantaneous readout), then I'll go back to the original speed (unless pressed for time!). It may be a bit nerdy (and meaningless in terms of real fuel savings ) but it keeps the auld brain active on my long drives down the M5 / M6 through the UK.

deman

The sweet spot in my '05 Primera 1.6 is somewhere between 95-100 kmph. It's easy to see on the dash display.

monkeybutter

thegreatchimp wrote: »

Does anyone have definitve figures on the ecenomical driving speeds of cars?

I drive a 1.6 litre ptrol mazda 323. I've been trying to save money on petrol by driving at a fuel efficient speed. However I've heard many different figures for this "magic" speed. Originally I was told 70mph (110kmh), but other people swear it is as low as 56mph (90kmh) and even 50mph (80km/h). That's a pretty massive variation.

I realise there's will be a variance based on engine size and power to weight ratio, but I would have thought that calculating this would be fairly straight forward; but amazingly I haven't been able to find anything definitive on the net. Not even road authorities and car manufacturers seem to know, and posts on forums are just as varied as the above figures. Surely there's some mechanic or engine expert out there that can shed light ont this...

Any replied appreciated.Thanks

Sean

Generally the higher the revs the more fuel is used, but you need to factor this against a longer journey. Acceleration is a killer and I doubt the Mazda is super efficient (older models anyway) at cutting back usage to zero etc when slowing down or breaking.

Having tested this rigorously, it is indeed 100k on the old Mazda 323, BH generation.

To go from 100k to circa 120k I need to up my average revs from 3200 to 4000, and this has roughly a 20-25% change in my fuel consumption across the same trip. This would be dublin to cork. So I can up the mpg from about 31 to 38.

Doing 120k would also usually mean more breaking and more acceleration over taking people etc. Whereas at 100k, I am mostly the one being overtaken

So 380-400 miles (if you are don't do any breaking) on a 10 gallon tank. So up and down on one tank or 76 euro.

The same trip takes roughly 30 mins longer, but the trip is more relaxed as I'd be chillin like Matt Dillon on Penicillin and possibly safer as I am going at slower speed and doing less lane changes.

If you can tuck in a mini convoy of cars it should also help economy but, at a sensible distance. Harder to measure this.

Zubeneschamali

monkeybutter wrote: »

Generally the higher the revs the more fuel is used, but you need to factor this against a longer journey.

No, the OP is asking what speed to drive for the best MPG, they're not asking to balance mpg against time. If you are after mpg, 100 kph is too fast in just about any car.

I'd bet 80kph will be better in every car that anyone on boards drives.

CiniO

Yakuza wrote: »

Not at all. When I say optimal I mean speed (and inversely, journey time) versus fuel consumed. Basically, after roughly 100kph in my current car, my consumption starts to increase at a greater percentage than increase in speed (I say roughly, depending on whether it's just me in the car, or the whole family / luggage etc). Yes, at 80kph consumption is better than at 100kph but I don't want to spend all day driving!.

One note here, as I think not many people realise.
Extra weight in the car obviously adds to the fuel consumption, but it differs a lot on driving conditions.

F.e in city runs where there is plenty of acceleratin and braking, it can make really big difference if you are traveling alone, or 5 people on board with planty of luggage. All because it's extra weight to accelerate and therefore extra energy needed.
Also when traveling in mountain areas, it need way more energy to push up the car up the hill if it's loaded comparing to if it was empty.

But at steady motorways speed on flat area (no mountain or hills) car load makes very little difference. As to keep constat speed, force which engine is producing must equal resistance forces. Air resistance remains the same no matter what weight you are carrying (and air resistance is the major force at motorways speeds that engine must work agains). Only what is increased if weight is bigger, is the rolling resistance, but at motorways speed that make really small difference.

So in general - if you load up your car with heavy weight to maximum, expect big MPG drop in the city or if driving in mountain regsion, while at motorways if you drive steadily in oppose to accelerating and braking all the time, MPG should remain nearly unchanged.

Top Dog

Zubeneschamali wrote: »

No, the OP is asking what speed to drive for the best MPG, they're not asking to balance mpg against time. If you are after mpg, 100 kph is too fast in just about any car.

I'd bet 80kph will be better in every car that anyone on boards drives.

I'd disagree with you based on experience. Our last car, a Laguna II 1.9dCI with the 6-speed manual returned an average of 50.2mpg when averaging 65mph compared with 47.6mpg when averaging 55mph (engine almost labouring in 6th where it was revving a little higher than you'd want in 5th for best economy). It also did the same 47ish when cruising at an average of 70mph.

CiniO

Top Dog wrote: »

I'd disagree with you based on experience. Our last car, a Laguna II 1.9dCI with the 6-speed manual returned an average of 50.2mpg when averaging 65mph compared with 47.6mpg when averaging 55mph (engine almost labouring in 6th where it was revving a little higher than you'd want in 5th for best economy). It also did the same 47ish when cruising at an average of 70mph.

Did you try what MPG would you get on 5th gear at 50mph?

Top Dog

CiniO wrote: »

Did you try what MPG would you get on 5th gear at 50mph?

Afraid the only time I did that on an extended motorway journey was with a caravan so the figures wouldn't be of any use.

Wetbench4

I think the confusion is the fact that just because your're using more fuel doesn't mean its less economical. Obviously a car will use more fuel at 120kph compared to 100kph, but that same car might use less fuel to cover 1 mile/km at 120kph than at 100kph because of gearing and torque etc.

CiniO

Wetbench4 wrote: »

I think the confusion is the fact that just because your're using more fuel doesn't mean its less economical. Obviously a car will use more fuel at 120kph compared to 100kph, but that same car might use less fuel to cover 1 mile/km at 120kph than at 100kph because of gearing and torque etc.

I think you didn't give a think about what you just said.