Discussion
We are often told to use high gears ASAP to save fuel, but how significant is it?
Obviously if you are driving at a steady speed down the motorway using an unnecessarily low gear will use more fuel. But what about situations when your speed is varying?
Threading my way through town in situation where I frequently have to use second I often leave it in second rather than keep grabbing third or fouth briefly then changing down again on the basis that I have better things to concentrate on than gear changes. So my revs are higher, but I don't have to put my foot down as far to accelerate, so I can't believe fuel consumption is much different.
Similarly in country nlanes on the bike I like to keep the revs up sufficiently to get a snappy throttle response. I spoke to someone who was in the habit of changing up at 3000 RPM, then went on an IAM run and was encouraged to rev higher. He was astonished to find no noticeable increase in fuel consumption
Obviously if you are driving at a steady speed down the motorway using an unnecessarily low gear will use more fuel. But what about situations when your speed is varying?
Threading my way through town in situation where I frequently have to use second I often leave it in second rather than keep grabbing third or fouth briefly then changing down again on the basis that I have better things to concentrate on than gear changes. So my revs are higher, but I don't have to put my foot down as far to accelerate, so I can't believe fuel consumption is much different.
Similarly in country nlanes on the bike I like to keep the revs up sufficiently to get a snappy throttle response. I spoke to someone who was in the habit of changing up at 3000 RPM, then went on an IAM run and was encouraged to rev higher. He was astonished to find no noticeable increase in fuel consumption
I generally just use whatever rev feels right for that moment... It kind of bothers me when people stick to say 2500 revs or 3000 revs ALL the time, especially when on the motorway. I think it actually uses more fuel as you're requiring the engine to be working harder and less efficient to pick up the right speed. I'm not meaning you should thrash it everywhere you go, just if you're going up a slope, or something, just keep it in second/third for that little longer... makes all the difference.
+ I've always found my MPG aint really effected with my driving style... Get about 200 miles out of 20L, which is roughly 40mpg? So not bad at all, considering I often use my rev range. I am a firm believer that good MPG is not just the revs itself, but overall smooth driving wherever you are, and sometimes to be smooth you must use your revs!
+ I've always found my MPG aint really effected with my driving style... Get about 200 miles out of 20L, which is roughly 40mpg? So not bad at all, considering I often use my rev range. I am a firm believer that good MPG is not just the revs itself, but overall smooth driving wherever you are, and sometimes to be smooth you must use your revs!
I've tried driving my present car, a Hyundai i30 petrol 1.4, very economically with low revs and compared it when driving normally (i.e. not quite red-lining it but close) and it made no real difference.
Depends on the car I reckon.
I usually cruise in the highest gear possible regardless.
Depends on the car I reckon.
I usually cruise in the highest gear possible regardless.
put simply, it takes (quite a lot of) energy to turn an engine over. if its turning over more regularly, then more fuel is being used. The point where engine inefficiencies due to low revs come in is, on the vast majority of engines, a lot lower than you think (there are some excepitons such as engines with very revvy cams). On a diesel, where the fuel is only injected at the very last moment (when the valves are shut), I'm not sure this point would ever come (but someone cleverer than me can probably explain).
this is a graph of an rx8's mpg. note that it is at its best when in minicab mode.
someone might come along and claim labouring an engine damages is, but again I can't see how this is the case, except for extreme circumstances. My diesel mondeo shed has done 60,000 miles in my hands (and 120,000 miles prior to that) - and probably spends 95% of its time below 2,500 rpm. Hasn't done it any harm
this is a graph of an rx8's mpg. note that it is at its best when in minicab mode.
someone might come along and claim labouring an engine damages is, but again I can't see how this is the case, except for extreme circumstances. My diesel mondeo shed has done 60,000 miles in my hands (and 120,000 miles prior to that) - and probably spends 95% of its time below 2,500 rpm. Hasn't done it any harm
i would say you have to feel on how's your car responding at different gear, keeping constant speed does give a better save on fuel especially on motorways.
i did experienced for example driving along the snake pass, during the uphill although you can put it on fifth on some parts but it does better on 4th to keep momentum and preparing to overtake some trucks over the stretch. ideally i would say keep it around 2500-3000rpm to keep responsive from weak torques.
i did experienced for example driving along the snake pass, during the uphill although you can put it on fifth on some parts but it does better on 4th to keep momentum and preparing to overtake some trucks over the stretch. ideally i would say keep it around 2500-3000rpm to keep responsive from weak torques.
5lab said:
put simply, it takes (quite a lot of) energy to turn an engine over. if its turning over more regularly, then more fuel is being used. The point where engine inefficiencies due to low revs come in is, on the vast majority of engines, a lot lower than you think (there are some excepitons such as engines with very revvy cams). On a diesel, where the fuel is only injected at the very last moment (when the valves are shut), I'm not sure this point would ever come (but someone cleverer than me can probably explain).
this is a graph of an rx8's mpg. note that it is at its best when in minicab mode.
someone might come along and claim labouring an engine damages is, but again I can't see how this is the case, except for extreme circumstances. My diesel mondeo shed has done 60,000 miles in my hands (and 120,000 miles prior to that) - and probably spends 95% of its time below 2,500 rpm. Hasn't done it any harm
The diesel Mondeo like many cars now - mine's a Mk3 - has a dual mass flywheel which is, I am advised, vulnerable to damage through high-torque rotational vibration if the engine revs are allowed to fall to too low a level. Discuss...this is a graph of an rx8's mpg. note that it is at its best when in minicab mode.
someone might come along and claim labouring an engine damages is, but again I can't see how this is the case, except for extreme circumstances. My diesel mondeo shed has done 60,000 miles in my hands (and 120,000 miles prior to that) - and probably spends 95% of its time below 2,500 rpm. Hasn't done it any harm
motco said:
The diesel Mondeo like many cars now - mine's a Mk3 - has a dual mass flywheel which is, I am advised, vulnerable to damage through high-torque rotational vibration if the engine revs are allowed to fall to too low a level. Discuss...
This, I presume, is why these cars don't even attempt to save themselves by adding throttle when they drop below idle like most petrols would, but immediately allow themselves to stall.The exhaust on my 350z blew the other day and I had to continue driving it like that for a couple of days whilst a replacement was on order. It was quite embarrasingly noisey so I was changing gear as early as possible to avoid attracting unwanted attention. I was amazed how much my fuel economy improved changing up at just below 2,000 rpm compared to the normal 3,000. The combined total on the trip computer and distance to empty display was over 500 miles at one point - I've never previously got more than 400 miles to a tank!
lauda said:
The exhaust on my 350z blew the other day and I had to continue driving it like that for a couple of days whilst a replacement was on order. It was quite embarrasingly noisey so I was changing gear as early as possible to avoid attracting unwanted attention. I was amazed how much my fuel economy improved changing up at just below 2,000 rpm compared to the normal 3,000. The combined total on the trip computer and distance to empty display was over 500 miles at one point - I've never previously got more than 400 miles to a tank!
Likewise when the snow was bad I was changing up and 2k rpm in my petrol turbo Saab, just before the turbo came on strong, and it made a remarkable difference to my mpg.lauda said:
The exhaust on my 350z blew the other day and I had to continue driving it like that for a couple of days whilst a replacement was on order. It was quite embarrasingly noisey so I was changing gear as early as possible to avoid attracting unwanted attention. I was amazed how much my fuel economy improved changing up at just below 2,000 rpm compared to the normal 3,000. The combined total on the trip computer and distance to empty display was over 500 miles at one point - I've never previously got more than 400 miles to a tank!
that's a 350Z, large torque at low revs meanwhile also a terrible fuel consumption, keep it less than certain rpm and that really save a lot of unneeded torque hence fuel too.lauda said:
The exhaust on my 350z blew the other day and I had to continue driving it like that for a couple of days whilst a replacement was on order. It was quite embarrasingly noisey so I was changing gear as early as possible to avoid attracting unwanted attention. I was amazed how much my fuel economy improved changing up at just below 2,000 rpm compared to the normal 3,000. The combined total on the trip computer and distance to empty display was over 500 miles at one point - I've never previously got more than 400 miles to a tank!
It'd be interesting to know if the difference actually was that significant, as opposed to the trip computer just guessing it was..That RX8 graph is certainly useful. Shows how much the fact that an engine makes the most efficient use of its fuel at peak power comes into play at higher speeds, the graphs don't drop off as quickly as I'd have expected.
Never ever braking is the best way, though
acf8181 said:
obviously keep revs low......but revs are less of an evil than throttle input.....better to use revs rather than press throttle more.
...is my theory and it seems to work for me (along with never ever braking)
I tend to think along the lines of "if it needs more than half throttle, change down"....is my theory and it seems to work for me (along with never ever braking)
This is a typical graph of BMEP against RPM for a typical petrol engine
The contours represent Brake specific fuel consumption and relate to economy
The most efficient point in the graph is where BMEP is close to maximum and revs are just over 2000. In other words, maximum fuel economy will be achieved when the engine is working hard at relatively low revs. If the engine is working less hard then a lower engine speed would be more efficient.
...
5lab said:
put simply, it takes (quite a lot of) energy to turn an engine over. if its turning over more regularly, then more fuel is being used. The point where engine inefficiencies due to low revs come in is, on the vast majority of engines, a lot lower than you think (there are some excepitons such as engines with very revvy cams). On a diesel, where the fuel is only injected at the very last moment (when the valves are shut), I'm not sure this point would ever come (but someone cleverer than me can probably explain).
this is a graph of an rx8's mpg. note that it is at its best when in minicab mode.
someone might come along and claim labouring an engine damages is, but again I can't see how this is the case, except for extreme circumstances. My diesel mondeo shed has done 60,000 miles in my hands (and 120,000 miles prior to that) - and probably spends 95% of its time below 2,500 rpm. Hasn't done it any harm
An RX8 that gets 60mpg?this is a graph of an rx8's mpg. note that it is at its best when in minicab mode.
someone might come along and claim labouring an engine damages is, but again I can't see how this is the case, except for extreme circumstances. My diesel mondeo shed has done 60,000 miles in my hands (and 120,000 miles prior to that) - and probably spends 95% of its time below 2,500 rpm. Hasn't done it any harm
MMmmmmm interesting stuff.
My wife had a Duratec engined Mondeo that while driving down to Devon I coaxed 45 mpg out of, by using a light throttle and allowing the engine to spin well before changing up, plus try to brake as little as possible.
Irrespective of revs I reckon big throttle openings slay consumption.
However my Granada Cosworth would show 31 mpg at 70 on the motorway but an awful 18 mpg at a steady 30 mph, now that puzzles me.
My wife had a Duratec engined Mondeo that while driving down to Devon I coaxed 45 mpg out of, by using a light throttle and allowing the engine to spin well before changing up, plus try to brake as little as possible.
Irrespective of revs I reckon big throttle openings slay consumption.
However my Granada Cosworth would show 31 mpg at 70 on the motorway but an awful 18 mpg at a steady 30 mph, now that puzzles me.
McSam said:
his, I presume, is why these cars don't even attempt to save themselves by adding throttle when they drop below idle like most petrols would, but immediately allow themselves to stall.
You might well think so, but I regularly manoeuvre mine on idle relying solely on the anti-stall. Mine is not a common rail TDCi but a humble TDDi though.Gassing Station | General Gassing | Top of Page | What's New | My Stuff