Meathead Posted March 29, 2012 Report Share Posted March 29, 2012 Hi Simple (ish) question. How much does your revs effect how much fuel you use? Now I know that obviously the more you rev, the harder the engine works and the more fuel you burn, but is there a formula to calculate how much? For example would driving at 5,000rpm (on the motorway doing almost 100 in 6th gear) use twice as much fuel than if you were driving at 2,500rpm (on the motorway doing about 60 in 6th gear) I dont think its wil be as drastic as twice the revs = twice the fuel, but there will be some mathmatical way of figuring it out surely. I ask because as fuel is getting more and more expensive i want to use as little of it as possible and usually i boot it along in low gears, lift kicking in on all the straights, loving the induction kit noise on the high revs blasting down the mororway being that dickhead that stays in the right hand lane and never gets overtaken... but as i do almost 300miles per week its costing a fortune on petrol, so if the math shows I am essentially burning a LOT of money, I'll take the lead out of my right boot! Link to comment Share on other sites More sharing options...
badj Posted March 29, 2012 Report Share Posted March 29, 2012 Its not specifically rev related but think about it like this... The lighter you press the accelerator, the less air will be flowing through the throttle body, and thus a smaller amount of fuel will be added. Simples Link to comment Share on other sites More sharing options...
adampat84 Posted March 29, 2012 Report Share Posted March 29, 2012 It's all relative... Some people believe accelerate slowly = use less fuel, but take longer to get to your target speed? Others Accelerate quicker to your speed then drop the revs which uses less fuel? I am in both camps.. depending on my mood!? Link to comment Share on other sites More sharing options...
Meathead Posted March 29, 2012 Author Report Share Posted March 29, 2012 surely there must be a way to calculate it thought? Link to comment Share on other sites More sharing options...
Wolfie Posted March 29, 2012 Report Share Posted March 29, 2012 With my work I really put the miles on over the weekends, 200-500 every week. The only way I can keep the fuel bill down is to sit in the inside lane doing 60mph in 6th gear. After an hour and half on the motorway I do get bored but the gauge hardly moves. It works for me. Link to comment Share on other sites More sharing options...
silver gt Posted March 30, 2012 Report Share Posted March 30, 2012 Simple (ish) answer. Fuel burnt is a function of engine revs and throttle position. For a fixed throttle position (eg full throttle/foot hard down) 6k revs will use twice as much petrol as 3k revs. The problem is there is no way of accurately judging part throttle openings and any given cruising speed will involve different amounts of throttle in different gears. For these reasons there is no chance of calculating expected mpg. However, the rules for maximising mpg are fairly straightforward. For any cruising speed of 30 mph and above, top gear will give the best economy. Hard acceleration WILL use more fuel. Try keeping the revs under 3k for normal driving. Try to read the traffic ahead. If the car ahead of you slows right down to turn off at a junction, you have to slow down and then accelerate again, costing you fuel. But if you can anticipate his move and back off just slightly at an earlier point, you can make room for him to make his manoeuvre without having to slow down to his speed. Link to comment Share on other sites More sharing options...
Gstraw Posted March 30, 2012 Report Share Posted March 30, 2012 (edited) Yeah you can work it out. Don't know the full equation but it's like 14.7:1 for full fuel efficient burning so 14.7 parts air and 1 part fuel. 1800 cc engine is 450 cc so assuming perfect combustion taking up the complete cylinder, you have 30.612 cc of fuel per cycle per piston. Now 30000 rpm is the speed of the crank I think (and the cams turn at half that speed so that's 1500rpm) now in a 4 cylinder engine a quarter turn of the crank is one full travel of one piston so one 1rpm will take 30.612 x 4 = 122 cc per revolution which x 0.001 is 0.1224 litres...BUT x3000 is 367.2 litres in one minute don't drive your caaar!! It drinks more fuel than a rocket ship! As I done this I've realised I've made a mistake somewhere lol I think it's in the part of cc of fuel per cylinder. In real like it will never be quite full Anyone else shed some light? Edited March 30, 2012 by Gstraw Link to comment Share on other sites More sharing options...
LiamGTR Posted March 30, 2012 Report Share Posted March 30, 2012 I find 2k is the most MPG friendly, below as the car chugs, above and it starts pulling more Link to comment Share on other sites More sharing options...
stevenc1603 Posted March 30, 2012 Report Share Posted March 30, 2012 getting good MPG (IMHO) is about getting the correct balance of speed and gear. 30 MPH in 5th is fine, until you hit an incline and the engine starts to bog down. Then you have to tromp on the throttle to maintain speed, or even downshift if the incline is enough to stall the engine. The car I drive just now has a trip computer that shows a calculated instant MPG. It is almost always constantly changing, even on cruise control as the engine will open or close the throttle enough to keep the speed constant. This means the MPG goes up and down all the time. I've never actually tried to work out where the best MPG speed is though. Link to comment Share on other sites More sharing options...
rpsmith79 Posted March 30, 2012 Report Share Posted March 30, 2012 Google Hypermiling if you want to learn how to drive economically Fitting a vacuum gauge is also a useful tool in the pursuit of Hypermiling Link to comment Share on other sites More sharing options...
sadlebag Posted March 30, 2012 Report Share Posted March 30, 2012 (edited) Yeah you can work it out. Don't know the full equation but it's like 14.7:1 for full fuel efficient burning so 14.7 parts air and 1 part fuel. 1800 cc engine is 450 cc so assuming perfect combustion taking up the complete cylinder, you have 30.612 cc of fuel per cycle per piston. Now 30000 rpm is the speed of the crank I think (and the cams turn at half that speed so that's 1500rpm) now in a 4 cylinder engine a quarter turn of the crank is one full travel of one piston so one 1rpm will take 30.612 x 4 = 122 cc per revolution which x 0.001 is 0.1224 litres...BUT x3000 is 367.2 litres in one minute don't drive your caaar!! It drinks more fuel than a rocket ship! As I done this I've realised I've made a mistake somewhere lol I think it's in the part of cc of fuel per cylinder. In real like it will never be quite full Anyone else shed some light? That could be what it is at full throttle @ 30000 rpm Edited March 30, 2012 by sadlebag Link to comment Share on other sites More sharing options...
splinter Posted March 30, 2012 Report Share Posted March 30, 2012 (edited) its all about work done by the engine. sitting in 6th gear low revving and labouring your engine can be highly inefficient. you will use more fuel and will not burn efficiently. Also the 14.7 to 1 is the stoiciometric point of total combustion (purfect ammount of air to fuel by volume), and engine is designed to vastly over supply air in order to help facilitate a good burn. I drive to how my engine sounds and feels, being in the right gear for the right speed for the right ammount of work required for say incline/decline will ultimatly get you the best combustion. Revs are all relative to the calculations, there is no set revs that are ultimatly more efficient, there are too many variables!! Edited March 30, 2012 by splinter Link to comment Share on other sites More sharing options...
Nurse Amy Posted March 30, 2012 Report Share Posted March 30, 2012 Pfffft Matt I've never known you ever drive for economy Link to comment Share on other sites More sharing options...
badj Posted March 30, 2012 Report Share Posted March 30, 2012 I could tell you how to calculate it based on what I've done for my degree and/or what literature I have, but theres not much point as its all theoretical because the calculations don't take traffic or engine load into consideration, i.e. up hill, driving into strong winds blah blah. Light right foot = better economy, basically. Link to comment Share on other sites More sharing options...
Peter150 Posted March 30, 2012 Report Share Posted March 30, 2012 Adding to this, the best MPG you can achieve for cars is somewhere between 50-60mph in the highest gear possible. This way you're travelling a decent speed to get the miles but RPM is still quite low for efficiency. I used to get an extra 75 miles ish out of my old car (Fiesta) by dawdling at 60mph along the motorway compared to normal speed (). Not noticed this so much in the Celica though, seems pretty consistent whatever speed I go. :rolleyes: Link to comment Share on other sites More sharing options...
Lex_GTS Posted March 30, 2012 Report Share Posted March 30, 2012 You have a T-Sport with a 2zz-ge... A high revving engine.... Fuel economy pretty much went out the window when you handed over the cash Seriously the rule of thumb is more revs = more cash. A higher gear at higher speed = better, but don't be in 6th gear trying to climb hills and shit, 6th is for cruising, not pulling. Link to comment Share on other sites More sharing options...
splinter Posted March 30, 2012 Report Share Posted March 30, 2012 to be honest, as Amy very kindly suggested, and Lex pointed out. 2zz and economy isnt really great. i nail the fuck out of my car everywhere, screw economy. if i wanted that id buy a.... uuuuurgh... deisel.... eeeeeeerugh!!! So no. i'll stick to an awesome celica thank you!! haha. i can live with poor economy, for awesomeness!! haha Link to comment Share on other sites More sharing options...
infernalzen Posted April 1, 2012 Report Share Posted April 1, 2012 easy acceleration saves me more fuel than a lower top speed to be honest, as Amy very kindly suggested, and Lex pointed out. 2zz and economy isnt really great. i nail the fuck out of my car everywhere, screw economy. if i wanted that id buy a.... uuuuurgh... deisel.... eeeeeeerugh!!! you'd be supprised by some tdis. riding in an audi 170bhp tdi ~250lbft. the thing pulls like a V6. they come in 4wd it can give you whiplash from low speed acceleration it is so quiet, you cant hear the engine on a dual carrageway. (tdi noise is noticeable around town) someone has one @220bhp +350lbft torque a few things that might shock some diesel-phobes Link to comment Share on other sites More sharing options...
badj Posted April 1, 2012 Report Share Posted April 1, 2012 easy acceleration saves me more fuel than a lower top speed you'd be supprised by some tdis. riding in an audi 170bhp tdi ~250lbft. the thing pulls like a V6. they come in 4wd it can give you whiplash from low speed acceleration it is so quiet, you cant hear the engine on a dual carrageway. (tdi noise is noticeable around town) someone has one @220bhp +350lbft torque a few things that might shock some diesel-phobes and bhp is a function of engine speed so in general terms you're talking the equiv of 300-400bhp petrol engines really Link to comment Share on other sites More sharing options...
Alan Clogwyn Posted April 1, 2012 Report Share Posted April 1, 2012 (edited) Simple (ish) answer. Fuel burnt is a function of engine revs and throttle position. For a fixed throttle position (eg full throttle/foot hard down) 6k revs will use twice as much petrol as 3k revs. The problem is there is no way of accurately judging part throttle openings and any given cruising speed will involve different amounts of throttle in different gears. For these reasons there is no chance of calculating expected mpg. However, the rules for maximising mpg are fairly straightforward. For any cruising speed of 30 mph and above, top gear will give the best economy. Hard acceleration WILL use more fuel. Try keeping the revs under 3k for normal driving. Try to read the traffic ahead. If the car ahead of you slows right down to turn off at a junction, you have to slow down and then accelerate again, costing you fuel. But if you can anticipate his move and back off just slightly at an earlier point, you can make room for him to make his manoeuvre without having to slow down to his speed. Nail on head. Get a manifold vac gauge, it's what newer cars use for their MPG meters. Higher vaccum = less fuel eing used. Edited April 1, 2012 by Alan Clogwyn Link to comment Share on other sites More sharing options...
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