Here are the questions (applies to manual tranny only)…
1)When coasting does it use less gas to have the car in gear… or out of gear at idle.
2)Say I am driving down the highway and I am coasting because there is an off ramp coming up, should I be in gear(5th) and braking, or out of gear and braking (what uses less gas).
3)I don’t know if this is the same question as the previous two, just reworded. But does gas mileage depend on how much the pedal is depressed, or what your rpm is?
I know these aren’t life and death to gas mileage but I think its interesting to know. Thanks!
if ut really that concerned about gas milage…ride a bike…but it really doesnt matter cause ur not accelerating…thats the only time u use more fuel…jusr because ur engine is at 5000rpm its doesnt mean its using any more gas than at idle because ur not on the gas the ecu will send fuel when throttle is applied…u can go up 9k and not use any more gas than staying under 3 and going WOT between shifts
At 9k you’re pulling in much more air than at 3k, and the computer will match the amount of gas you need with the amount of air you’re sucking in. Engines are most effiecient at high load, low RPM, so shifting early will help your gas mileage. Not as much as simply not flooring it, but even doing a part throttle run up to high RPM is worse than doing a part throttle run and shifting at 2500.
i always forget about the whole needing air thing…unless its the ECu leans the fuck out of it if ur not on the gas which prolly isnt the case…but to his original post…weathe u coast from 3k or idle down a ramp ur not burning a ton of fuel either way…me personally…drop to 2nd and see how fast the maxima will take the ramp…u could always get an SAFC and tune it
For questions 1 and 2: Coast when slowing. Either way you’re at closed throttle. The difference is that in gear your engine will be spinning proportional to road speed, say 3000 rpm for example so your engine will burn gas at 3000 rpm/closed throttle. Out of gear it will be at idle so you will be burning gas at, say, 800 rpm/closed throttle.
For question 3: Can’t really draw a conclusive answer. When you’re at partial or wide open throttle, the computer adjusts things like fuel delivery, egr on/off, etc. I believe I remember reading Zer0daze posting once that at 60% throttle Civics go into open loop mode. I THINK that means that the ECU stops taking into account O2 sensor readings and moves to a secondary fuel delivery algorithm… Or some shit. Hopefully he can chime in and clarify?.. But I’ve heard both sides argued: That partial throttle and slow accelerating is best for gas mileage to keep the throttle shut, or that WOT short shifting is best to keep revs down while minimizing time spent accelerating. :gotme:
Watch o2 sensor voltage readings. Down hill, they will read 0 volts if you are in gear (any gear, regardless of engine speed). Coast down the same hill in neutral, and you will get voltage readings as the car will be burning fuel to keep running.
In many states, coasting is illegal, though it’s even more difficult to enforce than the seat belt laws.
The minimum brake specific fuel consumption is typically at about 90% throttle and at an engine speed corresponding to maximum torque. In other words, the best thing you can do is get into 5th as quickly as possible.
(Some trivia: During the Mobilgas Economy Runs, it was common to see many of the cars using their windshield wipers when it wasn’t raining. This was because the wipers were vacuum driven and the F/A ratio was leaned while they were in operation.)
When a car get’s 30 mpg, or 158,400 feet per gallon how many off ramps do you need to coast to have signifigant change in fuel economy?
For argument we can consider a ramp size of 1/8 mile, also in this argument we will assume that the car uses no gas what-so-ever during coasting. 7.2 times in a 30 mile span to gain 1 mpg, but then again it would also mean that you have to accelrate up to the speed limit 7.2 times, which uses more gas than average mpg. :gotme:
And what, a .03% increase in fuel economy, a ~$0.0747 per gallon savings.
what kind of car do you drive? that g35 for yur icon? or is that a scion? i cannot tell either way , Hard accelerating is the worst thing for gas mileage…i agree with the previous poster about idling , the ecu is gonna dump fule to keep it running , while as if it were in the gear the tires are turning the motor over and making it run with no fuel being burnt up
I thought that if you leave it in gear when coasting you save more fuel because the wheels are driving the engine? Because if you’re in neutral, the engine needs fuel to stay at idle, but while coasting in gear, there is high vaccum, which means less fuel? Reguardless of rpm, the engine only uses fuel with on the throttle, and when it’s at idle, correct?
correct me if I am wrong, because i always leave my car in gear.
^Meh. Don’t really know. Doubt if you’d actually notice a difference anyhow. Just don’t bury your foot in the throttle all the time or cruise at high speeds on the highway.
Idle fuel flow increases with engine rpm. If it didn’t, the engine would have to be restarted after bringing the car to a stop with the transmission in gear. If you’re going to save fuel by coasting, do it right and disengage the transmission.
Incidentally, if you’re using the engine to save the brakes, this is no longer considered beneficial. Back in the forties and fifties, competition sports car drivers used the engine for this purpose, but, with the disk brakes and friction materials used today, this is no longer considered necessary. Oh, they’ll still take advantage of engine braking when convenient, but you no longer hear them coming down through every gear as they once did.
depends on how fast the car kicks over to the fuel cut …
the only way coasting in gear saves you fuel is if the cars EEC immediatly shuts off ALL fuel flow. for example, the EEC in the turbo sho does so … BUT only after a good 30 seconds of coasting in gear … and thats 30 seconds that fuels being dumped into the motor and not creating any accelation … thats a waste of fuel
engines use a certian amount of fuel / cylinder to maintain 14.7: 1, so @ 3000 rpm your ingesting more fuel then @ 800rpm … ALWAYS
good fuel economy comes from only using gas to accelerate as much as needed, when needed… burning fuel to slow down is as good an idea as using your brakes to accelerate :bloated:
or another way to think of this
does PEDALING down hill on a bike use less energy then taking your feet of the pedals ?
fuel trims are based on 2 things, long term and short term fuel trims… #1 rule of thumb, idle speeds (in a closed loop pcm situation) always require less fuel, #2 if your going 70 in 5th. your map, maf, tps, o2, all will read that your engine is at a higher rpm, if your moving at 70 at 1500 your car will use more fuel than at 800 at idle. #3 injector pulse width will increase only when tps is increased* #4*- ALTHOUGH if your are not on the gas, and in 5th gear at 1500 you still have more air entering the manifold because irreguardless of tps, when the engine is running the throttle plate is ALWAYS open just a little…
the maf will see this, and hence increase injector pulse…
the difference is slight, in a full tank testing you may only see maby .001mpg better, your talking about a few hundred rpm’s here, not 9grand…
— a little ASE test prep questions, i hope that helps
[QUOTE=HotRodKid]depends on how fast the car kicks over to the fuel cut …
QUOTE]
wtf… as long as your car is running, the car will never shut off injector pulse…
actually, in most Chrysler products (when observing a scan tool) after you let off the throttle in high speed situations (over 55 mph) there is a temporary fuel enrichment mode where the injectors go full rich even konw engine rpms are at “idle” they do this for cylinder cooling purposes… once again, as long as your car is running injectors are never shut of, they are syncd with spark timeing, your shut one off, you create a miss…
in the situation i mentiond the fuel IS shut off … meaning the only reason the engine is spinning is because its being driven by the wheels
trust me on this, ive seen the wideband reading, ive seen the injector pulse width datalog’s
proof … turbo sho on the way to the nyspeed BBQ. were driving down a rather steep hill, alex backs totaly off the throttle says “watch this” weight of the car is pulling the car down the hill at 40mph …
14.7:1 afr … backfire, backfire, backfire, backfire, wideband maxes out lean, reads “- - -” backfires STOP untill he just barely taps the gas again, repeat results … the car was shutting the fuel off because there was no need for it
also 3rd gen shos shut off the fuel during shifts
again, the car isnt technicly running, in this case its off for a split second in order to soften the shift and save the tranny
theres a difference between engine spinning and engine running
i dont look at them much, i tend to stay away from that crap
mopar has the best datastream…
ill look into that, i want to see it… maby its just a fault, i know factory computers shouldent do that…
I have been on the verge of running out of gas before in my car. If I throw her into N and my rpms were at like 800, if it was in gear it was at like 1k. not really a big enough differance when you get 13-14 mpg.
either way , Hard accelerating is the worst thing for gas mileage…i agree with the previous poster about idling , the ecu is gonna dump fule to keep it running , while as if it were in the gear the tires are turning the motor over and making it run with no fuel being burnt up