Better MPG: more throttle or more RPMs?

darthroush · Post by **darthroush** » Mon Oct 25, 2010 3:44 am

Been wondering about this for some time now. I live in the mountains, so on the way home from school and even around town, I have to drive up steep roads at times. Both options seem to be the exact opposite of what is "optimal," but each is also seems to be a trade-off with the other. Which is better, if either at all, for better MPG's:

Going up a hill at a higher RPM, but using less throttle or going up a hill with more throttle, but at a lower RPM?

Just curious...

RADARsx 2.0 · Post by **RADARsx 2.0** » Mon Oct 25, 2010 5:10 am

I would say...and i've been taught this when I started driving transport in the military...High RPM's and Low throttle.

Your not making the engine labour and fight to keep going...

Post by **racer12306** » Mon Oct 25, 2010 7:40 am

I would think you would want to minimize load so that would mean higher RPMs and lower throttle position.

Post by **occasional demons** » Mon Oct 25, 2010 7:59 am

RADARsx 2.0 wrote:..High RPM's and Low throttle.

racer12306 wrote:I would think you would want to minimize load so that would mean higher RPMs and lower throttle position.

This.

My mom's Caliber with the CVT will go to high rpm's when ever there is a load, even minimal. And it gets around 34 mpg with city/suburban driving. While it does get annoying, it is efficient. Of course the PCM can control the throttle position better than 90% of the humans out there for fuel economy.

Being old school, my brain wants to up shift, and give it the gas. It just seems wrong to hang at 5000 rpm, and little throttle. But that is basically what the e-throttle/CVT transaxles do.

trojmn · Post by **trojmn** » Mon Oct 25, 2010 8:56 am

IMO a steady throttle downhill (speeding!) and slowly easing on the uphill slightly decellerating is the best. NO CRUISE. Wahtever you do avoid flooring it going uphill and stay out of open loop. It really is all about staying in closed loop and keeping high AFRs. Thats why i say no cruise. Higher RPMs are probably more efficient just to stay out of an extended high load situation (pedal floored) and keep the computer from going into COT.

For instance, coming back from texas i got 29MPG in the T&C (no trailer), with 15.7:1 AFR LC1 wideband... through the Ozarks! On the same trip I hauled an empty 12' covered trailer one way to TX behind the t&C. It was worth 4MPG to stay out of the super rich COT cycle where AFRs drop to 10.5-11:1. MPG went from 13 to 17 just from paying attention and staying out of open loop for an extended duration.

Seriously put a wideband on it and do some logging. You'll see what I mean. Really handy to have the laptop open or guage in site while your going up a hill.

Troj

fixitmattman · Post by **fixitmattman** » Mon Oct 25, 2010 12:11 pm

Higher RPM, less throttle. Not only is it easier for the engine, you also get into more efficient operating ranges of the engine.

I still do just as good with fuel economy in the big hills as the flat land. Sometimes I've actually done better as I engine brake all the way down again (yay for fuel cutout). Not uncommon for me to be revving over 4400 in 3rd for 4-5 min climbing some of the steep grades on the Trans-Canada. If I'm lightly loaded I can do it in 4th humming above 3k, but when heavily loaded even 4th gear bogs down.

esteinmaier · Post by **esteinmaier** » Mon Oct 25, 2010 1:29 pm

It's all about managing load. If you need more than 1/3 throttle, shift. If you need more than 1/3 throttle after 1 shift, drive slower.

shale green thing · Post by **shale green thing** » Mon Oct 25, 2010 3:02 pm

One of the keys to this concept is engine vacuum. If you really want the best mileage, either get
A. a Scangauge, or
B. a vacuum gauge.
The vacuum gauge will show you where the best throttle position is for your driving situation. You will want to shoot for the highest, i.e. most efficient, vacuum reading.

darthroush · Post by **darthroush** » Mon Oct 25, 2010 3:02 pm

Higher RPMs and less throttle I've kind of noticed to be the better of the two as well. I just wasn't sure if the gas gauge was lying or not

I know they aren't super accurate. Unfortunately, with the amount of driving I do, I don't have cruise control, ugh. I do know not to use it to on anything but flat road because it has different priorities in mind besides gas mileage when hills come into play. It just wants to maintain the set speed whether it has to use full throttle or not. As for shifting, the best I can do is to shift it from OD to D, which does a great job though of bumping the RPMs up and lessening the throttle by quite a bit, or a lot depending which road I'm on around here, and making the climb much easier.

I have a vacuum gauge in each of my other two cars, and actually just thinking about it, when I bump the RPMs up on those by downshifting, the vacuum does go up. Makes sense all around I suppose!

Thanks guys!

Post by **occasional demons** » Mon Oct 25, 2010 5:27 pm

As far as cruise control, the 41te will do ok in moderate hills. It is one of the few ATX's that will down shift with speed control to maintain the set speed, keeping vacuum higher, and on a hard enough deceleration, the PCM cuts fuel.

heydockyle · Post by **heydockyle** » Mon Oct 25, 2010 5:37 pm

The way I always think of it is, the more you push the throttle in, the more you open the tb for more air. The more air is going in, the more fuel. So the farther I push the pedal down, the more fuel it uses.

iThane · Post by **iThane** » Mon Oct 25, 2010 5:44 pm

glad someone made a thread for this cuz it always had me wondering..

but still, wouldn't you be burning more fuel at 5,000 rpm as apposed
to running 2,000 rpm? accelerating for example, am i saving gas by
shifting at 3,000 rpm or am i burning just as much as if i were to wait
till it reached 5,000 rpm, gas peddle having same amount of applied
pressure?

Post by **racer12306** » Mon Oct 25, 2010 5:48 pm

It really depends on the situation.

Say climbing a hill in 3rd gear puts you at 5000rpm, or in 5th gear at 2000rpm. Your engine will be working hard to pull the car up the hill, it may even start lugging which is even worse.

It all depends on load.

iThane · Post by **iThane** » Mon Oct 25, 2010 5:50 pm

so say im on flat ground though, im using the same amount of gas
at 5,000 rpms as 2,000 rpms?

heydockyle · Post by **heydockyle** » Mon Oct 25, 2010 5:55 pm

iThane wrote:glad someone made a thread for this cuz it always had me wondering..

but still, wouldn't you be burning more fuel at 5,000 rpm as apposed
to running 2,000 rpm? accelerating for example, am i saving gas by
shifting at 3,000 rpm or am i burning just as much as if i were to wait
till it reached 5,000 rpm, gas peddle having same amount of applied
pressure?

On flat land you can shift early, and apply less throttle to go just about as fast.

RPM's is revolutions per minute. Doesn't have much to do with how much gas is being burned. The only reason the rpm's are that high is because the gear you are in. The car is seeing no more air then it does with the same tps % in another gear, so it isn't going to dump much more fuel if any.

Higher gear will climb speed faster really as long as you aren't hardcore lugging.

iThane · Post by **iThane** » Mon Oct 25, 2010 6:00 pm

ah i see, thanks!

RADARsx 2.0 · Post by **RADARsx 2.0** » Mon Oct 25, 2010 7:34 pm

heydockyle wrote: RPM's is revolutions per minute. Doesn't have much to do with how much gas is being burned.

Well actually on flat terrain RPM's have a lot to do with fuel consumption. The more rpms on flat surface the more fuel you consume. Remember every revolution is a shot of air and fuel and the exausting of that burnt fuel and air. So 5000rpm is more than 2000rpm's there is 3000 injections of fuel difference.

But going up hill. it's the same principal but your rpms being high keeps your engine from "labouring" and when your engine labours it will cause the engine to try and inject more air and fuel to get more power. But there is a fine line from when your car is just "sucking" fuel and air with no results. Thats why just putting your rpms high and keeping a light throttle allows you to apply more throttle to keep momentom up and keeps needless fuel consumption down.

fusion210 · Post by **fusion210** » Tue Oct 26, 2010 8:55 am

Low rpm/higher load.

You'll be at a more efficient spot on the brake specific fuel consumption map.

http://en.wikipedia.org/wiki/Brake_spec ... onsumption

If you're at a lower speed, you'll also have the advantage of less drag on your side. Not only from the air, but from rolling resistance.

For example, if you had to overcome 30hp worth of drag.

The smaller the number, the more efficient the engine is per unit of fuel. It's obvious that low rpm/high load is where to be. That map isn't of a neon, but it's generally how they look and makes for a good example. I wish I had the map of the neon's 2.0l sohc!

Donkeypuncher · Post by **Donkeypuncher** » Thu Nov 04, 2010 11:34 pm

I've always wondered what the real amount of fuel is being burned at high rpms with low throttle, as opposed to low rpms with high throttle.

I always assumed that the more the throttle is open (more air=more fuel) the more gas is being pushed through the injectors.

I'm guessing there is a sweet spot where there is a perfect throttle position at a perfect rpm to maximize fuel consumption.

Until somebody can actually measure fuel usage in a massive amount varying situations, I don't think we will ever know for sure.

Post by **occasional demons** » Fri Nov 05, 2010 7:09 am

Where the most torque for the throttle position at that RPM, is the most efficient. One reason for drive by wire, the PCM can maintain that sweet spot far better than our right foot.

The other reason is to kill us all.

Swordfish2Cowboy · Post by **Swordfish2Cowboy** » Thu Dec 02, 2010 1:13 am

esteinmaier wrote: If you need more than 1/3 throttle after 1 shift, drive slower.

Hey, lately I've been cruising around in 5th at 35MPH because it's so damn cold and the car isn't warmed up. I CAN'T drive any slower.

ZeroChad · Post by **ZeroChad** » Thu Dec 02, 2010 2:26 am

35 in 5th isn't good for gas mileage at all unless you're in overrun.

Something to think about too is weather the pcm is in open or closed loop. If you're applying enough throttle to trigger open loop, you're no longer using O2 correction and going off the richer WOT tables.

fusion210 · Post by **fusion210** » Thu Dec 02, 2010 12:31 pm

35 in fifth is great for gas mileage. Best mileage is achieved typically at the lower revs in whatever the top gear is. When it was warm, I was able to get a hair over 60mpg by doing so (in my slightly modified first gen, observed in real time with my scangaugeII, on level ground). As speed increased, mpg decreased. You can find a bunch of mpg vs speed charts here http://ecomodder.com/forum/showthread.p ... 15182.html

Swordfish2Cowboy · Post by **Swordfish2Cowboy** » Thu Dec 02, 2010 12:57 pm

ZeroChad wrote:35 in 5th isn't good for gas mileage at all unless you're in overrun.

Something to think about too is weather the pcm is in open or closed loop. If you're applying enough throttle to trigger open loop, you're no longer using O2 correction and going off the richer WOT tables.

I do it until the engine warms up, then I'm in 3rd at 35MPH.

Marcel · Post by **Marcel** » Thu Dec 02, 2010 2:16 pm

35mph in 5th

i've tried thad a few times but i hate how the throttle response is and how the engine sounds. When the engine is cold i shift at around 2000-2300RPM, which makes 45mi/h my moment to hit 5th gear.

but back on topic: higher revs; go easy on the engine in the low-rev areas, don't let it work so hard!

darthroush · Post by **darthroush** » Thu Dec 02, 2010 5:25 pm

Changing my dirty air filter helped a bit too

I changed it at it's normal interval, but it was much dirtier this time around. The only difference was no air silencer. Who knew it was actually good for something?

ZeroChad · Post by **ZeroChad** » Thu Dec 02, 2010 6:44 pm

fusion210 wrote:35 in fifth is great for gas mileage. Best mileage is achieved typically at the lower revs in whatever the top gear is. When it was warm, I was able to get a hair over 60mpg by doing so (in my slightly modified first gen, observed in real time with my scangaugeII, on level ground). As speed increased, mpg decreased. You can find a bunch of mpg vs speed charts here http://ecomodder.com/forum/showthread.p ... 15182.html

I guess I was referring to actually driving and not cruising.

I never trusted my scangauge's instant mpg. It had no clue what the injection pulsewidths were. In fifth gear from 55-70 I always got 30mpg in mine regardless of what it said.

Post by **occasional demons** » Thu Dec 02, 2010 7:54 pm

35 mph in 5th is going to depend on what year MTX you have. IIRC, '03 up were .72 and older are .81 Mine will do fairly well on slight hills in fifth in town. but if I had the .72, prolly not so much. I usually just use 4th, as it gives the engine less load, and better oil pressure.

The oil pressure is what I would be more concerned with, as there is higher loads with less lube.

fusion210 · Post by **fusion210** » Fri Dec 03, 2010 3:38 pm

ZeroChad wrote:
fusion210 wrote:35 in fifth is great for gas mileage. Best mileage is achieved typically at the lower revs in whatever the top gear is. When it was warm, I was able to get a hair over 60mpg by doing so (in my slightly modified first gen, observed in real time with my scangaugeII, on level ground). As speed increased, mpg decreased. You can find a bunch of mpg vs speed charts here http://ecomodder.com/forum/showthread.p ... 15182.html
I guess I was referring to actually driving and not cruising.

I never trusted my scangauge's instant mpg. It had no clue what the injection pulsewidths were. In fifth gear from 55-70 I always got 30mpg in mine regardless of what it said.

What is the difference in ''actually driving'' and cruising? My statement stands and my observation only backs it up. You said it isn't great for mileage, care to explain why?

Once calibrated, my scangauge has been right on under a wide variety of conditions. Anything from commuting around town to driving 10 hours and using a full tank. Once calibrated, it's great. I think the most it was off before calibration for me was 3.9%. It doesn't sound like your testing is too solid.

The the other charts make it plain to see that lower speeds in top gear is where it's at.

darthroush wrote:Changing my dirty air filter helped a bit too
I changed it at it's normal interval, but it was much dirtier this time around. The only difference was no air silencer. Who knew it was actually good for something?

Changing your air filter from a dirty one to a clean one does not have an effect on gas mileage in a newer car. http://www.fueleconomy.gov/feg/pdfs/Air ... 6_2009.pdf

darthroush · Post by **darthroush** » Fri Dec 03, 2010 5:01 pm

fusion210 wrote:
darthroush wrote:Changing my dirty air filter helped a bit too
I changed it at it's normal interval, but it was much dirtier this time around. The only difference was no air silencer. Who knew it was actually good for something?
Changing your air filter from a dirty one to a clean one does not have an effect on gas mileage in a newer car. http://www.fueleconomy.gov/feg/pdfs/Air ... 6_2009.pdf

The study says there is no significant effect on fuel economy, and it did not show 0 improvement 100% of the time. You cannot conclude that a new filter will NOT help at all based on the results, such as they did. I did not say the clean filter gave me 5MPG. I simply said it helped a bit. When you drive ~2,500 miles per month and don't have a decent amount of expendable income, every little bit helps. Dirt was falling off of the filter when I tapped it, as well as the element being a completely different color than the new one. It was well used.

Your beloved FuelEconomy.gov site also says that the Neon only gets a combined of 25MPG and 29MPG on the highway:
http://www.fueleconomy.gov/feg/noframes/20694.shtml

We must all be lying about the gas mileage we get too.