Tips for Improving Mileage, Most of Which Simple

[Alpine003]

I think your econometer may tell you otherwise! Engine is most efficient around peak torque, not peak power.

However both a low throttle opening and/or too low revs when the car is working is not good for fuel economy.

I tend to agree with this and always tell my friend's father that he shouldn't be starting out in second gear, especially since he doesn't drive a Viper.

[christophbmw]

I think your econometer may tell you otherwise! Engine is most efficient around peak torque, not peak power.

However both a low throttle opening and/or too low revs when the car is working is not good for fuel economy.

hmm, your logic seems more true than mine, i geuss at peak torque the engine is at it highest efficiency level on acceleration. what is the peak torque RPM level on our cars?

also another good pointer for improving fuel economy: switch out to a lower diff, like say a 3.91 or 3.93 ratio diff.

[sheepdog]

It has been documented that low flow/revs conditions do not allow for proper fuel atomization and fuel mixture. E.F.I. is better at it, but still suffers from it.

So you can actually hurt it from not enough throttle open.


And while you can run a Vette' down to 15000 and cruise, it has the torque to keep the car moving, we however, do not. Keep the rpm's up a bit. Watch your gage.

[sheepdog]

correction, it would be unlawfull by driving barefoot. it is only illegal if you get caught. (illegal means a state law enforcment agent has beeninvolved).

Very true, but there is good reasoning behind it, sort of like wearing a seatbelt.

[christophbmw]

Very true, but there is good reasoning behind it, sort of like wearing a seatbelt.

well not wearing a seatbelt is just plain dumb. i wear flip-flops in the summer (live in cali') and i cant drive with them on, i must take them off otherwise i become a road hazard. i know it is illegal and dangerous, if i get in an accident, but not wearing them keeps me safer in the long run (and hopefully helping me to avoid an accident should i ever need to).

[bmwman91]

well not wearing a seatbelt is just plain dumb. i wear flip-flops in the summer (live in cali') and i cant drive with them on, i must take them off otherwise i become a road hazard. i know it is illegal and dangerous, if i get in an accident, but not wearing them keeps me safer in the long run (and hopefully helping me to avoid an accident should i ever need to).

Same here.  I do not take off shoes, but sandals are impossible to drive in.  I guess there are some good reasons for not being barefoot though...I do not want to step on broken glass!

As for the shifting low thing, I am willing to accept that maybe it IS bad to shift there.  We will find out soon enough though...my current off-hours project is an econo-meter of sorts.  It will count actual injection events and allow me to log them along with vehicle speed and other things.

[christophbmw]

just remembered something else. for those of you who live at higher elevations (4-5k above sea level) you could use 87 octane instead of the usaul 91 since there is less oxogen. it doesnt help milage but it does save you a few bucks in the long run.

[ecpreston]

Careful drafting semi's.

It hurts their mileage

I completely agree with you that drafting a semi is inadvisable, but there's no way it hurts their mileage. Drafting is usually good for both parties. Your car being there SHOULD reduce drag for them, improving their mileage. I have no idea if anyone has tested this though, and I can't imagine the difference would be measurable for the semi either way.

the internal combustion motor is most efficient at its max HP rating

ICE is generally most fuel efficient at WOT, not at max HP or torque. Low RPM, WOT uses less fuel because it decreases the restriction provided by the TB, and the low RPM means lower pumping/friction/heat losses. This is slightly complicated by the fact that the car runs a richer fuel map at WOT, but something close to WOT in a lower RPM is definitely better than less throttle opening and higher RPM. If you're climbing a hill, you're better off at near full throttle in 5th rather than down shifting to a lower gear.

[sheepdog]

well not wearing a seatbelt is just plain dumb. i wear flip-flops in the summer (live in cali') and i cant drive with them on, i must take them off otherwise i become a road hazard. i know it is illegal and dangerous, if i get in an accident, but not wearing them keeps me safer in the long run (and hopefully helping me to avoid an accident should i ever need to).

You cannot avoid every accident.
Shit happens.

[sheepdog]

I completely agree with you that drafting a semi is inadvisable, but there's no way it hurts their mileage. Drafting is usually good for both parties. Your car being there SHOULD reduce drag for them, improving their mileage. I have no idea if anyone has tested this though, and I can't imagine the difference would be measurable for the semi either way.

You are forgetting a component in this.

In racing, it is about speed, the lead vehicle pays a penalty, but the others are carried along easier and can push the lead vehicle, making him faster. There are plenty of pits stops and leaders change.

Trucks, do this and it works as well, they use it for efficiency. The lead truck takes a loss, and the following trucks get a huge break on mileage. If you watch, they swap places now and then to balance it out, giving all of them a break. Birds do the same thing in flight.

However, you are too small to give any advantage by leading them, and you have no intention of leading anyhow, so you get the advantage and they get the disadvantage.


I have talked to a few truckers about it and I even drive truck on occasion. You can tell, and it can be scary for them.

[christophbmw]

You cannot avoid every accident.
Shit happens.

i know, i totaled my last 318i. ...but that was my fault, i was young and stupid.

I completely agree with you that drafting a semi is inadvisable, but there's no way it hurts their mileage. Drafting is usually good for both parties. Your car being there SHOULD reduce drag for them, improving their mileage. I have no idea if anyone has tested this though, and I can't imagine the difference would be measurable for the semi either way.

the semi would have to be doing at least 100mph for it to be effecient (they did a test on myth busters and the test was "busted").

and besides, i dont like my hood and windsheild all chiped up from tailgating semis

[Abrax]

As this is not a tip - it is interesting enough to place here, because it is very well measured.

So:

In 2004 me and my 2 racing friends got an idea to buy 2 318is cars in Germany as they liked my car's mileage compared to the efficiency and driveability.

Next day we went to Germany to buy theese 2 cars, they wanted to buy and we went there by my car which was 280000km car. As we've bought them, we got into the highway /in Germany there is no speed limit/ and we just started to measure the mileage.

The cars:
318is 1990 280k km
318is 1991 180k km
318is 1989 245k km

all in perfect condition and all with 10W40 oil.

The distance -> the same for all of us, 560 km by highways in Germany. We were driving from the heights of Stuttgart to our border in Zgorzelec. It is kindly from the hill, but:

As we have similiar driving habits, throttle coverage and we were driving just in the same moment not far away from each other, we were able to achieve nice mileage of 11.2 litters/100 kilometers -> I don't know how many litters fitts one gallon, so You have to recalculate it on Your own.

The point is that we were driving all the time just a little higher than 200 km/h.

So we made the distance in 3.05 hours stopping 2 times /about 12 minutes each stop/, one for a coffe and the other for refuelling. This gave us 187km/h of the average speed on the distance of 560km!!!

So buddys, we have one of the most economic gasoline cars capable of "cruising" with such a speed :-)

I felt really strange when we 've been overtaken by a policecar which passed us going like 250km/h when we were going 225 at that moment. While overtaking us one by one, they were looking really bored, they were eating sandwitches and drinking something...

The photo You see below was made during the coffe stop so You can see the three cars...

[b318isp]

You cannot avoid every accident.
Shit happens.

Such as? Might not be your fault, but at some point someone could do something.

[bmwman91]

Back to the maintaining lower engine speeds thing.

Try cruising at 35-40mph in 3rd and 5th gear.  In fifth, you will be at about 1500RPM and be getting (hopefully) 40ish MPG.  Downshift to 3rd and go steady.  You will be more around 27MPG at 3200RPM.  The motor has a LOT of friction with its moving parts and fluid pumps.  The higher you revv it, the more Coulomb friction you have from moving mechanical parts (linearly scalable to the engine speed).  The viscous friction from the coolant and oil increase in a 3rd order function I believe...so doubling engine speed results in 8x the friction.

So, I still maintain that shifting higher means that you spend time with the motor at these elevated speeds, therefore wasting more energy.  The volumetric efficiency argument really falls apart at part-throtle conditions.  The pumping efficiencies people have mentioned are at WOT conditions.  The throttle plate being partially closed adds a HUGE restriction to the system, making the other inefficiencies become smaller parts of the whole.  Shifting at lower RPM's means a smaller number of cycles that the pistons and all the other moving parts will spend oscillating and robbing energy via friction.  Most people do not accelerate at WOT ALL the time, so I still feel that shifting low when around town is a better choice.  Cheers all!

[Alpine003]

so I still feel that shifting low when around town is a better choice.  Cheers all!

What is your definition of "low"? With the high gas prices lately($3.70/gal 93oct here). I've been trying to shift around 3k from 1st to 2nd and around 2.5k there after.