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Home > Performance Mods > Engine Mods > Myths about MPG

Myths about MPG

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psiewert


Reg'd: May 05, 2007

Vehicle Designer

Hey there guys and gals. I was getting kind of tired of people proclaiming huge gains or drops in MPG by adding or removing stuff from their car, and I wanted to clear up all this mumbo jumbo.

Myth 1: Aftermarket Engine Modifications

Any modification you perform on your engine, intake, or exhaust, WILL NOT IMPROVE MPG unless you are either a) modifying fuel control to limit fuel consumption b) reducing volumetric displacement, or c) using a cleaner fuel.

Installing a CAI will allow higher volumes of air, and much colder air. This means that MORE fuel can be burned per revolution. Thus more power and more gas per mile.

Installing an exhaust will improve flow of your exhaust gases, allowing for slightly less backpressure. This means more air can pass through your engine. More air = more fuel = lower MPG.

Installing items like a pulley may sound like they improve MPG by freeing up rotational inertia, but thats only partially true. Less inertia = more hp = lower MPG. You may improve MPG a tiny tiny bit though due to the inertia and weight loss.


Myth 2: Aftermarket Drivetrain Modifications

Changes to your wheel diameter, suspension, ride stifness, height, or the like will NOT affect your MPG more than 1 MPG AT THE MOST! The only reason you are affecting MPG is because if you lower your ride or changes its profile, you are changing aerodynamics and reducing drag.

I've seen people say that wheel diameter will alter your MPG. That is 100% false. Just changing your wheel size will ONLY affect your speedometer reading. You can only improve or hurt MPG if they are magical wheels my friends.


Myth 3: You can determine MPG from your fuel level indicator.

The thing I can't stand the most is well people claim to calculate fluctuations of 1-2 MPG by filling up their car and seeing how far they get when it gets to half full. Here are some of the reasons why that isn't ever going to work.

1) Your fuel meter is most likely only accurate to within half a gallon. On our cars, thats 10 - 15 miles!.

2) The way your car reads fuel level is with a flap that sits inside the tank. Based on the flaps angle relative to the base, thats how it determines how much fuel you have. Fuel is constantly flopping around, that flap is always moving.

3) Every single time you fill up, your fuel mixture is slightly different. For example, BP and Sunoco do NOT have a similar mixture in the least! They all have different additives, different contamination levels, and so on.

4) Every time you hit the gas pedal, you burn fuel differently. If you are driving a different route every time you try to estimate your MPG, you'll never get it right. How many hills you climb, how often you coast, how many stops you make, and how quickly you accelerate have a HUGE difference on MPG.


Those are the three topics I can think of right now. Sorry, it may sound like I'm venting, but I get kind of irritated when people claim to find the secret of high MPG by only driving 4 miles and watching their dial.

The ONLY way to accurately measure MPG is to hook up a flow meter and hold the RPMs perfectly constant at about 3000. You will need to measure the EXACT amount of revolutions of the tire down to a couple of radians. As soon as your tired circumference equals 1 mile exactly, you need to record the gallons of fuel consumed. You need to repeat this 10-15 times to get a decent average.

-Sincerely psiewert


[#] Feb 25, 2008 01:39pm
jboy510


Reg'd: Sep 06, 2006

Scion Guru

nice post, good info

So what if my girlfriend drives a Supra, I'm more sexier than she is! =)

************* CHECK OUT MY GALLERY *************

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[#] Feb 25, 2008 01:51pm
psiewert


Reg'd: May 05, 2007

Vehicle Designer

thanks man....remember, 95% of the changes we will ever see in MPG have to do with the way we drive. I could easily get 50 MPG on this car if I drove at a max speed of 10 MPH, coasted everywhere, turned my car off while coasting, stopped on hills for momentum, and never used my brakes.


[#] Feb 25, 2008 01:53pm
justDstart3


Reg'd: Jan 06, 2008

Vehicle Designer

good helpful info..



"Passion is in the blood. Desire is in the will. Prepare to be mentally strong. Till the day the earth stood still. Everyone in the world will turn. Up as they glance upon you. Everyone will witness as your weakness will burn. As you will be reborn to someone true. The passion, desire and will is all you need in life." *by me*
[#] Feb 25, 2008 01:54pm
FrAnkRYzzO


Reg'd: Oct 21, 2005

$upervillain
for Hire

Yeah, good stuff. There's a couple points that I want to look into a little further, but the part about using the fuel gauge to estimate mpg is great. You could look at your gauge at the halfway point 10 times and you will have 10 different quantities of fuel in your tank. Period. Its not an accurate gauge, its there to provide an estimation of the fuel left in your tank.

Its like when your fuel light comes on. Its supposed to let you know that you have 2 gallons left. Do you have exactly 2 gallons left every time it comes on? Of course not. It just means that you have around 2 gallons.

Now I will say that if you drive a relatively consistent route, you should be able to get a decent estimation of your mpg's, but you could only use the tank fillups where you did nothing out of the ordinary during the course of driving til the next fillup. To do this you want to reset the trip when you get gas and then divide the miles by the amount of gas you replace in the tank during the next fillup. That's the only way to determine how much gas was actually used. But unless you are the most boring person in the world and go the same route everyday without deviation and live in a magical place where the traffic is always consistent, you'll never get anything more then an estimation. And for most people, I think even this estimation is probably much less accurate then you'd think.

Even Zombies can Search...
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[#] Feb 25, 2008 02:01pm
Orin


Reg'd: Dec 08, 2005


Excellent Post!

it's nice to see original user created material and it's also nice to bring some facts to the "...but my cousin has a CAI and he gets like 57 mpg..."

made a sticky in engine mods

[#] Feb 25, 2008 02:13pm
07TouringCoupe


Reg'd: Jul 14, 2007

Scion Guru

Nice.

A car is only as good as it's driver.
[#] Feb 25, 2008 03:28pm
Web


Reg'd: Apr 17, 2006

THE SHADOW

The only comment about this is that it is not the same for every driver...at least for my car. From day one where my only mod I had was an intake I was getting ~26 mpg...during the first 3000 miles before I added more items.

Around the 15-20K mark on my car, I had Intake, header, exhaust, and pulley....I was using STRICTLY Chevron/Texaco (same exact fuel mixture b/c they are the exact same company) 87 and averaging about 27 mpg.

From the 20K+ mark, I ended up adding my suspension setup to my car and from then I have been averaging about 30-31 mpg with my car.

I am not saying that any one mod in particular have added any mpg to my car, but I can't deny the fact that from bone stock to my current setup, my mpg HAVE gone up. I drive my car harder now then I did the first 20K. I used to hold an average of 65-70 on the highway and now it's easily between 75-80+.

For the most part, I agree with some of the post above, but I also can't deny the fact that my car (with my mods) HAS received better overall mpg.


I know not with what weapons World War III will be fought, but World War IV will be fought with sticks and stones. A.E.
[#] Feb 25, 2008 04:31pm
psiewert


Reg'd: May 05, 2007

Vehicle Designer

Well then you need to explain exactly how you measured that MPG. For example.

What was your driving distance?
How many gallons did you use?
How fast were you driving?
Was it the exact same route?
What was the ambient temperature?
What were the road conditions?
How many stops?
etc...
etc...
etc...

You're talking about 3-5 mpg difference. You can get a 3 MPG difference by just reading the fuel gage from a different angle.

This is the way they measure MPG professionally.

1) Run engine till all fuel is consumed.
2) Measure exact fuel put into tank down to the ounce.
3) Run the engine at a predetermined torque and RPM with a programmed ramp up rate.
4) Measure RPM at the wheel contact surface.
5) When all fuel is consumed and engine stops completely, measure the distance traveled down to the foot.
6) Distance Traveled / Gallons consumed = MPG
7) Repeat 10 times for an average MPG.
8 ) Perform steps 1-8 at different RPMs and torques to determine City and Highway MPG.


[#] Feb 25, 2008 05:55pm
Web


Reg'd: Apr 17, 2006

THE SHADOW

Well, that's the thing.......my post stated "average" mpg for each specific situation. Within each scenario, there's always a varience of +/- ~2 mpg. So, from stock, my varience is between 24-28 mpg (26 being my average)...and now, I hold steady around ~30 mpg (vairence being 28-32). Simply using an average of what I used to drive to my DAILY commute for the past year, I can say that I have gained a few mpg but cannot say which mods did it specifically....they all seemed to have contributed in some way.

Also, I measure the mpg like they do in the "professional" list you have. I fill up my tank at a given level and record the amount. After I burn that fuel, I fill up and for how much I fill up, I divide my miles driven on that tank by that fill amount. I do this for about 3-4 times and record that at most, I deviate from my 29-30 mpg by no more than .5-.6 mpg each 5 fill up average. Watching the needle, I agree, is NOT the way to do it.

I can only comment on what I am actually seeing my car do so my numbers are my actual findings for MY car and my driving habits.


I know not with what weapons World War III will be fought, but World War IV will be fought with sticks and stones. A.E.
[#] Feb 25, 2008 05:59pm
k-woodtc08


Reg'd: Feb 01, 2008

Detailer

You straight up killed all the haters on the "i get this many miles on a tank of gas" ticket. HAHAHA

[#] Feb 25, 2008 06:01pm
psiewert


Reg'd: May 05, 2007

Vehicle Designer

But Web, you are still using the in dash fuel gage to measure your consumption. Those are only accurate withing 3-5% at best. Say my gage is at half full, I can travel 4-5 miles without my needle even moving the tiniest bit. You'd have to fill your car up and run it till it stalled every time to have an accurate MPG.


[#] Feb 25, 2008 06:07pm
Web


Reg'd: Apr 17, 2006

THE SHADOW

But again, that's why all my numbers have been "averages". Even the EPA estimates you get on your car are NEVER accurate. Those are all averages and can vary from 3-5 mpg city/highway.

I just know that for my setup, I have performed the same exact measurements over and over and even if the gauge isn't 100% accurate, there still is a fluctuation in mpg. For me, it has gone up in numbers and that's why I can only say that for my particular situation/car, my MPGs have actually gone up.

That's why I'm also not 100% going against your findings b/c I can't speak for everyone else's car/driving habits...only my own.

Believe me, you do have valid points....but at the same time, my car setup shows me otherwise.

I will stay neutral on this topic as I have seen otherwise from the current information provided.


I know not with what weapons World War III will be fought, but World War IV will be fought with sticks and stones. A.E.
[#] Feb 25, 2008 06:10pm
FrAnkRYzzO


Reg'd: Oct 21, 2005

$upervillain
for Hire

psiewert wrote:
But Web, you are still using the in dash fuel gage to measure your consumption. Those are only accurate withing 3-5% at best. Say my gage is at half full, I can travel 4-5 miles without my needle even moving the tiniest bit. You'd have to fill your car up and run it till it stalled every time to have an accurate MPG.


I understand where you're coming from on this, but its not exactly accurate. If I fill my tank and reset my trip, I'm at 14.5 gallons. (granted there is a little variance here, it isn't much.) I then drive for 320 miles until my light comes on. I go to the gas station and refill my tank back to 14.5 gallons, and the pump tells that I've added 12.832 gallons. Using this system you know that you used roughly 12.8 gallons (due to the slight variance in the 14.5 cutoff measurement) and traveled 320 miles. For this tank you got 25 mpg. The greatest variance in averaging your gas this way is in driving habits, traffic conditions, and routes traveled, not in the calculation method. I agree that the tests conducted by the epa are more accurate, but the average joe doesn't need the calculation down to the nearest hundredth.


Even Zombies can Search...
..............
.......................................................................Why Can't You?!?!?!
[#] Feb 25, 2008 06:43pm
psiewert


Reg'd: May 05, 2007

Vehicle Designer

thats a plausible method, but still, you guys need to understand that there are WAYYYYY too many variables to be able to calculate down to 1 or 2 MPG. If that is the way you measure, then you have to say you get 30 MPG + or - 3 MPG. Like I said before you have to take a ton of things into account.

1) Ambient temp
2) Road Conditions
3) Speed
4) Number of stops, turns, and hills
5) Batch that you get your fuel from (Yes, every batch of fuel is different, even if it still the same manufacturer.)
6) Weight of the car
7) Accessories on or off
8 ) AC on or off
9) Condition of your oil
10) Air in your tires between tests

The list goes on an on. All of those individually are pretty insignificant, but added together its a huge set of variables. Hell, just the difference between winter and summer driving can change 1 to 2 MPG.

And you're right about driving habits. Gunning it from a stop to 60 MPH just once will burn just as fuel as it takes to drive 1 mile at 30 MPH.

I've read an article about a guy who averaged 150 MPG in his corolla over the span of a week based on how he drove.


[#] Feb 25, 2008 07:32pm
RuniCtC


Reg'd: Apr 02, 2008

Tire Changer

psiewert wrote:
Myth 1: Aftermarket Engine Modifications

Any modification you perform on your engine, intake, or exhaust, WILL NOT IMPROVE MPG unless you are either a) modifying fuel control to limit fuel consumption b) reducing volumetric displacement, or c) using a cleaner fuel.

Installing a CAI will allow higher volumes of air, and much colder air. This means that MORE fuel can be burned per revolution. Thus more power and more gas per mile.

psiewert



Ok, so here's a thought...

If you increase air consumption, thus increasing fuel consumption, but you are making the same load on the engine; you should'nt loose mpg. Yes more fuel is spent per revolution, but the work output should be greater, right?

[#] May 09, 2008 11:56pm
Web


Reg'd: Apr 17, 2006

THE SHADOW

RuniCtC wrote:
psiewert wrote:
Myth 1: Aftermarket Engine Modifications

Any modification you perform on your engine, intake, or exhaust, WILL NOT IMPROVE MPG unless you are either a) modifying fuel control to limit fuel consumption b) reducing volumetric displacement, or c) using a cleaner fuel.

Installing a CAI will allow higher volumes of air, and much colder air. This means that MORE fuel can be burned per revolution. Thus more power and more gas per mile.

psiewert



Ok, so here's a thought...

If you increase air consumption, thus increasing fuel consumption, but you are making the same load on the engine; you should'nt loose mpg. Yes more fuel is spent per revolution, but the work output should be greater, right?


With more A/F, you are NOT making the same load on the engine. The revolutions increase thus, increasing output and requiring more input to keep that power level up. BUT, increasing input without demanding much from the combustion process can, inturn, help reduce fuel consumption by preserving irradic fuel burn off from high engine rpms.



I know not with what weapons World War III will be fought, but World War IV will be fought with sticks and stones. A.E.
[#] May 10, 2008 11:17am
dsm3383


Reg'd: Mar 14, 2007

Scion Guru

psiewert wrote:
Well then you need to explain exactly how you measured that MPG. For example.

What was your driving distance?
How many gallons did you use?
How fast were you driving?
Was it the exact same route?
What was the ambient temperature?
What were the road conditions?
How many stops?
etc...
etc...
etc...

You're talking about 3-5 mpg difference. You can get a 3 MPG difference by just reading the fuel gage from a different angle.

This is the way they measure MPG professionally.

1) Run engine till all fuel is consumed.
2) Measure exact fuel put into tank down to the ounce.
3) Run the engine at a predetermined torque and RPM with a programmed ramp up rate.
4) Measure RPM at the wheel contact surface.
5) When all fuel is consumed and engine stops completely, measure the distance traveled down to the foot.
6) Distance Traveled / Gallons consumed = MPG
7) Repeat 10 times for an average MPG.
8 ) Perform steps 1-8 at different RPMs and torques to determine City and Highway MPG.


First of all I have observed the government MPG rating tests on new vehicles first hand. It is similar test to what is stated above.

The one thing to know especially if your using this as a case and point is that between each run is a large variance. I have seen 5 - 8 MPG differences between each run. The testing done to report a vehicles MPG rating is not accurate at all. That is why an average is used cause that's the only information you really can give a consumer.

Beer is proof that God loves us and wants us to be happy.

-Ben Franklin
[#] May 20, 2008 01:31pm
dsm3383


Reg'd: Mar 14, 2007

Scion Guru

One extra tid bit of interesting info on fuel economy!

Approximately for every 100 pounds of weight reduction in a vehicle results in only a fuel consumption decrease of 0.1 MPG.

Beer is proof that God loves us and wants us to be happy.

-Ben Franklin
[#] May 20, 2008 01:48pm
FrAnkRYzzO


Reg'd: Oct 21, 2005

$upervillain
for Hire

^^And roughly 1/10th of a sec faster in a 1/4 mile

Even Zombies can Search...
..............
.......................................................................Why Can't You?!?!?!
[#] May 20, 2008 01:55pm
falkore24

Reg'd: Mar 30, 2008

Master Mechanic

Here's a thought just to throw this discussion for a loop:

Our cars cut off the fuel injectors almost completely whenever you are not pressing the gas at all .... so with I/H/E/P/Flywheel, you CAN get better mileage due to less parasitic drag when coasting.

[#] May 20, 2008 01:59pm
Dr_Isotope


Reg'd: Sep 02, 2005

Chief Mechanic

^^^ Incorrect. DFCO (Deceleration Fuel Cutoff) is only in effect between 4k and 2k RPM, and only when the ECU is not registering an engine load (as in, going downhill in gear). You'll never coast long enough for it to impact your fuel economy in any way whatsoever. I tested the impact of DFCO over 10 tanks of fuel, almost 3000 miles, and while attempting to use it to it's best advantage, it did not affect fuel economy even 1/10th of a mile per gallon.

To address the OP's points, the notion about tire changes is perhaps partially correct. A heavier wheel/tire combo will negatively impact economy quite noticeably, as rotating mass is the enemy of... well, all things good. Ask any of them blingin' SUV drivers with 22s. Increased contact patch will also hurt economy. I know that when I went from 215s to 245s, I lost about 1.5mpg overall, consistently, over 30k miles traveled.

And to the "power mods"-- if anything you put on the car makes for better economy, it makes for less power. You can't have both. Just double-agreeing with the OP on that one.


Custom parts for your Scion tC: Doctor Isotope dot Com
[#] May 20, 2008 05:55pm
falkore24

Reg'd: Mar 30, 2008

Master Mechanic

Thanks for the info on the cutoff doc! How about this one?

Mods make your wallet lighter! Weight reduction FTW!!!

[#] May 20, 2008 06:00pm
Dr_Isotope


Reg'd: Sep 02, 2005

Chief Mechanic

Now that one is so true it's physically painful.


Custom parts for your Scion tC: Doctor Isotope dot Com
[#] May 20, 2008 06:03pm
Mayo


Reg'd: Feb 14, 2007

The Guru

falkore24 wrote:
Mods make your wallet lighter! Weight reduction FTW!!!


Hahaha!! True true. Time to increase my fuel economy by spending now.


"I have an idea! An idea so smart, my head would explode if I even began to know what I was talking about." - Peter Griffin
-- Want to post a "For Sale" thread? Check out the rules first.
[#] May 20, 2008 06:10pm
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