mpg and weight

[McPatrick]

I was wondering if there is a theoratical weight to calculate the mpg at a given weight at the same speed if one mpg number and one weight number is given.

Say I know I get 50 mpg at 65 mph with a car that weighs 2200. I look at my car and wonder what the mpg would be at the same speed if I manage to take off 100 lbs of the weight. Is there a formula for this at all?

[goodella@hartwick.edu]

I haven't seen any formulas for any type of mod, since everything is different each tank of gas. It's not like a regular science experiment where you can charge a single variable.

Regardless, I was wondering the same thing. Generally, if everything else stayed the same, I would think you could calculate it like this:

old weight / new weight * mpg at old weight = mpg at new weight.
2200 / 2100 * 50 = 52.38

In any case, you might get +2.38mpg, but in reality it could be less. Can't hurt though!

[McPatrick]

Well, chances are the graph would not be linear. It would probably be doable adding 100 lbs extra and then 200 lbs, then get the mpg's for a controlled trip and then extrapolate (is that English) the graph in the other direction to see where this would get you. Just thought of this.
The reason I started thinking of this is the gastank that I usually fill up to be able to calculate my mpg, but which I usually only drive till half a tank, so I carry 4 to 5 gallons in extra weight with me. Add to that a spare tire that could be thrown out (hey I am an AAA member anyway) and some other things.It made me wonder if it would be worth it...

[skewbe]

Real hard to guess, you can do an A-B-A test where you remove 100lbs (or add 100 lbs) and get a pretty good feel of the effect in your driving environment.

In P&G in my metro, I haven't seen any correlation, sometimes the extra weight of passengers actually seems to help?

[McPatrick]

In P&G in my metro, I haven't seen any correlation, sometimes the extra weight of passengers actually seems to help?

Well that doesn't make much sense theoratically does it? But then again maybe it works that way in Middle Earth

[skewbe]

Well, there's a lot of variables. But in my tiny metro, I can glide further with the extra weight, so maybe I don't have to relight the engine as many times???

Note, I see you haven't discovered the joys of kill switches and engine off coasting

[Guillermo]

Mass only affects acceleration not constant speed driving.
Force=Mass X Acceleration

It takes more energy to accelerate a heavier car at the same acceleration as a lighter car.
Or using the same energy, (fuel) the rate of acceleration of a heavier car is slower then that of a lighter weight car.

The amount of force (or fuel) it takes to drive a car at a constant speed depends on air resistance, rolling resistance and mechanical losses (friction in transmission, bearing, engine etc..) The weight of the vehicle only slightly affects rolling resistance when traveling at a constant speed. But this factor is insignificantly small.
So the equation you are looking for does not and cannot exist in a way that can be applied to different cars.

[Biffmeistro]

And the P&G working better with a heavier car does make sense.
Edit: Well, no reason there would be a lessening of FE in P&G with weight.

Though you may be using slightly more gas to accelerate during the pulse, you can coast at least enough longer to cancel out that loss. Extra weight equals extra momentum.

For example, if you apply 10 seconds of acceleration to a light car, you'll get going faster but also slow down sooner once those ten seconds are up.
You apply the same force for that same 10 seconds to a heavy car, you won't go as fast, but you'll take a lot longer to slow down.

Theoretically, in an ideal environment, you do those tests next to each other, the cars will go the exact same distance, but the light car will get there sooner.

Of course, this is all if I remember my physics correctly.

[Erdrick]

skewbe: Yeah... EOC is excellent. I was a bit scared of it at first, but now I actually PREFER the feel of no power steering and non-assist braking! If only we could figure out a way to ACCELERATE with EOC (on an upgrade)! haha..

Bonus of EOC is that you can be moving a ton or 1000lbs... doesn't matter. Each one gets infinite miles to the gallon. Definitely one of the most important techniques for hypermiling!

[Bill in Houston]

Yeah, I have seen a calculator where you could put n a few different things and it would calc the mpg for you. You would have to look up, estimate, or back into some stuff like Crr and Cd x A.

[lca13]

I've made up a spreadsheet that allows you to play with weight at Cd and see the theoretical changes. It also has coast down input fields to get the experimental equivalents... then you can correlate to get a pretty good picture of your car... once you have that, varying things like weight should be pretty close. It doesn't do mpg directly, just force instead, but from that you can get to work and power pretty easily. If anyone wants to try the spreadsheet, drop me a mail with your email and I will send.

[lca13]

>>Mass only affects acceleration not constant speed driving.
>>Force=Mass X Acceleration

Only a theoretical truth. In practice, once is constantly accelerating and decelerating around your target speed.

[landspeed]

Look here :

http://www.gassavers.org/showthread....hlight=rolling

It is a simple spreadsheet I made - the original is from the link at the top of the thread. It lets you add certain details about your car, and then work out the power being wasted by aero and rolling resistance (the two main sources of drag for a car).

It is very useful for deciding what speed to pulse & glide around, and so on.

One proviso - the engine efficiency drops off as speed decreases, so, the slower you go, the more pulse/glide you need to do. I find my car is best doing P&G from 30 - 40 (because below 30, I can't use 5th gear, and above 40, engine enrichments move to a more extreme mapping).

[Bill in Houston]

Originally Posted by lca13

>>Mass only affects acceleration not constant speed driving.
>>Force=Mass X Acceleration

Only a theoretical truth. In practice, once is constantly accelerating and decelerating around your target speed.

I think that one usually multiplies mass by the Crr and some kind of velocity term to get rolling resistance. So, it does matter at a steady speed.

[GeekGuyAndy]

I really don't think that adding weight could ever improve mpg, even considering that you could EOC further. From the physics I remember, it would make sense that less weight means less acceleration needed, but less glide. More weight needs more acceleration, but can glide further. However, with more weight, there will only be more friction. IF you had no friction, than EOC would help immensely, but that can't be the case. I would vote for a lighter weight always having better mpg.

[McPatrick]

I would be inclined to vote for that as well. Why else would a company like Honda make the VX as light as they could to make their fuel sipping model?
This whole heavier/lighter question is in the same league I think as what is better for FE, hilly country or flat land...

[Biffmeistro]

The reason for lighter weight generally having better fuel economy is that people use brakes.

The added RR from weight is negligible, the problem that weight has is that you can't use as much of the momentum as with light weight.

With a light weight car, when you coast to a stop, you slow down quicker, and therefore when you finally apply the brakes, you're wasting little forward energy, because you wouldn't have had much past that anyway.

With a heavy car, when you apply the brakes, you waste far more of your forward energy. Therefore that extra energy that you spent getting that car up to speed cannot be reclaimed with it's longer coasting.

Heck, with my current 3500+ pound t-bird, I've been highway cruising at 60mph before, came to the "1 mile to rest stop" sign and right there popped it in neutral and turned off the engine. When I finally got to the rest stop, I was still doing almost 30mph (On level highway). I could never have done that with my old 2300 pound 89 Corolla coupe.

So not necessarily in a world without friction (Because then FE would almost be infinite. Get to 60 mph and coast for the whole trip across the country), but in a world without braking, weight wouldn't have any real effect on FE.

Edit: Heck, when I just went on vacation, I was loaded down with 2 extra people plus at least 200 pounds of luggage, bringing my total weight to almost 4500 pounds, and I was still getting the 35-45 MPG highway (at 65+ mph) that was my ideal before I just changed my O2 sensor.

[skewbe]

I think saying extra weight never helps is a bit of an oversimplification. Even if you were pulsing and gliding on a flat track with no wind, it STILL takes a certain amount of energy to restart your engine (an mph or so), which will be again lost when you kill it, so there is definitely some return in cycling the engine less (pushing longer and gliding farther).

Having enjoyed RC sailplanes for a few years, I can tell you that I was making a mistake when I was thinking everything had to be as light as possible. My first longest flight happened after I decided to add a couple OZ to my 1.5 meter RES. Prior to that it was like trying to fly a piece of tissue paper. After the extra weight, I could penetrate the wind and cover more ground quickly and "coast" to the next bit of lift. Most sailplanes come with adjustable ballast to adjust to current conditions, don't want too much or too little ballast.

[baddog671]

There are so many variables in this that I dont think it could be experimented properly. The grade of incline/decline, weather, traffic, everything would throw this way off...

Extra weight WOULD help you glide further and EOC further, but this small gain would be overwhelmed by the effort it would take to accelerate, especially from a stop. And, so, you can glide further downhill, but you still have to go back up it. The mpg gained coming down will not overrule the mpg lost going up..

Either way, lighter is the way to go. Why else would every prototype and race car be light?

[Biffmeistro]

Race cars are always light because rate of acceleration is more important than FE. Prototypes because they're cheap and don't put everything a production car needs into it. Plus they need to be able to push them out onto the showroom floor. Hehe!

But I'm not at all denying that lighter cars get better FE. That's proven time and time again. But that better FE is in city driving. On highways, or any situation where you wont be touching the brakes, weight doesn't make a difference.

But honestly, you'll never make a trip where you don't touch your brakes. It's only on segments of the trip you don't touch your brakes. Such as P&G. In P&G, weight doesn't matter. In nearly all other driving, it does.

[baddog671]

How does weight not matter in PG? Extra weight could increade the time it takes to pulse back up to speed, and extend the distance covered in glide. I doubt these would balance each other out, so it would matter...

Look at the statistics of F1 race cars. They have extremely high reving huge engines going extreme speeds and get insane mpg. F1 cars are LIGHT, they dont even paint them to avoid the extra weight of a millimeter thick of paint....

[Biffmeistro]

But the longer glides do cancel out the more costly pulses.

And the F1's FE is due mostly to aerodynamics, not necessarily weight.

[baddog671]

I have lightened my car nearly 100 pounds off the factory dry curb weight and have seen an increase in FE and overall power. I can cruise in higher gears than I have before, and that is a direct weight to FE improvement. I can also leave stoplights with less effort on the engine.

There are no flats around here, so I cannot vouche for "flat-driving", but I promise you, weight has a dramatic affect on mpg when you start cruising up a hill.

[Biffmeistro]

I know. I never denied it made accelerating easier on FE to be lightweight. I never denied it had an overall improvement for FE to be lightweight.

I just said it made no difference for constant, level diving, or more appropriately, P&G.

[McPatrick]

But the longer glides do cancel out the more costly pulses.

Do we know this for sure?

[rGS]

I think that one of the factors that hasn't been mentioned yet is what I've come to believe as a fundamental "FE driving technique" which is the "when driving up hill, set the power (RPM) and accept whatever MPH you get." For those who mentioned Newton's Second Law of Motion, that's like having a constant Force. So, given a constant Force, if Mass is relatively low, then Acceleration will be relatively high. Similarly, given a constant Force, if Mass is relatively high, then Acceleration will be relatively low. What that means is that for a heavier loaded car, it means it's going to take a longer time to get to say 55 MPH than a lightly loaded car. However, since fuel efficiency is related to the engine RPM reading on the tachometer more than the MPH reading on the speedometer, I think the "set the power (RPM) and accept whatever MPH you get" driving method basically cancels out the weight factor. Now, if you live in a part of the country with minimum speed limits then you wouldn't be able to "accept whatever MPH you get" then that would necessitate a higher throttle setting in order to move the relatively heavier car at a proportionally higher acceleration to achieve and maintain the minimum speed limit. In that case, yes I agree that a heavier car would have a negative effect on your fuel economy.

So to summarize those who posted before me, the answer is "it depends."

[Biffmeistro]

For my car, yes.

Usually it's at 3600 pounds or so (With me in it) and on trips where I'm going a consistent 65 mph, I get an average of 35-45 mpg.

When it was loaded down to nearly 4500 pounds for that vacation, I got that same average of 35-45 mpg at 65 mph.
(And that was over 600 miles of highway cruising, so it was a pretty good way to average. But note, I'm ONLY counting the lengthened interstate trips where I didn't touch the brakes. The total average for the trip was 29.7MPG. Th 100 miles of city driving I did in that one trip lowered my MPG by that much! If I were lighter, they wouldn't have)

Nearly a thousand pounds extra weight, and it didn't change my highway MPG.

Wreaked havoc with my city MPG because I had to use the brakes and I couldn't reclaim that extra fuel used to accelerate in longer coasts.

The physics works. My real world experience works.
On the highway, weight plays little factor in MPG, so long as you don't touch the brakes.

You won't go as fast, but you'll go the same distance on the same set energy. And that is counting friction into the equation. The rolling resistance is higher for heavier cars, yes, but not by enough to really matter. With equal drag coefficients and equal force being applied, the weight won't effect MPG.

If you don't ever touch the brakes.

That's the key sentance. And therefore, since people always will touch the brakes.

Lighter cars will ALWAYS get better real world MPG than heavier cars. Because you will ALWAYS use your brakes.

[skewbe]

Originally Posted by Biffmeistro

...If you don't ever touch the brakes...

That's the key sentance. And therefore, since people always will touch the brakes.

Lighter cars will ALWAYS get better real world MPG than heavier cars. Because you will ALWAYS use your brakes.

That's the key here, one of the best hypermiling tips is to drive like you dont HAVE any brakes, which is not "status quo" driving, so lets not make statements like "ALWAYS" or "Real World", ok?

But I would agree in general, heavier cars will get worse mpg (unless the extra weight is an efficient energy recovery system or something)

But for a given car, it *may* be possible to "fine tune" the weight to local conditions for best mpg.

[Biffmeistro]

To be utterly honest, when has anyone ever made a trip without touching the brakes once?

If anyone can tell me that they ever ONCE made a trip where they NEVER touched the brakes, only ever used coasting to slow themselves down, I will most definitely eat crow.

[skewbe]

"Never" is not the upper limit, nor a good choice for a debate. But I use them infrequently enough and as little as possible that I probably have made a few trips around town without using them, sure.

Think about all the variables, and how they might be different from one situation to another (like the distance between hills, how much the wind is gusting, how well trained the driver is, how is the car geared, ad-infinitum),

Once you know everything in the universe then it is safe to use words like always and never