Having MPG issues, whats wrong?

This was already brought up above but I wanted to expand upon the point, speed plays a huge role on mpg during highway driving. The reason for that is because the power required to overcome aerodynamic drag has a cubic relationship to speed. Meaning if you double your speed ie 30mph to 60mph it takes 8 times as much power to overcome the aerodynamic drag. Yes you read that correctly, 8 TIMES! Considering aerodynamic drag is only one component of total drag you can see why speed plays a big factor.

Your tires also will have higher rolling resistance than OEM tires which will also contribute to lower mpg as well as increased weight which others have mentioned above.

Sorry for the long rant, I just hadn't seen a technical explanation about speeds effect on mpgs anywhere on tw before so I thought this as good a place as any.

 

pair that with the brick like aeros of the taco and thats why i will see a 4+ mpg drop when crusing at 75-80mph vs 60-65

 

Exactly! Couldn't have said it better myself

 

Ok, so I've modified my driving habits and was able to boost my MPG to 18.6!!

 

Also, I'd like to expand on what has been said above (technical stuff). Being an engineer (PE Mechanical) I can expand on these concepts.

The energy to move the vehicle is E=m*v^2 so any increase in velocity is exponentially increased.

Regarding tires, the Torque required to rotate the tires is T=tJ/r where the rotational moment of inertia (2nd moment of inertia) is affected by the equation J=(pi)(r^4)/2. So you can see that additional weight towards the outside of the tire is added at a rate to the 4th power and being at the outside it has the greatest effect.

Sorry for the nerd rant but my inner engineer had to speak up (just got my PE about 6 months ago).

With all that said, nobody really buys a truck for great MPG but I did get this truck to get better MPG than my last one.

 

Great job!

Also, check your tire pressures. When you move from a P rated tire like the stock tires to a LT tire like a KO your nominal tire pressure should increase by quite a bit.

For example, a SR5 or TRD OR/Sport will run tire pressures around 30 PSI according to the door tag but a Baja or Pro will run at 46 PSI according to the door tag. This is because the load inflation curves for P and LT tires are very different. Yes an LT tire can handle a higher maximum load but at any given inflation pressure an LT tire handles a lower load than a P tire. LT tires handle higher loads only by running much higher pressures.

So it is possible if you installed your BFG KO LT tires but are running 30 PSI pressures as the door sticker says for your stock P tires that you are actually running under inflated and that could also lower MPG by a bit.

 

All wrong. Entirely wrong. The *power* to move a vehicle (energy is irrelevant) with no rolling or aerodynamic resistance is zero. Conservation of momentum. You've confused the kinetic energy of the vehicle with the power to maintain speed.

Of course there is rolling and aerodynamic resistance. And so the relationship of power to maintain speed depends on how those resistances change with speed.

In general air drag goes as the velocity squared, but this is a bit of a simplification. It assumes that the drag coefficient of the vehicle is itself not velocity dependent which is not strictly true. But square law is a good enough approximation.

The larger issue is that air drag is not the only resistance, there is the rolling resistance associated with the contact with the road and the drive train. This is not square law, typically it is less than square law. The result being the overall resistance is dominated by rolling resistance at lower speeds and air drag at higher speeds.

So when considering 65 vs 75 for driving the result is slightly less than square law. The higher the speeds the more closely square law is a good approximation.

But none of this has anything at all to do with E=0.5m*v^2

Indeed, and this can drastically effect city mileage where there is lots of starting and stopping of the wheels. On the highway the impact is far less clear from equations and fundamentals.

Exactly. Though I just threw KO2s and a camper shell that overhangs the cab on mine and ouch as far as MPG goes on the highway!

 

As a quick way to experiment with velocity vs fuel efficiency you can use this online tool:

http://ecomodder.com/forum/tool-aero-rolling-resistance.php

Some very rough parameters for a Tacoma:

Weight - 5000
Crr - 0.12
Cd - 0.5
A - 25

Leaving the rest at defaults gives us:

55 mph - 22.5 mpg
60 mph - 20.2 mpg
65 mph - 18.0 mpg
70 mpg - 16.3 mpg
75 mpg - 14.7 mpg

Obviously the parameters loaded are not exactly correct, but they give a rough idea for the balance between rolling resistance and aero resistance. At 65 mph the aero resistance is about 70% of the total resistance and so the relationship is not quite as bad as square law.

So yes, slowing down helps quite a bit. And 65 vs 75 should expect around a 3 mpg difference.

 

I drive 80mph and floor it at lights...17mpg.

 

I appreciate the feedback but the science is sound. You are referring to 'Work', which is power over time. However once you compare two vehicles over the same amount of time then the amount of time becomes irrelevant.

 

I drive 60-65 and accelerate like an apple. 17 MPG.

 

No the "science" is not sound and energy vs. power is not the issue.

You are using the equation for the kinetic energy of the vehicle. That is only relevant in the case of changing speed (i.e. accelerating from a stop to say 60 mph). At the same speed the kinetic energy remains constant and thus by definition no power. Recall power is change in energy divided by time. You've got an equation that while square law has nothing to do with the correct answer.

The correct answer relates to air drag and rolling resistance. Drag is square law, rolling resistance is not.

I appreciate you are proud of being an engineer for all of six months and wish you a productive career. But you need to go back and review your Physics 101 text and think about the problem some more. Kinetic energy does not play any roll in the solution. If you want to have a successful career as an engineer you need to make sure that you are using the correct equations and correct fundamentals when analyzing a problem or system. Just grabbing an equation that happens to have nearly the right power law is not sufficient.

 

Lol, you're so close but not quite there. You are correct that the truck has to overcome air drag and rolling resistance but in both equations (air drag vs kinetic energy) the velocity is square so the truck's speed is affected the same (ideal conditions) based on speed. Basically, both equations are changed by the same factor by a change in speed. THUS, both equations give you the same answer when wanting to comparing one speed vs another (this equation is used mainly in Ballistic Analysis). Maybe once you've taken advanced statics/dynamics instead of just googling aerodynamics you'll understand the concept of how power and energy and drag are all related. FYI: I've been an engineer for over 7 yrs but just got my PE 6 months ago.

 

Did I miss where you mentioned what size tire and psi you are running?

 

rabble rabble rabble buy a truck for mpg rabble rabble Prius

 

Thanks, engineer for 23 years. You don't know what you are saying. Not going to waste any more time. The web is full of half baked statements, yours is just another one. Won't bother fighting a losing battle.

You used the wrong equation (kinetic energy) which happens to be square law which almost answers the question correctly. Wrong fundamentals to get the "right" answer is just clueless sloppy engineering demonstrating no actual understanding of the problem.

 

50% city driving like a stoplight warrior / 50% highway 80mph

avg 15 mpg

 

Oh no, not again. With thusual audience commentary.
I'm happy with my 18ish mpg.
Remember what happened last time.

 

Right, '23 yrs' and no license? Gotta be doing something wrong there. Let me know when you're ready for a lesson on basic aerodynamics and force balance equations.

 

You get better mileage than my smaller liter 3.4 V6 so you should be happy. I get about 12 combined on any given day, but I drive pretty assertively too.