Is the computer really that innacurate?

I always set the pump at the slowest notch and stop the first time it shuts off. I'm scared to try to put more in. How can you tell when it's right at the bottom of the fill tube? Peer in with a flashlight? Anyway, I know I may not always be filling the same, and I have not filled enough times to average out the differences, but for what it's worth:

I filled within a couple of miles of the last bar starting to blink and put in 8 gallons. I had driven 316 miles, so 39.5 mpg. The car said 38.8 mpg for the tank.

For what it's worth, odometer inaccuracy would not cause a difference between reported and calculated mileage, since both are using the same odometer

 

I think a lot depends on the cost of the system as to accuracy. My Jaguar XJ8 computer was sometimes spot on with the calculated and posted mileage (approx 19 mpg). At other times it would be off by as much as 1 mpg, go figure. I think the temperature of the gas has a lot to do with it since that's probably the largest variable not accounted for by the computer. The gas comes out of the ground cool but, on a 90 degree day, it must start to cook in the tank between the twin exhaust pipes. Long term mileage (over 20k miles) the calculated and computer mileage did a mathematical homing in on 19.2 mpg. Not bad for a 4.0 litre V8 with rocketship performance.

 

By the way, my 2004 Prius is flirting with 50mpg on the Washington commute. At 1400 miles it's definitely loosening up and ( I think) getting quieter.

 

Wasn't there something about this on the yahoo forum months ago? I thought people said that if they averaged the MPG readings from MULTIPLE tanks calculated by:

1. fillup amounts and odometer (calculated MPG)

and

2. computer MPG reading (displayed MPG),

both would match very closely. I vaguely remember people speculating that this indicated that the displayed MPG was probably very accurate, but the problem is that each time you fill up the bladder does not take the same maximum amount of fuel.

Since calculated MPG depend on the amount of gas you can pump in the tank, if the amount of gas the tank accepts is variable, then your calculated MPG will be innacurate (for any single tank of gas). i.e., the amount of gas the tank accepts is NOT exactly the amount of gas you used since the last fillup.

Could this be the reason that displayed and calculated MPG figures are off for single tanks of gas?

-Kevin

 

The discrepancy can also be caused by the type of gas or something as small as the gas pump you used. You dont know exactly how much gas your car is really getting unless you fill it up so much that it is comming back out (wouldnt reccomend that). It may not be the car at all, but rather the old technology gas pumps!

 

i think we are all looking at the wrong culprit perhaps?

to accurately judge fuel economy, we would have to have the battery at the same charge every time we filled up.

the temperature would have to be the same.

we would have to use the same fuel pump. (they may be required to be calibrated on a regular basis, but that by no means gaurantees their accuracy.

and finally, we would have to monitor the actual quality of the gasoline being used. gasoline quality from one tank to another is not static by any means.
Gas when it is transferred from refineries to the main holding areas before being distributed to the gas stations all use the same pipelines to transfer gas. so the difference between reg and premium is blurred.

after all, gasoline is a mixture not a compound. impurities are introduced into the gasoline quiet easily simply because of its highly volatile nature.

so gas mileage that seems to vary quiet a bit may actually be varying for many different reasons.

We also need to remember that as gas prices go up, honesty will slowly erode. a few years ago, a news magazine show did a survey of gas pumps in the Chicago(i think) area and found that about 15% of the pumps were short changing the customer. The show also stated that the new electronic pumps were supposed to be tamper proof, but as im sure we all know, if money is involved, nothing is impossible.

 

The battery is only good for a couple of miles, so on a full tank of gas the SOC would have negligible effect on calculated mileage. Only 1/4 mpg difference, if you get 45 mpg for 8 gallons, but then get another 2 miles because the SOC went from full at fill-up to empty at the next fill-up.

 

OK, my turn to chip in on this topic. Total fuel used is calculated during any period as injector on time * injector rated pounds per hour * 4 injectors. Inaccuracies result from viscosity changes in the fuel with temperature, fuel pump pressure variation within tolerance, and allowable delivery tolerances for each injector. The resulting computation is truncated, and the cumulative truncations result in some progressive error. Distance is simply the vehicle speed sensor input average speed * time. This value is also truncated to each OEM's desired specifications. The result is, if someone does a good job during development of creating a fudge factor based on consistent shortages or overages, the end resulting calculation is within 3%. Believe it or not, most "regular" cars have about +/- 6% relative accuracy. I don't like this either, but it is reality.

The fuel gauge, and I was involved in this at one point of my career, is another matter. The fuel tank engineer designs the ideal fuel tank, in part to address the last system's inaccuracy and resulting complaints. Each OEM has a reserve specification which often varies from model to model. The ideal tank is then fitted to the vehicle and the fuel tank engineer craps his/her pants. The tanks never seem to go in at the intended fore/aft and side to side pitch. Now comes the scramble. The fuel pickup is designed to work on inclines and declines as well as level ground. Baffles within the tank help keep fuel near the pickup as well as attempt to keep a reasonable "average" fuel level at the fuel sensor location. This all works for a period of time. Driving down the road the fuel is sloshing, causing the fuel level to vary. The engineer attempts to make changes to the baffles, pickup, and level sensor to once again optimize the design, ultimately resulting in several compromises. Now comes the current technology of digitally filtering the fuel level sensor's output to minimize sudden changes in fuel level. Some algorithms work better than others. The filtering results in delayed updates, whether they be increases or decreases. Heck, I even remember one bright engineer's algorithm that would not allow a fuel level increase unless the ignition was cycled. That's right...drive on up on E, stop engine and turn the key to run to watch the gauge, fill it up, start it up, and the gauge remained on E until you actually shut it down. Those who broke the rules and refueled with the engine running were totally confused. Kids attempting to simply bring the gauge back to 1/2 or whatever were foiled.

The point is each tank has a rated capacity. Take my Grand Prix for example. 17 gallons rated, low fuel indicators on when only 14 gallons can be added. Running the car to what the computer says is empty and then some, I once got 16 gallons in, but usually only can get 14 or 15. The tank is tipped such that nearly 2 gallons are unusable, more if you are making tight right hand turns or going up hill. The Prius does not seem far off from design practices I am used to. 11.9 rated capacity, standard 2 gallon reserve, some of which may not be useable depending on tank manufacturing tolerances, tank installation tolerances, and fuel sender tolerances. Then there is the bladder. Getting a low fuel warning when 8 or 9 gallons is used would appear reasonable to me. The fuel fill variability seen due to the bladder clearly adds another gallon or 2 to the equation, depending on temperature, rate of fill. etc. While I would agree there is a perceived reduction in tank capacity, again believed to be due to the bladder, the only way to conclusively prove this is to fill up the tank, then drain it dry by dumping it all out and measuring the actual fuel inside. I think too many people are caught up on the 11.9 versus the intended "normal" use range, which per Toyota advertisements is 500 miles per tank, 10 gallons worth at 50 MPG. Using the EPA numbers, one can quickly see that this is achieved at the 51 MPG HWY rating, the 55 combined, and the 60 City with fuel to spare. The fact that some are not getting that last gallon in during refuel because of the bladder may be frustrating. But my Grand Prix warns when I have 3 gallons left. This could be anywhere from 0 to 75+ miles worth of range. The Prius warns when it has anywhere from 0 to 150 miles worth of range left. What your range really is depends on the temperature, type of driving, etc. Which gets us back to the fuel tank engineer, who is told not to allow anyone to be unexpectedly stranded due to an out of fuel condition, regardless if it be from incline, decline, low temperature, heavy HVAC use, etc. This person is not banking on the 50 MPG minimum scenario during these last 3 gallons. Or that we are monitoring the fuel use as closely as many are. They are betting on worst case, just in case.

 

is it breezy out today?

well it wasnt yesterday.

a 5mph headwind on an AVERAGE car can change gas mileage by 3-8%. on the Prius, that value will most likely be much higher. the reasoning is simple. for the super high mileage figures to be obtained, it requires a type of driving that takes full advantage of interia and momentum. therefore even a small breeze will noticeably affect your mileage. the above stated figure is primarily for smaller cars. smaller cars do provide a smaller plane for the force of the wind to act upon, but their lack of weight and therefore lack of inertia results in a greater loss of momentum.

also i agree with MDAC in that i have owned over 2 dozen cars in my life and have yet to see any of them get their rated amount of gas in the tank. that includes several times when i ran out of gas and then immediately filled the tank with as much gas as i could get in there (a habit i have always practiced)

 

Without saturating the vapor canister, right? :_>

 

lol... have the cars i owned probably didnt have one.

 

Thank you, mdacmeis, for the information about gas tanks.

 

your physics is flawed.

im not spouting stuff off the top of my head. there are many factors that apply when considering levels of entrophy in any closed energy system.

when considering forces acting on a surface, we have to consider the amount of force, momentum and surface area of the force. in the average case, the higher weight of the vehicle will out weigh the disadvantage of of the greater surface area at which outside force act upon it.

also you are confusing issues here. obviously overall gas mileage is give and take consideration. so the heavier vehicle will expend more fuel just to attain speed. but that is not a consideration in fuel economy reduction when considering the effects of outside forces on the vehicle such as headwinds, tailwinds and slope of the road.

the statistic quoted only considers the effect of a headwind on vehicles and the reduction in fuel economy. what is not taken into consideration is the fuel usage of the vehicle. if you stop and think about this, it will soon become obvious.

to illustate.

vehicle A weighs 5000 lbs, gets 10 mpg creates a force equal to 5000A

vehicle B weighs 2500 lbs gets 25 mpg creates a force equal to 2500B.

both are driving into a headwind of 10 mph. now even when considered the difference in coefficient of drag, which even with the best versas the worst is not a lot to begin with, the smaller lighter vehicle will experience the greatest loss of momentum percentage simply because it is a lighter vehicle.

in this case the weight difference is 100%. the difference in the coefficient of drag can be 200% but it still wont overcome the weight difference. i know that last statement will be hard to swallow, but plug in the numbers and check it out for yourself.

the fact is, adding 10 mph headwind is esentially the same as driving 10 mph faster. the bigger car with more weight will use more gas but it already uses a lot of gas. the lighter car has a smaller engine and although it wont use no where near the gas the larger vehicle uses, the percentage of fuel it uses goes up much faster than the bigger vehicle that at lower speeds is not running in it optimum power band.

the Prius according to the manual, its optimum speed is 62 mph. now a 10 mph headwind will be effectively like driving at 72 mph. although it is a 16% increase in speed, the gas comsumption will be much higher simply because a large car with a V-8 is not driving at its most efficient at that low a speed where the Prius is. that accounts for the smaller car losing the greatest percentage of mpg.

now that does not mean that driving faster will get better gas mileage. but what it does mean is that a large car may go from 10 to 9.5 mpg whereas the Prius might go from 50 mpg to about 43 or less. (this is my average from observation on trip of 700 miles)

 

now to add to my previous post.

if what you are saying is true, then the slower you drive the better your gas mileage will be.

well that is a linear relationship as we all know. in fact, at low speeds the difference in economy is neglible.

otherwise if you get 50 mpg at 62 mph, then we shoud get 100 mpg at 31 mph.

well it doesnt work that way. the why is that other forces are at work. and when considering the debilitating effects of outside forces on movement, weight is still the dominating factor.

take an extreme case with a compact versas a semi. if both were to coast to a stop who would stop sooner?

trust me if what you said was true, then there would be no advantage to larger vehicles to transport heavier loads.

under your asumptions, you claim that at constant velocity the result would be equal.

well there may be constant velocity, but to overcome things like wind, rolling resistance and gravity, you must have acceleration.

 

you cannot compare weight with Cd.

the coeficient of drag is a ratio not a constant. weight is a constant provided we exclude radical altitude changes or interplantary travel.

a moving vehicle has weight and coeficient of drag

a stationary vehicle has weight but no coeficient of drag.

now heavier objects do not fall faster, but they hit the ground with more force and that is what we are talking about.

you need to separate the difference of constants, ratios, and given physical properties.

a vehicle traveling at a constant rate may appear to have zero horizontal acceleration, but it still has vertical downward aceleration equal to the acceleration of gravity (we are assuming that he remains on the ground)

now that downward acceleration creates a downward force that slows horizontal acceleration by increasing rolling resistance of the tires where the downward force is directed. therefore to have constant horizontal velocity, you must have acceleration to counteract the downward force of gravity which in turn creates additional drag on the moving vehicle.

ok got that??

now we also need to realize that velocity, acceleration are vector quantities. (i think im starting to lose you...and me) momentum is a measure of angular velocity.

when we drive down the road the direction of force is not parallel to the road. if that were the case, we be not moving. the actual direction of force is slanting upwards to compensate for gravity. that is what torque is. torque is the force that is the vague equivalent of acceleration but works in 3D space. acceleration only works in 2 planes.

 

I've got brain fade right now, so I am not going to say this exactly right, but I recall an equation that dictates that as speed increases, the supporting horsepower needed squares. Thus, driving 60 does not require twice as much power as driving 30, but much, much more. CD is indeed a ratio, and the faster you go, or the faster the wind blows, the bigger the equivalent brick wall you appear to be. This may not immediately seem logical, but any Indy car, Nascar, or funny car driver will show you that it take a ton of HP to get just a few MPH more out of their vehicles. Think also of airplanes. To push or pull through the air as speed increases requires increasingly huge amounts of power. Jets improve efficiency and increase speed by climbing to higher altitude, where the air is thinner, resulting in less drag.

 

MDAC:

yes you are right. actually it takes more than the square of power in to get effective power out.

but any moving body has to overcome gravity which is 9.8meters/sec².

that is part of the reason why it takes the extra power. then we add rolling resistance, wind resistance, friction of all the moving parts in the engine and then multiply that by the energy efficiency rating to get effective horsepower output.

as you can guess, after all that is taken into consideration what we have is in effect

a lot going in, and a little coming out. the Prius is more efficient simply because it takes energy that had previously been discarded and re uses it. that energy is inertia or momentum.

microengine technology is working on using more wasted power from combustion engines, namely heat. visions include mini electrical power plants using super efficient heat exchangers and the presurized water concept to generate power. the smaller scale of the microengines will hopefully make the idea feasible on any scale. but that is another thread.