As we all know, to accurately calculate MPG you need to know two things precisely:

1. The distance traveled, and
2. The amount of fuel consumed.

How does the xB2's computer know how much gas you're using? Can you input the number of gallons when you fill up? Is it somehow measuring the exact amount of fuel in the tank, or monitoring the fuel flow as you drive?

I guess what I want to know is, is it accurate, and how do we know it's accurate?

r

 

It has to be monitoring the fuel used as it uses it. The xB1's computer calculates this as well, you just have to attach a gauge for it to tell you. I use the ScanGauge 2.

 

It could be monitoring the the rate of fuel going through a given point somewhere in the fuel delivery system for the instant mpg, and then keeping tabs on that in some predetermined incriment to calculate an average.

 

That's not how the Scangauge works. You have to tell it how much fuel the car used between fillups, and how big the tank is. It uses those numbers plus the odometer data to calculate MPG. As a result, there can be significant inaccuracy in the ScanGauge's MPG reading, resulting from variations in exactly how much fuel you put into the tank during a fillup.

The only thing the xB1 "knows" about fuel state is when the low-fuel sensor trips.

Obviously, in the xB2 you don't need to tell it the fuel capacity; that would be hard-coded since the computer is specific to the car. My question is whether it, like the ScanGauge, uses derived numbers to calculate MPG (derived from you supplying the amount of fuel you put in at the pump), or whether it has some way of actually measuring fuel flow (which would be something unusual and worthy of note).

r

 

It could be. If it is, that would be an unusual bit of automotive technology, worthy of note. Thus my question.

r

 

So you're trying to tell me that the xB1's computer doesn't know what the rate of fuel flow is? You're trying to say that it only knows how much gas is used by you telling when you fill up and how much? WRONG. Those numbers are good in setting the correction factor, but it does know the rate of fuel flow. One of the readings it will display is Gallons Per Hour, so obviously it knows how much fuel it is using as it uses it.

 

You don't have to input anything. It calculates it on the fly and can be reset manually (it also resets automatically when you fill up.

I am an automotive engineer, and if I had to guess, I would say it probably uses the duty cycle information from the fuel injection (how long the fuel injectors are firing), combined with the known fuel injector flow rate, and the vehicle speed.


That way it would know exactly how much fuel is going into the motor and can average fuel use over time and distance. It probably does this constantly, but it updates the display every 10 seconds or so.

Like I said, I don't work for Toyota or anything, so I am just guessing.

 

You can increase the refresh rate on them. No need to wait ten seconds. I have mine set to 'fast' which refreshes about every half second or so.

 

thats right. pretty basic stuff

 

I was talking about the avg MPG display, instant is faster; and I don't think the refresh rate is adjustable without a tuner...is it?

 

I am talking about the MPG display on the ScanGauge 2. somewhere in there is a refresh rate setting. I did not use a tuner or anything to get it to refresh more often, it is a setting within the unit.

 

Ohh...thought you were talking about the new xB.

 

I'm just guessing one way to do it, because I don't know enough about the systems. I doubt they put anything groundbreaking into the system as the mpg gauges are rather common and have been around for a considerable time. They could just run an algorithem based off the speed and rpm as variables. They could measure resistance somewhere in the system, I'm not a tech so I don't know. BMW has had analog mpg gauges for at least 20 years, and the Prius has had them since it first came out 10 years ago; so I doubt Scion did anything new.

 

but the problem with running that is customers will come in complaining that their MPG sucks whenever its windy. or complaining that its in accurate

yet they do that anyway already.

the systems basically look at the injectors duty cycle (how many times its opening and closing, letting in fuel), current speed and engine load (strain put on engine). the more load (ex: hills, wind, towing a 10 ton boat behind the xB) will cause the engine to work harder to maintain the same speed. more fuel is needed dropping mpg.

anyways, its not a totally accurate gauge since to max it out all you have to do is take your foot off the gas and it spikes to 99mpg

 

what I ment by resistance was electrical resistance. There are so many electrical components on a car, that it doesn't seem a stretch to me to have a one or two be effected by gas milage somewhere. Not sure. My first answer seemed to be more logical.

Either way, it doesn't really matter, it does it some how or another and it's most likely a basic off the shelf system used on a hundred different vehicles.

 

As a retired electronics engineer, I can assure you that that would be the most cost effective way to go. A unit that accomplishes that can be done (designed and manufactured) pretty cheaply (in production quantities), so if Toyota lays out a couple of $$ per unit to do this, would make sense to go that route.

The average MPG and instantaneous MPG would be powered off the same logic unit ( a discrete module) and actually, the avg would just be a summary of the instant.

Nope, don't work for Toyota either, just designed computer hardware for over 30 years here.

-Mick, EE

 

As you scroll through the options it's the first that says a number with MPG in the corner, right?

 

Actually, I think its labeled Avg MPG. Wife is at work now, so can't check on it, but from when I have been in it, that is what I THINK I recall it saying on the monitor screen.

 

For the first three tankfuls, I have averaged 30 mpg. But, the xB's calculation shows 2-3 mpg better. This is for a 5 speed in a mix of expressway (55-65 mph) and small town country back roads.

 

Checked the car last nite to see what the display actually says for average MPG.

To see the Average MPG you are getting, toggle until you get "AVG" in upper left hand of display and "MPG" in lower right hand side.

The instant MPG seems to be all over the place when driving and am not sure what use it really is, except you could find the speed at which you were getting the best MPG when using the cruise control on a trip. Outside of that, not sure what else its for.