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Old 05-12-2008, 10:58 AM   #1 (permalink)
EcoModding Apprentice
Join Date: May 2008
Location: South Dakota
Posts: 117

GMC Sonoma - '94 GMC Sonoma
90 day: 36.97 mpg (US)
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Question regarding Scangauge MPG readout & DIY device

Question regarding Scangauge MPG readout. I do not have one and have never even seen one so I can only assume what the mpg display is showing. My question is, what does the instantaneous MPG reading look like? I will assume it is fairly constant if driving a constant speed, constant throttle, flat road etc. But if you tromp on the accelerator I assume the readout will drop very sharply, fairly quickly. If you then decelerate (let off the pedal completely) I would expect that the readout would rise very high very quickly. If you then slip it into neutral but while still moving fast I would expect the readout to be extremely high.

So, is this in the ballpark of what the MPG display shows or am I way off base? Also, what might be a typical mpg readout if you floor the gas pedal while moving slowly (30 mph) in high gear?

What would be the readout if you were moving fast (75 mph) and let off the pedal completely, or even shift to neutral?

The reason I ask is that I'm planning to build an FE gauge for my non-OBDII 4 cyl Sonoma. It will take the fuel injector's pulse-width-modulated signal and change that to a voltage which increase with the width of the injector ON time. It will also take the 50% duty cycle square wave of the VSS (will use a 555 timer IC to configure a frequency to voltage converter) which will output a voltage relative to vehicle speed.

Both of these voltages, speed and fuel injector duty cycle, will be input to an op-amp. The speed voltage will go to the non-inverting input and the injector voltage will go to the inverting input. The output of the op-amp will increase as speed increases but it will decrease as injector duty cycle increases. Thus a higher speed will show better mpg but at the same time more fuel going thru the injectors will counter act the speed's increased reading.

What I expect to see will be a constant voltage while cruising, flat road, constant speed. If I increase throttle I expect the voltage to drop, at least until the vehicle speeds up to match the increased fuel/air the engine is now receiving.

I expect that it will take a bit of speeding up and slowing down to find the "highest voltage" on my display, thus signifying optimum MPG for the conditions I'm driving in.

For me the building of the electronics is the fun part of the project even if it does not turn out to be a very useful device in the end, but to the point; does all of this sound reasonable with regard to my assumptions on what the scangauge MPG meter tells you?

Last edited by Gregte; 05-12-2008 at 11:03 AM..
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