I've thought of this and looked it up - you would need to make circuits to protect the laptop soundcard, especially as injector pulses can hit +50/+100v due to inductive effects when the power supply is cut off (at the end of the injector pulse).

I found somewhere, a USB device that had about 16 A-D convertors and D-A convertors too. It was about $150 though. Shortly after this I found the SuperMID on this site (in fact, it was research into homebrew MPG gauges that led me to this site!)

Yah, worth mentioning, don't want people frying their line in ports.

I would think taking the signal from a pair of diodes in parallel, which are in series with a largish value resistor which leads to , oh nevermind, let me draw a picture :) Were talking pennies worth of parts to limit (clamp) the voltage to .7 volts.

https://www.gassavers.org/attachment....1&d=1183986302

I've got some old assembler routines I wrote for high resolution recording in dos (windows recording always make skips), I can dredge those up, mebbe a linux port would be in order.

I think we have enough info for a laptop based mpg/mph/trip/and rpm program, once you tell it some info about your car like:
1. how many vss pulses per wheel rotation
2. how many injectors
3. how many cylinders (if it is tbi)

but dang if a laptop isn't a lot of luggage for an mpg gauge :(

Yah, it isn't that simple. there are many OBDII protocols, only iso (asian cars)can be handled by the laptops serial uart chip with only the scantiest of circuitry, and less and less laptops have such a serial port available. The other protocols take $100 adapters (and you might as well get a scangauge/supermid at that point).

I'm looking for cheap and simple, that can be adapted to any electronic fi car (regardless of obd status) using surplus parts/etc.

I was just talking with the guys at work about doing something similar to that. Goal was As long as the vehicle has a vss and efi the gauge will work. Just have to do some calibrations for fuel use. I just want a DIY route for my 94 metro since it is all DIY so far :) + I'm cheap. The scangauge I bought should offer enough to be worthwhile on my newer gas hogs :o .

a vss is easy to fabricate if the car doesn't have one, basically zip-tie a small magnet to a drive shaft or axle and bracket up a coil to pick it up.

Some cars will have one for a cruise control even if it isn't an obd car.

I'd love to see what you come up with, as i would like to make one for my 94 VX...

Let's recap, assuming you don't want to tap the ODB2 codes (would work for all cars!):

For MPG you need actual distance and gas consumption (duh!)

- distance: count wheel axle rotations

Count rotations R with a magnet or an optical reader
Calibration: multiply count by the circumference Ctire of the tire in inches. That's easy to get with a chalk mark on the tire.

D = R * Ctire /63360

- gas consumption: collect duty cycle of injectors

Can we track it by monitoring the duration of the electromagnetic pulse on the injector electric wires somehow? Maybe with a pickup from the speaker of
an old walkman headset taped to an injector wire? Else tap the wire directly to measure voltage shifts.

Calibration: Need the max volume of gas Vmax the injector can feed divided by the max duty cycle DCmax. Those parameters are probably known by the racers.

V = Vmax * DC/DCmax

In a way, you're making it too tough. You really just work off a couple of assumptions. 1- injector pulse width is proportional to fuel flow 2- number of pulses from the vehicle speed thingy is proportional to speed. And then, you just lay a constant on top of it.

Instant MPG = A constant * speed / flow

Less elegant than your idea, though. :-)

that is incorrect, not all cars by a long shot. It won't work on pre-96, didn't even work on my 97 until I swapped ECU's (car worked fine though), it didn't work on my neighbors car. It's ugly too as it is several protocals under one name, some needing more circuitry than others. Check it out.

Having a layer of ECU and whatnot between the mpg gauge and the actual sensors also means a lot more can (and will) go wrong, in my experience.

Note, however, my metro uses ISO (I'm using an OBDII specific term here, so be careful), and you can use a standard serial port (found on older laptops and even old blackberries/palm pilots) with ISO for a few transistors, i.e.: https://prj.perquin.com/obdii/ So there may be an old palm pilot gorilla glued to my dashboard in the not too distant future anyway :)

revisiting the analog stuff
 

so rethinking the analog stuff and reusing a tach dwell as a mpg gauge, I found this reference about using op amps to divide.
https://www.national.com/an/AN/AN-30.pdf


So we would use the rpm circuit to create a voltage proportional to the speed (like the rpm circuit does already to drive the meter, only it is connected to an amplified vss signal), and take the dwell circuit to create another voltage (like the dwell circuit does to drive the meter, only it is hooked up to an injector instead of a coil) and divide the speed voltage (E1) by the injector duty cycle voltage (E2) using op amps in a logarithmic mode with a fudge multiplier (calibrator) on E3. Easy cheesey, right? :)

https://www.gassavers.org/attachment....1&d=1184124933