Best method to measure fuel consumption
 

Hello! I am new to this forum and which brings me here is an interest and a need to measure fuel consumption in both diesel and gasoline passanger cars and light commercial vehicles. This topic is new to me, so any information or ideas can be useful. As I dont necessarily need to know the exact MPG, but the fuel used between time intervals. Getting to know the exact distance traveled is not that important.
What is possibly most accurate way to count fuel used? As I know, it is possible to calculate fuel usage from different sensors of the vehicle, but this data is likely volatile.
May the best possible method be using liquid flow sensors/meters installed both between feed and return fuel lines? and in this case, what kind of flow meters? I've done a little bit of research and have found few of them but the specifications dont fit my needs. Most of them are made for bigger machinery, size is big and measuring starts from high flow rates, also the ambient operating temperature is not suitable to use in nordic climate where I live. I would also like to ask from which l/min quantity minimum flow rate i should be looking for, concidering we are talking about european/japanese cars & commercial vans which doesnt have very big engines.
If you guys could recommend me a decent flow rate sensor with reasonable price, I would be thankful. Device itself should be eletronical and display is not needed. Also no specific requirements for the ouput signal, any will do.

A reasonably calibrated Scangauge will give you a fairly accurate reading of the volume of fuel used. On my Escape Hybrid the variance is usually within 2/10 of a gallon over an entire tank.

ScanGauge - Trip Computer + Digitial Gauges + ScanTools

Being too old to have an ODBII port, my 1989 Civic can't use a ScanGauge. Instead, I'll be installing a vacumm gauge.

MPGuino is an option for those cars too old for OBDII.

MPGuino trip computer - MPGuino

It might be more effort than the vacuum gauge to install.

Wish I could. I did buy a MPGuino. Hooked it up. Got a few seemingly inaccurate number readings out of it, but it isn't working perfectly.

THEN someone said an MPGuino won't work with my base model, 1989 Honda Civic due to it being a "dual point injection" or something like that. The "multi point injection" was used on the non-base models.

Thanks for trying to help, though. :)