I want to log how much fuel my car is using under what driving conditions to do some efficiency experiments. Are the any pre-made digital flow meter modules i'd be able to plug into arduino via SPI or I2c? Google isn't giving very promising results, but I wanted to be sure. I'm also unsure about how a slight impedance in the fuel line will effect how it runs. Is it doable?
Hi,
What model car do you want to measure your fuel with.
Most cars have a circulating fuel system, fuel is pumped up to the motor under pressure, the fuel then passes the fuel injectors or carburetor, though a pressure valve and flows back to the tank.
This ensures cool fuel is being delivered to a hot engine, to stop the chances of fuel vaporization and to keep constant fuel pressure.
So just measuring the fuel flowing to the engine will not give you your fuel consumption.
If your car has OBDII, then it may be possible to find the fuel consumption on the data bus.
Tom..... 
I have a 2005 VW jetta with a 2.5L engine
You could take a look at this:
http://www.robotshop.com/en/adafruit-water-flow-sensor.html
I planned on using it for a gardening project. It should be noted that I have no idea if it will work with fuel.
Brgds
Hurtigpelikan
If you want to stay away from the car's electronics, perhaps it's an idea to measure the exhaust instead? Assuming good combustion the flow + CO2 content could give you a good idea of the fuel consumption. Not sure whether (hot) gas flow + CO2 content measurement is more or less feasible than measuring fuel flow though.
Perhaps it is possible to measure the level of fuel in the fuel tank. It could be done using a ultrasound sensor and would take care for the circulation problem described by TomGeorge. I dont know about the precision of this method though.
But if the data is to be used for analyzing fuel consumption a number of other parameters like rev, speed, gear etc. must be recorded.
there are a lot of OBDII interfaces written for the Arduino. just check if your car offers fuel flow.
or air flow.
rpm is definitely a data value available. as would be air flow
rpm and air flow would offer power curves, you would have to do some calculations
but the car not only has all the sensors, it has all the wires and offers a way to connect to grab that data.
One way is to measure the pulses to the injectors. The longer pulse (duty cycle) the more fuel.
My new GMC diesel shows fuel consumption that it calculates. I assume it is using the information from the injectors (since it is controlling them!). I have been very impressed after 8,000 miles just how accurate the "fuel used" is (it is within a few percent most of the time). If that information is available on the bus, then that should give you a reasonable way of calculating fuel consumption. Don't forget to reset it when you fill up though !!