Veijavi ,
Im interesting your project ,

also I think you dont need rpm data ,
if you know injectors duty cycle, calculate per hour fuel consumption .
for example ,if your injectors 200 cc/min ,duty cycle is 100 percent (of course this is example ,otherways most car injectors duty cycle is maximum %70-%85 ) your consumption is 200 cc per minute /per injector ..

So ,
How are you calculate injectors duty cycle ?
I dont know well ,but people do it ,only 1 injectors input ,calculate duty .

Here you can check this,I have this kit ,they are only use a 16f84a pic microcontroller and only 1 injector data ,than calculate easy and its true ,I use and check it .

http://www.jaycar.com.au/productView...=&SUBCATID=347

Other friend say about rail fuel pressure ,
yes its true ,because fuel pressure regulator is change fuel pressure ,mostly on turbo car ,it is a problem also ..

Maxi