Seems that farnell one is Ok but flow range may be a bit narrow, seems that one works with the inertia of the fluid hitting the rotor. Also I can't see anywhere on sheet whether it is suitable with petrol.
I may make a similar thing just with a close fitting rotor, more like constant volume pump to better measure at low flow rates.
Why hall effect rather than opto? I've got some experience using them for speedo setups on the bikes (in fact will be one involved in same thing) but never at small scale like this.
I was thinking if I went opto I could use an ally case and drill right through, LED one side, sensor the other and pot them both in there with some epoxy.
Actually my first serious electronics project. Thinking of going with Atmega8 chip linked to a 2x16 lcd screen. I want to display current mpg, trip mpg, litres used and battery voltage. I think there should be enough capability in there for that.