A hard and fast rule, nope, not really. The old rule of thumb is about one ounce of fuel per minute of flight for an average. There is almost no difference between the .46 and the .55 as far as fuel consumption but the .55 has a bit more torque. You can compare them just by reading the specs on the OS site.
I'm betting John has done the fuel testing on these two engines for long range or cross country flying and will post up with some facts.