We're hesitant to build any new features on top of EPA estimates for a number of reasons. We only have the pre-2008 estimates for many cars because those are the only numbers that the EPA makes available in a data-import friendly format. They also only cover US cars, so the feature would only be available for those people who have a good EPA match on their vehicle.
We have the "EPA Estimates" section under the Log Book on vehicle pages where there's an EPA match. You could take a look at the +/- numbers under the estimate, the "Avg. Price/Gallon" amount in the "Cost Analysis" section and get a quick sense of how your price would be affected.
And additionally because Fuelly don't go down to the detail of individual engines within a range (ignoring displacement etc), there isn't a 1:1 relationship between the fuelly vehicle model and the entry on the EPA register.
Hence even the EPA figure as shown is not necessarily correct or appropriate for your vehicle.
That's not necessary true, MMUK. We let people choose among the EPA descriptions available for any particular model. They might not choose the right one, but we do make all EPA models available to choose from.