Judging milage estimates

I'm looking at 2 vehicles, both have EPA highway estimates of 29 MPG. One is a 6 cylinder and one is a 4 cylinder. Those estimates are based on 55 MPH. I know that bigger engines get lower milage, and I know that as speed increases milage decreases. My thought is that while they may both get 29MPG@55 as speed increases the 4 cylinder will become less efficient faster than the 6. So my question is, am I right? and at what highway speed does a six cylinder become more efficient than a 4 cylinder?

 

While you may have a point, we don't have enough info to figure it out really

First off, the estimates from the manufacturer for the EPA are done by specific tests that are not (likely) repeatable in the real world
They are not actually running the vehicle @ 55 MPH
It's more like 100 seconds at X RPM, then 300 seconds at Y RPM, then 1000 seconds at Z RPM, that type of thing
All cars use the same test

Secondly, the weight of the vehicle has a direct bearing on the mileage

Third, the shape, or coefficient of drag, has a direct bearing on the mileage

So you if you have the exact same engine in a small 2-door and a truck you would get different mileage

Also your 6 cyl engine and your 4 cyl engine (or two 4 cyl engines) will have different sweet spots, where you get the best mileage on the highway, depending on the factors mentioned above, how the engine was made, and also the gearing of the vehicle
This sweet spot may have been completly avoided by the testing procedure
(I myself have one vehicle, a four cyl, that gets the best mileage at exactly 74 MPH)

So your answer is:
It could be, depending on the vehicle and engine, but by no means is it a given
And the answer to part two is unknown, if at all, and would be depending on the vehicle and engine and gearing
It also would not be a given