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Well, the speedo is just telling YOU what to do. So let's say for sake of argument that your best target speed is 60. But you're actually doing 60 when it reads 65. Gets confusing. Surely it's equally inaccurate at other speeds, right?
What will throw off FE calculations is when the odometer reads incorrectly. Depends which way. If it thinks you're covering more distance than you really are (like the speedo reading faster than real speed), then you have a somewhat artificially high 'miles' in the miles per gallon ratio.
Tire size - height actually - does in fact affect speedo readings. Since it counts revolutions and the formula must assume stock tire size, a taller tire would simulate a taller final drive gear. It'll bring down your revs at a highway cruising speed. I've read a few threads about it on here. If I recall correctly, the general consensus was that the lower rpm might save fuel (despite the increased frontal area) as long as you're vehicle has enough power to prevent constantly downshifting for small hills. Read: underpowered vehicles might lose FE with tall tires if it results in more downshifting. I'm sure you can find out better info than my brief synopsis around here somewhere.
To get really nitpicky, a nearly bald tire will be a bit different from a brand new tire of the same size, make and model. Fun, huh?
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