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My personal real world report:
I've been hypermiling my old Saab for some years now, starting quite easy with reducing speed and changing over all gearing ration by changing tyre dimensions and experimenting with tyre pressure. The changes were quite marginal, but it appeared as larger circumference and higher pressure gave a small improvement of fuel efficiency. I had plans to go for the extremest possible with high and thin tyres on DIY-built rims.
All this suddenly got reduced to a "don bother" when I started hypermiling for real with engine shut-down and acceleration in one minute cycles. My mpg climbed like crazy! After I had learned to fine-tune this driving technique I got to a quite steady level of about 50% of the usual fuel consumption of this particular type of car. I then widened the rims heavily and replaced my 165R15 with 195/65R15, lowered the car quite a bit and went for a stylish rat-look.
I had already a heavy right foot so no change there. The dramatically improved road holding allowed me to take the corners a lot faster, but I still drove with safety as the number one priority. The interesting part is that the lower but wider tyres now was impossible to track in the fuel economy. When I earlier tried smaller tyres I noticed a slight loss of fuel economy.
I guess the answer is that tyre dimensions CAN change the fuel consumption, but the driving style do so much more that it's actually marginal. Then offcourse, there are tyres (threads in particular) that are really hard rolling. Some older winter threads are notorious, and there's probably no surprise that many offroad tyres are too. For modern "normal asphalt tyres" I think you can put almost anthing to the car and get a much larger benefit from changing driving style!
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