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Er, smaller tires better fuel economy? I think it's the other way around...
The reason 90 could be worse than 105 is because despite the extra power required, your engine efficiency could be going up a lot from 90 to 105, possibly enough to provide a net benefit. A simplified way of thinking about it is the engine has some amount of friction that you can't remove, and higher load only increases this friction by a relatively small amount. If you're only asking the engine to give you barely any torque, then most of the power made is actually going to overcome the engine's friction. If you ask the engine for say twice the torque, then you don't need twice the fuel, because most of the fuel was going to waste in the first place. If enough fuel was going to waste in the first place, then it's possible that the increased load could improve your mpgs.
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