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yes more energy to CHANGE their speed but not much more energy to "maintain" a fixed speed. a larger tire is like putting a lower gear transmission in the car (you travel a greater distance per revolution of the tire)
SO in city driving or mountain roads you want tiny light wheels but on the HIGHWAY you want tall skinny wheels. the taller the better as long as it does not bog it down.
being TWICE as heavy is a pretty significant difference that might eat up the gear change savings but the higher torque of the diesel should eat that up.
then add them being wider with aggressive tread. it might be a wash.
I never go over 45mph. except on the expressways then 55 MAX 50 if traffic is light. :-) UNLESS I am semi surfing then its any speed I can make the car go and keep my car in the semi's suck zone (60-100 feet)
its how i get 65mpg in my metro and its how I hope to get 40mpg in my tracker :-) and how I got 24mpg in my 88 cherokee (4" lift full skids rs900's all around 31" meats etc.. etc..)
I drive 40,000 miles a year (down from 55+ a year) so mileage is really critical to me.
have you ADJUST your odometer readings for the larger tires? you might be getting much better economy than you think you are ??
with the larger tires on my metro the odometer is off by a factor of 1.14 on the cherokee its also 1.1) ie in the metro every 100 miles the odometer says i went I have actually gone 114 miles !! that adds up to a lot at the end of a tank !
if your getting 30mpg WHILE DRIVING 70-80 mph regularly that is actually REALLY REALLY good !!
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