If your concerned about the effect your air cond has on fuel economy, maybe you should be walking. As for air cond increasing fuel consumption by 15-25%. I'd challenge that in real world driving. I've got climate control on all of my cars and a/c never gets turned off. on my VT I get close to 600km to a tank, so of I turned off my a/c I'd get 750? I don't think so.
We could get way off topic here but try watching the injector duty cycles or fuel use L/hr in traffic at idle when the A/C switches on vs off. The difference might surprise you.
http://www.nrel.gov/docs/fy00osti/28960.pdf
"Current air-conditioning systems can reduce the fuel economy of high fuel-economy vehicles by about 50% and reduce the
fuel economy of today's mid-sized vehicles by more than 20% while increasing NOx by nearly 80% and CO by 70%." (From the abstract.)
That whole report is pretty interesting. It covers a few other things related to A/C too, like low heat transmission glazing, recirc. vs fresh air even occupant clothing. (I think that at one stage California was looking at legislating around paint colour (color?) as it related to varied heating of vehicle interiors.)
Turning off the air conditioning helps save fuel, Swiss study finds
"Automobile air conditioning systems do not run "free of charge." In fact in the hot parts of the world they can account for up to thirty per cent of fuel consumption. Even in Switzerland, with its temperate climate, the use of air conditioning systems is responsible for about five per cent of total fuel usage, rising to around ten per cent in urban traffic."
Note that the fuel consumption penalty is temperature dependent; on hot days both the power drawn with the A/C on and the compressor duty cycle are greater.
It depends on how sophisticated the climate control is in the VT but it might be interesting to see how often the A/C is actually on for a given desired temp. and external temp. Probably not much at all for low ambient temp.'s. I don't think the VT system is particularly clever but I would assume that some systems would vary the portion of recirc. vs fresh depending on in car temp. vs ambient.
The effect is possibly lesser in percentage terms with a large engine but in one of my cars (for all of my cars I have full records for the fuel used for each fill and observations noted on things like driving conditions and servicing, mods etc.) a pretty typical difference between A/C and not is 7.1L/100km vs 6.5L/100km. (I know that is 10%
). And I have no way of separating out extra cooling fan operation due to higher ambient temp. from A/C use but A/C 'on' cooling fan operation overcools for the extra heat from the condensor (= less engine cooling fan operation due to high eng. temp. than without A/C).
At (say) $3000/yr in fuel costs it doesn't take a big change to save real money either. Even a 3.5% improvement, which may not sound like much, is >$100 over a year. Would you walk past that if you saw it lying in the street? If you drive in such a way as to take advantage of it, kinetic energy recovery A/C is virtually free.
There's no magic to doing this (kinetic energy recovery) vehicle manufacturers are going to be using it (if not already) but taking it further with A/C evaporators incorporating latent heat storage (mostly for cooling through engine off on stop start systems) and A/C operation integrated with the engine management.
