The problem with diesel vs petrol is whether they use the same amount of fuel per 100km's driven the same way. I find a lot of people actually lie about there fuel consumption to justify it to themselves or to make it look better for the people that might want to buy it. My 200 series runs about 16.5L per 100km on normal driving gets down to 14L on a long haul (if your lucky) and 26-28L with the boat behind it, if the petrol was using the same amount per 100km's I would think the petrol would work out much the same, I just find it hard to believe they would be using the same amount. My SS V8 wont get 16L per 100km's driven quietly and its halve the weight and I have had an older SS that was worked and driven hard you could get 11L per 100km's on a trip to ChCh.
I think you would have to put a tank of fuel through one to really find out how much they use and work it out from there.
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