Fuel consumption with petrol engines varies much more than it does with diesels, for example between city and country driving, so any comparison can be slanted according to what you want to prove. 
But fuel cost is not the only consideration. Even 20% better economy translates straight into 20% longer range, which can either mean not carrying extra fuel or simply in town mean more choice where and when to buy it. 
And even if servicing is cheaper for a petrol engine (is it?), as a general rule, diesels are more durable.
But - what is the biggest cost in running a vehicle? No, it is not fuel. It is depreciation, by a long way (usually at least twice the cost per kilometre that fuel is). Now as you may have noticed, "V8" is a dirty word when trying to sell a second hand car. Yep, right - depreciation can be expected to be a lot more on a V8 petrol. In general, this will swamp the relatively small difference in fuel costs. (and does this perhaps suggest why he is talking about it anyway?)
John
				
			 
			
		 
			
				
			
			
				John
JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol
			
			
		 
	
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