Is the only reason that people buy an IS300h for its economy or its perceived economy? Driving say 12,000 miles per year at 50mpg would require 240 gallons of fuel. Doing the same miles at 40mpg would require 300 gallons of fuel. At about £1.20 per litre of fuel in the uk and 4.5 litres of fuel in a (UK) gallon, that's a difference of about £325 over a full year or about £27 a month. Call it £1,000 over a 3 year period. Over the same 3 year period a 36,000 mile IS300h will lose I'd guess about £15k although we could probably argue whether it was only £12k or as much as £18k. Then you pay £hundreds per year on insurance, an average of £300 per year on servicing, four new tyres in that time too and then incidentals on top. Fuel economy doesn't really matter that much. It's always interesting to see what is achievable but it ain't the be all and end all of motoring.