Fuel consumption test

156-mile round trip to the Autosport Show at the NEC. Good show. Put 42litres of Optimax in at Thrapston on way out and used left tank for a fuel check: 19.7litres in at Thrapston on return 109 miles later = 25mpg, assuming odometer is accurate. Juddering at 70mph, wheel balance? Screen washer packed up; tried to suck out blockage. Halfords Apple fragrance tastes dreadful.