25mpg isn't too bad for a heavy car with a 2.3 doing a stop/start journey... I'm getting around 22mpg in my 1.7tonne S90 3.0 24v and I don't drive it like Miss Daisy, on motorways I get 25mpg @ 75mph
4.5 litre = 1 gallon(uk) I put 40 litre in and I get 200 miles before refuelling (40ltr = 8.88gallon, 200 divided by 8.88 = 22.5)
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Current: 05 Mercedes C55 AMG, 97(R) V40 1.9 TD, 02(51) S40 S
Past: 86(C) 360 GLT, 93(K) 940 Wentworth, 95(M) 440 Si, 94(L) 940 Wentworth, 91(J) 460 GLEi Auto, 98(R) S90 Executive II