If there is a flow meter that is accurate to a half percent I would be pleasantly surprised. They are sometimes close, but not accurate.

t walgamuth got it right. The ONLY way to accurately measure fuel mileage is to fill it up, make note of mileage, drive it, fill it up and then divide miles driven by gallons used. Trying to calculate fuel mileage by the fuel gauge is NOWHERE CLOSE to being accurate. If you get within 10% it is totally by accident.

40 MPG would be possible if you filled it up at the top of Pikes Peak and eased it down the mountain, then filled it up and calculated mileage. Under normal driving conditions this would be a total pipe dream.

Have a great day,

Fill-up & note milage, Fill-up angain & note milage then do the math.

Works great as long as your odometer is accurate. On my 240D, I have increased the tire size to 195/70R14. The DOT put a speed detection sign up for a while by my house. When my Speedometer says 40, according to the DOT radar sign, I was only doing 36. If the speed is off, I assume the milage is also off? Would this be a correct assumption? If so, then my calculations would be incorrect as well.

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1981 240D, Auto Trans, 160k miles -- CRASHED!

assumption. for your case, youwill have to check the odo on the highway. take it to the interstate and do ten miles by the mile markers, then see how far your odo says. in your case the odo will say less because of the big tires. this i call an optomistic odo. lets say that the mile markers say 10 miles and your odo says 9. it is off 10%. now to check your mpg each time you do the math you have to add 10% to the distance you travel...then calculate your mileage with the corrected distance.... and while your at it you should make a note of the speedo error too... and this is the one way you can get a speeding ticket driving a 240!

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

if only my odometer worked Guess I'll have to tackle that one soon

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87 300d Turbo 241K with original #14 head (blue)
87 300d Turbo 198K will run great again someday (silver)
84 300sd sold ( buyer drove it to Paraguay )

My 87 300D gets a consistent 30+ MPG, or around 500 miles per tank. After the reserve light comes on, I fill, but I've never been able to get a full 18 gals of diesel in there, mostly around 16+ gals.