most efficient speed? 55mph or 75mph? MPGwise...

So I've always been told the most efficient speed is about 50-55mph. I've been trying to calibrate my new MPG guage (amazingly it finally arrived only 40 days late, an no attempt from the company to contact us, but that's another story). The guage says the Smart has "MAF" (or was it MAP?) and my guage reads really low, so far I've got it set to +65%, +70% was reading too high. @ 0 it reads that I'm getting 12mpg. The instructions say to drive at 50-55mph (the most efficient speed) and adjsut until the guage reads the advertised Highway MPG, 41MPG, which seems to be about 65%.

So while doing my calibration, I've noticed that I seem to get the most efficient reading at 75mph? That doesn't seem right? If my guage is correct @55mph (41MPG) than driving @75mph yields around 48MPG, these are rough estimates for now. Again, I have no way of knowing if my guage is right or not, at least not until I can do a single trip filling up at the beginning and again at the end to calculate gas in/miles out and more tests, its only been 3 days. Every time the car shuts off the thing starts a "new trip," so I don't know what it is reading overall for the tank of gas. Wish it could realize when the tank is filled and track it that way also, it does read the fuel level after all.

Anyone tested it? 55mph or 75mph? Which is more efficient MPG wise?

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Comment by Robert Stout on February 12, 2010 at 5:29pm
Someone previously reported their best Scan gauged MPG at 47-48 MPH in 5th gear...60 MPG is not a wild claim at that speed on a level surface without wind or air conditioner use....
Comment by james g on February 11, 2010 at 11:11pm
when the ambient temps are over 60F i get 50+MPG @ 55mph, @ 85F @ 55mph for 40 miles (avg speed 43mph), i logged 61.6MPGs with a 18 month old smart. unfortunately i won't be doing these highway runs anymore (they started selling powerball tickets in my state 2 weeks ago). in stop and go driving, 12 miles an avg speed of 23mph, i've hit 55MPG when it's in the 80's.

in 2 months i'll be adding a SOLO performace exhaust and a K&N air filter, both should add a little to my mileage with the same driving style/techniques. my smart is just over 2 years old with 19K miles on it, so it's "broken in". i use a Scan Gauge 2 and find it extremely accurate and informative, when refueling @ 7-8 gallons, it is either spot on or 1/10 a gallon off.
Comment by Dan LeValley on February 10, 2010 at 4:40pm
I rarely drive my car less than 70 when out on the interstate. We've pretty flat here in Kansas so the strain on the motor is probably more constant than in more hilly areas. However....wind can be a factor since we are rarely without wind. My car gets about 37-39mpg @ 70 mph. After a year of consulting with people in our club that's maybe a tad high for that speed. I'm not familiar with the gauge that your talking about but I can't imagine that driving in the 50 mph speed range would not get you a heck of a lot better mileage. How you start and how you stop is very, very important also. I've heard some wild claims about mileage and I pretty sure they're not driving 70. Dan
Comment by Stuart Berman on February 9, 2010 at 10:34pm
Sorry - I have no experience with the guages. 55 mph is much more efficient than 75 mph due to wind resistance. I get around 40 mpg at 55mph and in the 30's for 75 mph.

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