Friday, December 16, 2011

High Fuel Economy?

In 1973, the Datsun 1200 was the car with the highest fuel economy sold in the USA. It was EPA rated at 37.9 miles per gallon for the highway test cycle. Many owners report getting that mileage, although many others report less.

I checked my mileage carefully, keeping records of gasoling purchase and miles driven. After driving mine for several years and averaging 42 mpg when on the highway -- according to my records -- I realized the speedometer in my 1200 was inaccurate. It was reading about 10% too fast. I began checking the odometer against the "test" sections of the highway, and it too was reporting about 10% more miles traveled than actual. All of this even though I was running with the factory size tires. I also learned from my Ford service manual that Ford's specification was that a speedometer could be +10% to -5% and still be acceptible.

So when checking your MPG, be sure to test both the speedometer and more importantly the odometer. Today's roadside "your speed" signs make checking the speedometer easy. For the odometer, a stopwatch can be used. One mile @ a steady 60 mph takes 60 seconds. If your car takes 66 seconds, your gauge is 10% slow. Or just use the highway test sections which are usually 5 miles.

So what mileage did my Datsun 1200 really get? After correction, it was delivering 38 mpg, right with the EPA estimate.