Olds 307 mileage

I have an 89 Caprice wagon with an Olds 307. Does anyone know what the original mileage was for this car? I'm currently getting about 12mpg combined city/highway, which seems low. I had to unplug the overdrive because it would not stay engaged. I know this will reduce highway mileage but only by about 2mpg.

1989 Chevrolet Caprice wagon, 307 engine, 4-speed automatic, EPA estimates 17 city, 24 highway

source: http://www.fueleconomy.gov/feg/noframes/5906.shtml

Thanks for the information. I thought the original mileage was a bit higher so what I'm getting isn't as far off as I imagined.

Would you know the reason why my overdrive won't stay engaged? Is it simply worn out or can it be fixed without taking the transmission apart?