1985 318i mpg dilemma

Hello everyone,

so a few months ago I bought a 1985 318i automatic sedan. So the M10 added to a automatic transmission makes it pretty slow but I dont mind because i bought it pretty cheap as a daily driver. However what I do mind is the gas mileage. When I bought it I drove it 300 miles home and it averaged 30 mpg, which is great. However since then it has been averaging 15 mpg. It seems to me that a 4 cylinder should do better on gas, especially considering that my 1987 325i vert gets around 23-24 mpg and my 1986 325e gets 27-28 mpg. Anyway any ideas or advice would be greatly appreciated,

Thanks

1985 318i mpg dilemma