Date registered: Feb 2008
Vehicle: 1987 420SEL
Location: Ontario, Canada
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30-MPG is optimistic at best; there's plenty of ads up here and elseware that state this is normal for these cars. With the engine idling going down a mountain with a 80km/h tailwind, then maybe.
The best I ever got was 24-25mpg, and generally the W123 forum places figures between 22-26mpg.
For starting in the cold, the first preventative maintenance task you should perform is a fresh set of glow plugs. Use an anti-gel agent in the fuel tank each fill-up (Canadian Tire, you can purchase a jug for about $12 and it should last the better part of the winter). A block heater will make a world of difference and be easier on the car too.
If you can't use a block heater all the time, cycle the glow plugs twice/3 times before cranking to ensure it'll fire right up. You certainly don't want to be cranking away on an old diesel; the batteries are big, but not that big.