Joe---15mpg is about the lowest mileage you should get, and this would
be all city driving. The octane number doesn't mean much here so far as
mileage goes. In fact you could probably run trouble (ping) free with a
less pricey blend, with your 7.5: 1 compression, assuming your engine is
'stock' in this area. Driving habits and road conditions, along with
what speeds you up-shift all go into your mpg.
It doesn't sound like you're running 'rich' from the way you describe
your plugs.
Thick oil in the engine and drive train all take their toll on mileage
when the weather is cool, and runs are short. If your engine is not
mis-firing, and your ignition timing is correct, you should be getting
closer to 18 in the city, and 25 on the road, without OD. But this could
be taking some of your fun away, if you enjoy what Peter Egan calls "the
zoom factor".
Dick T.
CF11143
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