MPG change, where to start looking? Options

[914Bryan]

I know this isn't an exciting high performance question, but a question just the same.

I'm having an issue with the mileage my 74 2.0 DJet and am wondering where to start looking for the problem. I commute 60 miles each way to work (120 total). My driving is 98% freeway, non rush hours so a steady 75 mph, motor is turning 3200.

I could always count on 32mpg, day in, day out. Over the last two months, it has slipped to 28mpg. I burn a tank a week, so this has been about 8 tanks of gas that I have noticed this.

Engine is running fine, no change in performance.

Would a tardy valve adjustment cause this? I'm a little over 6k since my last one.

Any ideas?

[Katmanken]

Winter gas mix has more aromatic volatiles in the mix than summer gas, and the volatiles, such as butane, acetones, etc, have a lower energy density. The addition of volatiles makes the winter gas easier to ignite when everything is cold.

So not only do you get an energy drop from the addition of alcohol, you get another one through the addition of winter aromatics to the gas.

The aromatics are pulled out of the summer gas to reduce emissions as they tend to boil out of the gas in your tank during hot weather.

So yeah, expect worse mileage.

Hope that helps.