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?

[Dave_Darling]

It is normal for your fuel economy to suffer when the weather gets colder, for a variety of reasons.

- It takes more energy to get the engine, transmission, and tires up to temperature. That energy comes from your fuel.
- Your tire pressures will drop somewhat, and that gives them more rolling resistance. Check the pressures.
- Colder air is slightly denser, so it takes more energy to push through it.
- The denser air means the engine spends a little bit more energy to pull that air past the mostly-closed throttle, which costs a tiny bit extra fuel.
- Here in CA, colder weather generally comes with rain, which takes even more energy to push through (in the air) or move aside (with your tires).

This year, I believe they changed over to "winter gas" in October due to some supply issues. So the change in fuel is probably not a cause for your particular drop, unless you noticed it starting in October.

That said, it can't hurt to check the stuff above, and also the tire pressures.

--DD