(I couldn't post to your messgae below for some reason)

I used to register 28mpg with my '88 735i in suburban (town and country, but not city stop-and-go) driving and over 29.5 hwy both on the OBC.

After disconnecting the battery for unrelated maintenance I noticed at some point my mileage fell to 24.5mpg suburban and only 26.5 hwy. I couldn't figure what was making my mileage so crappy compared to what it had been. It was driving me nits. I checked everything and found nothing wrong. I tried different brand oils, and changed my tire pressure to 38psi to no avail. I was bummed.

I was considering dumping the car. I just can't mentally justify less than 25mpg (I kept my old 528e that gets 30mpg local and 34mpg highway so wouold have just started driving it again.).

One tank, I forgot to reset my OBC MPG so did the manual calculations from my mileage and gas purchase at the pump.

SURPRISE! I was still getting almost 28mpg local driving and back to 30mpg highway (if I limit my cruise to 70mph*). I check every tank now and it's the OBC that's screwed up, not the engine.

These might not be Hondas at the pump, but as long as I can get 26-30 mpg I can justify driving a more comfortable car. At below 25mpg, I feel like I am wasting money.

If you're getting bad mileage like I was, do the math at the pump. Maybe it's just the OBC.

Bob
'88 735i 5spd
*at 75mph Interstate the mileage drops to only 27mpg :<(
That's a 10% penalty for only 8% faster cruise. At 60 mpg I get 31 mpg average, a 3% increase over 70mph for only 14% decrease in speed, so I drive 70 on Interstates.