Weird AVG MPG dispaly activity

[Woodie]

Well, on my drive on the interstate this morning, I glanced down and it was about 35. HUH??? As I kept driving, it kept climbing. All the way to 37 before I exited and then went down the side roads. By the time I got to work, it had dropped a few tenths due to stops and acceleration, but it's still 36+. I know driving from driving the interstate for some distance will put me in the low 30s, but this I've never seen before. Is it the gas? Doubt it. Did my engine suddenly become much more fuel efficient? Doubt that too. A few weeks ago, I filled up on my home from a weekend trip about 120 miles from home. When I arrived home, the gas gauge was still just a hair above full. After 120 miles. It had come down from the initial full level, but after 120 miles, I should have been down to about 3/4. Unfortunately, I didn't calculate the mileage on that tank but I think I went about 400 miles on the tank.

So on my way home today I had to drive for a few miles on side streets. 35 mph or less. The AVG MPG dropped as expected, down to 34.5. Once back on the interstate, it started climbing again. Up to 35.5 before I got off. This is weird. Also, the DTE was over 400.

I find this absolutely normal.

You didn't do all the around town driving you usually do on the weekend, putting your tank average in the hole from the beginning. I often see 37 mpg displayed, when I start driving errands it drops, usually down to about 32. If I get back on the highway, it goes back up, but very slowly because it's got a lot of down miles to average in. I usually get 400 miles to the tank.

[Ronzuki]

I'm leaning in the more normal behavior direction. That said, it wouldn't hurt to get someone to clamp on to the battery and give it a test just so you know its condition and to rule out the possibility if you truly believe something is amiss. Murcod's explanation is totally plausible given the car sat for a couple days.

[LPSISRL]

I get out of the original post:
- vehicle filled on a Friday and then parked in garage for 2 to 3 days
- start vehicle on Monday(?) and head straight onto "interstate" (so a freeway), then notice average is 35mpg
- average keeps increasing until heavier traffic encountered, then reduces

It fits with the starter battery being on the way out. You will lose the trip computer average consumption when the vehicle's engine is started and your battery is on it's "last legs". The vehicle sat for a couple of days (so the battery had time to drain/ self discharge) before the next start. The first time the average consumption is noticed is on the interstate- so the trip computer could have reset at that morning's start, but wasn't noticed until after the short drive onto the interstate. (37mpg is 6.4l/100km and could easily be obtained as an average if the computer was reset just prior to entering the interstate.)

Well, the battery is new (a month old) so it's not low battery. I can also tell by cranking speed if the battery is low. I'm sure that the computer did not reset between Friday and Monday. Friday's reading would have been about 20 since that's what is common for the trip home after a fill up.
Since I have several miles between my house and the interstate, the display always starts out low and then climbs. Slowly. Tops at about 28 at best unless I'm outside my routine. My surprise was looking at it and seeing 36. The only time I ever see that is after a fill up and reset, then right back on the interstate or if I reset while rolling on a highway. Then I'm intentionally trying to see big numbers by messing with it. This was not messing with it trying to show big numbers. This just happened while in my normal routine.

This morning after dropping while on secondary roads to about 34.3, it began rising again while on the interstate. Up to 35.3 by the time I got off. It is not normal to have the display RISING when it's already 34. Maybe there's something different in the gas.

It is normal for it to stick on full, but mine is usually dropping off the full mark after about 50 miles. The 120 miles and still at full is just way weird.

[bootymac]

Wouldn't the clock reset too if the battery was too low?

[LPSISRL]

I really don't think that somethins amis, just out of the ordinary. I'm pretty in tune to my cars so when something is a little differnt, I usually know it. Just thought I'd share the wierdness with everyone. The forum is boring if nobody posts stuff!

[murcod]

Wouldn't the clock reset too if the battery was too low?

Not necessarily. I had the trip computer resetting when my battery was struggling a bit, but the clock was fine. (Perhaps the clock has better short term memory backup than the trip computer?)

[LPSISRL]

This morning: Dropping while on the side roads. Climbing on the interstate. Back up to 35.3. The dropping and climbing is normal and expected. It just usually does this around the 27-28 mark not the 35. Especially this far into the tank.

[bootymac]

Which gas did you use? Has the DTE changed drastically?

[sx4rocious]

Just throwing this out there: did it possibly get reset to km instead of miles? I'm not sure how, or even if this is possible, but It happened on my wife's old HHR and we thought the same thing...

[LPSISRL]

Which gas did you use? Has the DTE changed drastically?

Same gas as always from 7/11. Different bank of pumps than usual but other than than, normal. I suppose I could have hit the wrong button and got premium, but I'll have to check my receipt.