Gas mileage dropping

dealership didn’t tell me anything about octane minimum. Filled up today with premium and now gas mileage is dropping sitting in the driveway. Started truck,12.6 mpg,every 10 seconds mileage drops .2 mpg. While not moving. went down to 10.2 then I started driving. Mileage went up to 14.4 after a 2 mile trip.

 

Mileage is calculated by distance driven versus fuel consumed. Mileage will always drop when you're sitting still and not moving forward.

 

I sat in my driveway for 1/2 hour with the truck running and this didn’t happen. Figuring out how to use the head unit.

 

Toyota has some pretty good insensitives on the Prius

 

The truck knows when you fill up. My tank average will vary more after a fill up compared to when I'm at half a tank.

 

Gotta call BS on that one. Accrued mileage displayed will slowly tick down simply when sitting at a red light. You're guaranteed to tick down idling parked for 30 minutes.

 

You're Tank Average Mileage resets with each fill up. You will see it move a lot just after filling. Toward the end of the tank, it will be very stable. This works the same for any vehicle that gives you average MPGs, although some will do a 20min or so average.

I think it was said that a warmed up 3rd Gen uses 0.2gal/hr when idling. Use that to gauge what is going on.

 

He mentioned filling up. You might not see a change with a long idle toward the end of the tank.

 

But after idling for over 30 minutes? Still skeptical on that, and my tank average has gone way up since I had it reprogrammed last year. Even if I sat idling at the end of the tank, the displayed mileage would drop.

Then again, I'm getting between 20 and 22 empeegees currently (city driving, huge shocker there) and not 12.

 

I'm not really interested in testing that, but the math sort of proves it?

Say 21mpg after 400mi. That is 19.05gal used. Idling for a 1/2hr is 19.15gal. 400 / 19.25 = 20.9mpg. So depending, it might not register due to rounding.

Like you noted, it is even less with lower MPGs:

Starting at 17mpg after 325mi is 19.12gal used. Idling for a 1/2hr is 19.22gal, so 16.9mpg?


Also, considering a 0.1mpg drop versus how much it changes early in a tank, the OP might consider than as not changing? A lot of speculation, I know.


BTW, I did get a small bump with the TSB tune (about 0.5mpg / 19.4 to about 20). I also got a 0.5mpg bump with OVT no matter what tune that I tried. I am really pleased with the mileage that I get.

 

I never though someone would wonder why their gas mileage isnt taking them at expected distances when not moving.

 

I timed it, drop .2 MPGs per 7 seconds from 12.6 to 10.8. Then.1 MPGs per 12 seconds. This was 5 minutes ago. In driveway,not moving. Not calling B.S. on anyone. No mpg drop when idling for 1/2 hour. We can’t use common sense when diagnosing a faulty sensor.

 

I don't see what your issue is. Your truck is using gas sitting idle. It's still consuming fuel so your gas mileage will drop. It's not a sensor issue.

 

Math is hard?

 

It’s not an issue it’s a question. The truck is doing something this week that it wasn’t doing last week.

 

Miles Per Gallon = Distance traveled divided by fuel consumed

Whenever you fill up a new, fresh tank, your MPG average on the dashboard is going to be all crazy, because it hasn't collected enough data (you drive 10 feet from a stop by slamming the gas. If that's all it has, it's going to estimate based on you driving with a lead foot). As you drive on the tank, the number gets more accurate because you're driving greater distances in a more normal way.

When you burn fuel and don't move, your fuel efficiency goes down. That's what MPG (fuel efficiency) is. The average miles you are getting is dropping because you are using fuel and not moving.

This happens literally every time I fill up. Because it's math.

 

I'm trying to help you with an answer, but you have to help me with the question. What exactly is your truck doing that you think it shouldn't be doing?

 

Did you also sit in the driveway idling for 30 minutes? Scientifically, we are missing a control variable.

 

Nothing.

 

The missing variable is a hobby.