How accurate is it? When I bought it last September it was showing 20mpg. Then it crept to 21 a couple times but now it's reading 17. I do regular maintenance and Lucas added each fill up. I average about 250 miles a tank

 

In my 7.5 years of ownership, the trip computer on my 2007 base TL (A/T) has always been about 1-2 mpg higher than the fuel-up app on my iPhone. Also, my trip computer readings tend to be pretty consistent (when my driving-style is consistent) and not jump around like yours. Are you really driving that differently between fill-ups?

 

First of all quit using the Lucas crap. It isn’t necessary on a routine basis and is simply a waste of money. The 2nd thing you should do is calculate the mileage manually. That is the only way for comparison sake if accuracy is needed.

 

It typically reads 1-2 MPG higher than if you did the calculation manually. But there's no way to tell how accurate your manual calculation is, due to differences in each pump and each fill up.

You're getting worse MPG now because its winter. You probably live in a state where it gets cold, and where they use winter blend gas for a few months.

Winter blend gas has less energy content. So you get worse MPG.

You also get worse MPG since the ambient temp is lower. So the car has to come up to temp before it reaches peak efficiency.

If you warm up your car by idling it, you're getting 0MPG while you do so.

Everyone gets worse MPG during winter.

 

very interesting, was not aware of the difference in fuel during the winter

 

Yep, depending on your region, summer blend gas is typically up to 10% ethanol, or 100% gas.

For areas that receive winter blend gas,they add butane in place of other components to help ignite the fuel at cold temperatures.

Cold weather has a big effect on efficiency. Colder, denser air = more fuel to balance the mixture. And the fuel itself has a lesser energy content.

So you get a doubled up effect on top of all the other factors that typically occur due to driving habits, etc.

 

Article from 2017, but still applicable and likely more than you wanted to know. Summer vs. winter gasoline

 

I would highly recommend searching the forums for MPG details. This has been beat to death over the years.

1.) Calculate manually and compare
2.) Compare your MPG to your AVG MPH…direct correlation that usually clearly shows either low AVG as in lots of idling or stop/go traffic…or excessive speed

Over the course of 13 years, my MID was usually pretty spot on with manual calculations.

 

my MID will reset the MPG itself at random. I am very skeptical of the long term average it shows. not sure if mine is an isolated case or if anyone else has noticed this too. '08 base, 145k.

Typical scenario: running errands, stop briefly to run into a store then when I get back in and start up the MPG will read out something between 0 and 10. then over time it usually works back up to 22 or so.

 

Its not quite random.

But if you park on a hill or a slant, the fuel will slosh to one area of the tank. So the fuel level sensor thinks that you've just filled up fuel.

You probably have the MID programmed to reset on one (or both) of the trip odometers every fill up.

I set TRIP A to reset every fill up.
I set TRIB B to never reset unless I manually reset.

So the MPG displayed for TRIP A represents MPG per tank

TRIP B represents long term MPG.

I typically only reset Trip B when I change the oil. So it also serves as a way to keep track of how many miles I put on the oil.

 

Many variables can affect mileage. I make a trip monthly from Philadelphia to Long Island and very consistently get about 27 mpg. But a few weeks ago I made that trip in a downpour and only got about 23 mpg, and that in spite of reduced speed due to the weather. The reason is it takes quite a bit of energy tp push that rain under the tire to the side of the tire contact pattern. Don’t believe it? Ever wonder why your car looses traction from hydroplaning. The generated force is sufficient to lift the car.