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I have a 2018 XC90 and recently had the auxiliary battery replaced. Ever since I got it back that day, the fuel gauge estimator (the part that tells you approximately how many miles you have left for the tank and the average mpg portion) has not worked right. I used to get about 330 miles to a tank, but now it constantly changes and moves around. I'll have over 1/4 tank left and it'll tell me that I only have 50 miles worth left and it will constantly keep trying to give me directions to a gas station. The dealer has said multiple things about it. First they said it was normal and that's how the car always was. Nope! Then they said that it should be fixed after a couple tanks of gas. It has now been 5 tanks of gas since they replaced the battery and nothing has changed. Now they're saying that it will take 6 months to correct itself. Has anyone else had this issue? Is this just the car's new normal?

Also, my auto start/stop feature used to be able to be turned off and it would stay off. After the auxiliary battery was changed, I now have to turn it off every time I start the car. They also told me that it is how it always was.

Thanks in advance! I really appreciate any help!

Odometer Speedometer Gauge Tachometer Trip computer
 

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As far as I know, permanently turning off auto S/S was not possible in MY2018 cars.
At least that was/is the case in our XC90.
Perhaps auto S/S wasn't activate because the axilliary battery wasn't fit and now, with the replacement, it is?
 

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The auto start/stop feature was not persisted off after engine start on MY2018. I think it is only on MY2019+.

The mile-to-empty number is largely related to the speed/fuel consumption in recent past. What if you drive a longer trip on freeway before the fuel level goes to 1/4 tank, and check what number it shows?
 

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The mile-to-empty is a bit off. It shows 1/3 of tank. If you have a T6, that's more than 6 gallons. Did you drive around 10 miles / gallon recently?
 

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Perhaps auto S/S wasn't activate because the axilliary battery wasn't fit and now, with the replacement, it is?
+1 as the very likely cause. As the aux battery's performance wane over time, it would indirectly ensure that the start/stop feature won't activate most of the time. Also, changes in driving pattern can make it more difficult for the battery to recover itself to the sufficient state of charge for the start/stop feature.

As for the "Distance to empty" and the MPG estimate on the bottom right, as far as I know, they are somewhat independent, and there are 2 versions of the MPG estimate, the TA and TM ones. TA and TM calculates things differently. Per my model year's user guide, "The Distance to empty" takes MPG from the last 20 miles and latest fuel tank level into account for this estimate. TM and TA use different logic for the MPG.
 

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I think your car is working fine. And looks like new battery make S/S work again. It was not possible to disable it permanently in MY18
 
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