The Mileage tracker was recently added to QuickBooks Online UK, debrho21.
Right now, this tracker is only available to iOS devices. Our engineers are still working to add it to Android and on the web version. You can follow this article to stay updated with the progress of this feature: Track mileage in QuickBooks Online.
You can also check out the article about tracking mileage for vehicles for additional reference.
If you have more questions on the tracker, send us a short reply below. Have a good day!