I think you'd have to have some idea of the mileage when you started using oversized tires and that those tires would have had to have been consistently oversized since. If you've used all sorts of different sizes I think you're just out of luck. But if you know the first thing and the second has been consistent...google map a route to get the exact mileage and then drive it yourself. Do this a few times to give yourself a decent set of numbers to work with. So, let's do five trips.
| trips | Google Mileage | Your Mileage |
| 1 | 50 | 30 |
| 2 | 75 | 45 |
| 3 | 15 | 9 |
| 4 | 27 | 16.2 |
| 5 | 54 | 32.4 |
| Total | 221 | 132.6 |
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So, the theoretical mileage you record is .6 of the actual mileage. So, let's say your car started with 30k miles when you started using oversized tires. It now has 150k miles. So take the 120k and divide it by .6 for 200K miles. That
should be how many actual miles you've put on the car with the oversized tires if I've done this right.
You can do something similar if you're just trying to figure out your speed. Google map a trip, drive it at a consistent speed and start doing math for how long it should have took you at the speed your car is saying you drove versus the amount of time it actually took and it should give you a rough idea. I mean, if you take third minutes to go thirty miles, boom. 60 mph. If you
really wanted to you could do the math and make up a custom face to overlay the speedo in the dash. That sounds like a ton of work, though. If you really want to know, I'd just try to find a couple of benchmarks to go by (like 30 and 55), maybe mark in sharpie, and just guestimate the rest as best you can as you drive around.
Yeah, driving just becomes a bigger chore as I get older. The whole self-driving car thing can't come soon enough in many respects.
