On a long drive recently, I checked and re-checked the odometer and
speedometer in my 82 Vanagon.

I found that to get an accurate mileage, I had to multiply the indicated
mileage by 107%.
To get accurate speed, I had to multiply the indicated speed by 110%.

My tires are 195/70 R 15. I don't know if those are standard size or not.
Can anyone explain these discrepancies?

Thanks,
Paul in New Mexico