2001 Outback LL Bean speedometer is off 10 mph!

[Tonyp884]

I bought a really nice 2001 LL Bean Outback today. Really Clean inside, outside and underneath. The leather driver's seat shows virtually no wear, which seems kinda rare on a 17 year old car. It has 138,014 miles on the odometer and that agrees with the carfax, just incredible nice car. Except the Speedometer is off 10 mph! I am ok with 3 or 4 mph! But, 9 or 10 is a bit much! I drove it home from the Asheville, NC area to Monroe, NC over 140 miles. I clocked the speed using the measured mile signs on the interstate, my GPS, and even had my wife along side in another car. The car has the 225/60-16 tires. I Assume it has the original transmission, it is a four speed auto... If I reset the odometer and drive 1 measured mile on the interstate the odometer reads 1.1 miles! What could possible be the cause of this? Could someone have changed the front and rear diffs to a different ratio? Will this hurt the transmission if the ratios are different? I am going to try to count the driveshaft revs per tire rotation today, and determine what ratio I have. Should be a 4.11?
My son has a 2004 OB LLB that I assume has the same ratio, so maybe I can compare them when rolling down the road at different speeds today.
I mean I know it cannot be adjusted... or maybe a yellow box?
I do have a new set of 205/60-16 tires and wheels from a 2003 Outback, but if I understand the math, those tires would actually speed up the odometer reading even more.

Any Ideas/Comments are Welcome and Appreciated!
I am 58 yrs old and this is my 12th Subaru.
Thanks,
Tony p
Monroe, NC