CarFax Accuracy?

[Steve Sherman]

Found what I think (thought) was an excellent 91 200 20v, and pulled a
carfax report on it.  It came up clean, except that from the emissions
inspection reports (which include mileage) it looks like the speedo/odo
was replaced in '97.  Mileage was 34k in 95;  97 inspection it was 1600.
 Mileage goes up reasonable from the "replaced" 97 number thru the present.

Current owner may not have known (I told him I was going to pull a carfx
report and he did not mention anything).  My questions for the list are:

1. Any reason to doubt carfax reports?

2. Assuming carfax is right, what is an extra 40K miles worth off a car
(83K vs 123K miles)?

TIA