I thought that the NAPA OP6638 was the 80 PSI sender and the OP6636 was a 60 PSI sender. Checking the NAPA web site both show 80 PSI which may be a mistake on their part. It makes no sense to me to have 2 part numbers 2 digits apart for the same item. That said I would go for the one that ends in 8. If you have to get the other one OP6636 I think I would connect it to say 40 or 60 PSI air pressure and read it's resistance before installing. Then you can prove me right or wrong.

Ken B.