Thanks for the reply.
I followed the advice of another Stovebolter and took my GPS with me and was amazed at the difference the new diff may in my actual speed. I will make up a little chart so I know how fast I am really going. Not only do I have to correlate the new speed to the old speed, I have to convert to kilometers per hour (Canada, eh) - an awful lot for an old brain to do on the fly.
I should have twigged to the jumping speedo needle. Since taking over this old truck last year I am beginning to realize how much I have really forgetten from the old days when I bought cars for $30 and patched them up!
Sorry for the ramble.
