Speedometer error

[silver buffalo]

I have switched tires from the original (to me) Goodyear 8R19.5 to General 225/70R19.5. Looking at the Goodyear web site, it says there will only be a 2.25% difference in the spedo reading at 75mph. Well I have about a 10 mph
difference, the speedometer shows 75 and my actual speed is around 65. I don't know how accurate the speedo was before I changed tires.
I think the input for the speedometer is a gear in the transmission (?) and was wondering if anyone knew if all I had to do to correct it was to change the gear and also if anyone knows what the correct gear might be?

[Fyrzowt]

I used to work on GM products and they do use a gear in the trans to adjust the speedo. Unfortunately it's been long enough that I don't remember the formula. It was something like one tooth per 2 MPH change either way. The gear is easy to change, should not be big $$ if it's done the same as GM. Not sure who makes your trans or if adjustments are the same. Your dealer should be able to help.

[overlander63]

The geears are different colors, you will need to pull your gear out so the parts person knows what you have, and give you a chance of getting the right one first time. If you can't get a gear that's close enough, you may have to go to a speedometer shop so they can make a reduction kit for you, the last one I had done cost about $100.