Speedometer questions

[DB556]

I know this is been asked a hundred times minimum....
I've got the other thread with the parts numbers bookmarked

But here is where I am, as a sanity check before I order the wrong adapter.

I currently have 9.00-20 tire (power kings) which measure just just under 39" with a tape measure (but on their spec sheet should be 40.9")
using my cellphone(gps) and GPS(garmin) to check speed when I'm driving at exactly 50mph on the speedometer it clocks in at 54.7 on GPS.

I cant make out the part number on the transfer case adapter for the speedo it got some bed liner or epoxy on it. (I'm going to have to take it off and use a wire wheel on it)
But the adapter does not look to be the same age as the rest of the gear down there (much 'cleaner')

I haven't looked at the actual speedometer but I'm guessing from the layers of paint its original or atleast it was in place for the last few rebuilds.

Now I'm going to goodyear MVT 395/85R20 tires which are 46"

From the speed difference I'm guessing I have the adapter for 11.00-20 tires?
Does that sound correct?

 

[Barrman]

Tire size. 9.00 means 9 inch tread width and 100% side wall ratio. So, the side wall is also 9inches. 9 + 9 for each sidewall plus the 20 inch rim means 38 inch tall tire. 11.00 x 20 should be 42 inches.

2600 rpm with 9.00 tires should have you indicating 56 mph. What are you indicating at 2600 rpm?

 

[DB556]

I can never hit 2600 exactly with this truck only bounce around in the range 2400-2500 which is were is indicates the 50mph.
Filters are fine, and the smoke screw was already turned by the fire dept I got it from, first thing I did was install a pyro it is way to easy to get to 1200*

 

[Flyingvan911]

I switched from 9.00x20’s to G177 11.00x20’s and my speedometer shows 2mph slower than actual speed now. It’s not a big deal. The radial tires are great.

 

[DB556]

Well if my speedometer is almost 5mph slow now with 9.00-20's it will be almost 10mph slower with the 395's