This isn't going to do it as the speed off is a percentage not a hard number. The faster you go the more off the speed. Sure you can get it maybe less off, but it is still off.This is going to sound a bit CRAZY... But there MIGHT be a much simpler "ANALOG" solution for this problem... as long as you intend upon remaining with using those Larger Tires... and Change things BACK to Nominal if you intend upon Selling the Vehicle. Here goes nothing:
(1) Take a Test Drive on a Level Road with a Passenger using a Reliable GPS Unit to read your Actual Speed closest proximity to some Known, Posted Speed Limits ... like at at 30-45-65 MPH... and have him be ready to write down what you are about to say for each accurate GPS MPH reading. Perform this test at LEAST 3-5 Times so you can record an Average of these Speedo- vs. GPS variances
(3) The Driver should call out the Speed of the Truck -SUV showing on the Speedometer just as soon as the Passenger with the GPS Unit advises that the Vehicle is moving at a Steady Posted Speed Limit.
(4) Take the Truck/SUV Home and after taking a picture of the Dash Panel Speedometer Dial... Get a Cup of Coffee and compare the notes the Passenger was taking of the Speedometer-vs.-Actual GPS Speeds.
(6) Then... Pull your Instrument Cluster Panel out of the Truck/SUV and bring it inside where you can disassemble it and work on it where you can concentrate.
(5) Using those Recorded MPH Differences... Count Off How many Line Segments are present on the Speedometer Dial in between those Two Speeds.
(7) CAREFULLY... Measure the Rotational Distance using the same Line Segment Counts in between those Two Speeds and lightly place a piece of Tape in line with the last Speedometer Gauge Dial Position at REST. Then Place a Second Piece of Tape in line with the EXACT what the Desired Speedo Dial Position will be in AFTER this adjustment in the Analog Speedometer Dial is achieved.
(7) Then Physically and CAREFULLY ...Slowly Rotate the Red-Orange Dial Armature either Clockwise to DECREASE the MPH or Counter- Clockwise to INCREASE the MPH reading using EXACTLY the Same Number of Line Segments distant in between the Driver recorded Speedometer Readings versus the GPS MPH recorded by the Passenger.
(8) This might be Very Tedious to do... but from that Adjustment Point on... your Speedometer WILL be an Accurate Analog to the actual Posted MPH speeds in line with the Larger Tire Outside Diameter variance. If this Idea Works... No More GUESSING at how fast you are traveling and having to Look Down at the Speedometer and Calculate with MPH Math, taking your eyes and your attention off the road any more than the Average Driver.
Hope this Works!