If you are certain the EXACT distance is 3.0, seeing 2.9 on the odometer says now that your TS is too small, by about 0.1/3.0 x 100 = 3%. So, you may want to add back in about 70 mm making the TS equal to 2480 and see how that works. My guess is though, that the distance may not be precisely 3.0 miles.
It's a bit of trial and error to get it dead right, but it CAN be done. On your road trip, pay attention to the highway mileage markers. Over a longish distance, they are quite accurate. Here in Arizona, I calibrated my speedo by noting what the odometer recorded after 100 miles of interstate driving. I was within 0.1 miles, which is an error of only one-tenth of a percent. Using long distances minimizes error.
Another way to get it "close" is to "pace" someone with an accurate speedometer. Use the readout for speed on the Gryphon (the dash speedo will always be about 2 mph above that). If your pace car is at 70, and you read 71, your tire size is about 1/70 x 100 = 1.4% too big.
I'm guessing that 17 inch rims were standard on your truck and that possibly your current tires are OEM size? If so, what was the Ford recommended pressure for the tires? It should be shown on the VIN sticker on the driver's door and there might be a sticker on the fuel door.
- Jack
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