Hypertech inline set up question and actual tire size

So i sold my old hypertech on here a few months back waiting for them to release a new model for the 18+ tundras... no luck. So i ordered a new one today and it arrives tomorrow.

Called the local 4WP store to see if they had the software (mac guy and my work laptop wont let me download software) and the guy told me to measure my tires before hand. I have 275/65/20 (34.1x 10.8 x20) and when I went out to measure my tires i am getting- 32 7/8 x 9.5 (just the tread) x20. Assuming this is due to the weight of the truck? Wheels are method NV 20x9

Which size should I enter? On my last few tundra's with the hypertech I always had 275/65/20 as my setting, but thinking the actual tire size might be better?

In other news I am feeling severely robbed.... I thought my tires have always stood 34in tall and roughly 11in wide.... might be time to drop down to 17's and 295/65/20's

 

Many tires arent as tall when mounted. I had pro comp 35s years ago, shrank like a cold day at the pool when mounted on truck. Toyo 35s are damn true to size though

 

I have 295/70R18's (34.5"). It took me somewhere between 32.5-33" to get an accurate reading with the Hypertech.

 

I had to use a Hypertech on my F-150. I started with the reported "tire size" and then drove around with a GPS measuring my speed. Tested using cruise control at 30mph and 60mph. It took some tweaking, bu I eventually got my speeds to match. The actual size was smaller than I started, so not sure if that's the truck weight or what. My guess are most tires are like this.

 

Anyone have on truck measurements for stock platinum wheels and tires?