Big Tires

My Ford Bronco was fitted at the factory with 30 inch diameter tires. That means its speedometer is calibrated for 30 inch diameter tires. I "enhanced" the vehicle with All Terrain tires that have a 31 inch diameter. How will this change the speedometer readings? Specifically, assuming the speedometer was accurate in the first place, what should I make the speedometer read as I drive with my 31 inch tires so that the actual speed is 55 mph?

Forego the discussion of whether it is safe to drive a Bronco that fast . . .

One would expect some use of ratios here . . .

Tire Wear

New 30 inch tires for the Bronco come with approximately 12/32 in. of tread depth.   Over time and miles, the tread depth wears away and a tire is legally considered worn out if less that 2/32 in. of tread remains.

One deep-thinking Bronco driver argues he can drive with his speedometer reading above the speed limit because he has lots of miles on his tires and therefore the tires are slightly smaller in diameter.

Assuming his speedometer is accurate for new 30 inch tires,  how much faster can his speedometer read when he has 3/32 tread depth left and still be actually travelling 55 mph?

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