The bicycle industry has no standard or guideline for how tire widths are measured and indicated. So as a result, unfortunately, Joe can call his tire a 1.75" regardless of how it actually measures. This is not to say that Joe is being deceptive, just that width measurements can differ depending on the rim used and amount of air pressure in the tire, and since there's no guideline on how to measure, the actual width vary among different tire brands even though the size indicated is the same.
This means that the tire's actual width at its widest point, which could either be the casing or the knob depending on the tread/knob design, is within +/- 2mm of its indicated size. Our measurement standards have been unchanged for over 30 years so a 1.75" wide tire developed in the 1980s measures essentially the same as one developed today. We are not suggesting that we are right, just that we remain consistent with our sizing.
This, perhaps, causes the most confusion – the answer is no, it is not the same. It's important to note that in keeping the width true to indicated size, the O.D. may differ among same widths of Tioga tires due to differences in tread/knob design. This is a bit confusing but the difference in O.D. is because bicycle tire width is directly proportional to the height of its sidewalls, which in general means that to make a bicycle tire wider without altering tread design, its sidewall height must be increased. For example, a knobby tire like the Comp-III 1.75" measures 45mm (1.77") at its widest, which is from its side knob to side knob, has an O.D. measurement of 498mm. On the other hand, the low profile PowerBlock 1.75" is 44mm (1.73") at its widest, which is at the casing/sidewall, has an O.D. of 506mm.So in essence, due to the combination of tread pattern/design and knob height differences among various models of tires, the O.D. can differ while the width remains the same.
When creating the PowerBlock, Tioga engineers thoroughly studied the proper O.D. to maximize the tread's ability – particularly when used on the rear. Once all factors are considered, two sizes are made available for the rear, the 20x1.75" and 20x1.60", each intended to take advantage of the strengths of a particular type of racer. For those with more powerful leg strength, the larger diameter PowerBlock 1.75" rewards the racer with greater speed and distance with each stroke of the crank. For nimble, technical riders who prefer quickness, the PowerBlock 1.60" is virtually the same O.D. as the Comp-III 1.75", which continues to be the standard O.D. of a rear racing tire, and delivers fast out of the gate acceleration.
Whichever size you choose, rest assured that the PowerBlock is the most efficient, fastest BMX racing tire available.