Won't matter. If it changes mph 1-2 by just a few pounds like Adam was saying, a tire that flat should make it very visible.
Like I've said, I've gps verified my tracker at 30psi and 8 psi and there is no difference in mph at speed.
Post 33? Quoted Goodyear (educated gues talking about a 40" tire) over course of a mile role out distance was 13' with a 5% psi difference.. that's 600 or so revolutions to a mile... 1-2 revolutions and a thumb/eyeball isn't going to pick the difference up..
Basically what I gathered is they, good year, took tire X loaded it and set 100psi and knowing 5280' is 600 revolutions.. verified it. then dropped psi 5% or 5psi from 100psi, in the same tire, went 600 revolutions again and came up 13' short of 5280'
Most GPS especially civilian are not that accurate