Just tried one of these little devices and did a 30-90mph pull in my stock 07 G35X Sedan. This is a fast and accurate GPS based meter. Even cruising it responds very quickly and shows your real/true speed which is often as much as 5-10% different from what the dash shows.

Starting in second (5at) using manual shifting I got a 9.55sec. There's probably more to be had starting in first and accelerating at full speed through 30, as in second the rpm is fairly low.