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They're not incredibly accurate; used more for bragging rights than anything.

IMHO, best use for them is as a (relatively) cheap way to see if a vehicle mod affects acceleration -- you can establish a pre-mod baseline, test, do the mod, then test again to see what result you got from the mod. It'll be accurate enough against itself, but it certainly isn't a dyno-in-a-box like the marketing makes it out to be.