MPH to 1/4 mile time conversions

[datdude]

Does anyone know how to convert a mph to a guesstimate in 1/4 mile time...according to the Gaffrig water mph gauge I am running about 85. How accurate are these gauges? Thanks for any help.

[Jet Hydro]

Low 12`s if its hits it in about a 10 count.
Use a GPS and count how long it taks to top out. Go 1001 1002 1003 and so on. Post how long it takes and we can give you a better idea. Just guessing I`d say 12`s in the 1/4. Gaffrigs are good but not as good as a GPS