recently changed battery in 03 ranger will not pass smog now. how many miles does it need to be driven to reset the emission monitor codes ?

I was told 20 miles by a mechanic.

yes, though there is no set mileage, I often had to drive in excess of 100 miles on average..every monitor has to reset, with out a scan tool for obd2 there is no way to make sure they are set, and once you fail at emissions, it is much harder to go again, I would drive it roughly 100,125 and go back to the mechanic and ask them to check i.m. readiness unless the light comes back on by then.

I was informed its a combination of miles and engine starts. I was told I'd have to drive a total of at least 25 miles and have restarted the engine at least five times driving a minimum of three miles between shut offs. This has happened numerous times to me on my 2001 Ford Taurus six cylinder. It never happens in cool weather always in the heat of summer. I think its more a matter of the gas than a system problem.