Sea level pressure IS assumed as the "0" reading on every pressure guage
designed for automotive use.  As an example of my meaning; a new tire, once
mounted on its wheel rim, before ever having any air pressure added to
inflate it to spec, contains ambient air pressure of the atmosphere around
you.  This would be measurable as 14.5 to 14.7 PSI (close enough, as most
automotive guages won't even read this finely) at sea level. Any pressure
guage reading must assume to be a value above standard ambient sea level
atmospheric pressure, or 1 bar.  Read the following quote from the wiki link
to help you understand what I'm referring to;

"Pressure gauges often measure with reference to atmospheric pressure (which
is around 1 bar). This is gauge pressure and denoted by barg, often written
with no spaces, spoken "bar gauge", and sometimes using symbols such as
'bar(g)'. For example, if someone says that their car tyres are pressurised
to 2.3 bar they usually mean bar gauge: the pressure in the tyre is really
3.3 bar, but only 2.3 bar above atmospheric, which is the scale a tyre gauge
would read. When absolute pressure is desired, it is sometimes denoted
'bara' or 'bar(a)' for "bar absolute". The alteration of units of measure
for this purpose is now deprecated, with qualification of the physical
property being preferred, e.g., "The gauge pressure is 2.3 bar; the absolute
pressure is 3.3 bar".[1]

  And sea level has nothing to do with pressure
> gauges - altimeters yes, that's why when you get the barometer reading
> from the airport weather station, it is Not the actual atmospheric
> pressure, it's the equivalent sea level pressure, corrected for local
> conditions of altitude and air density.  Absolute pressure
> gauges/transducers are referenced to vacuum (i.e. read 0 at full
> vacuum), and gauge pressure gauges/transducers have to be calibrated to
> read zero at station pressure (i.e. ambient atmospheric pressure).  At
> all elevations and atmospheric pressures, ambient pressure is 0 psig.
> Absolute pressure gauges/transducers, in ambient conditions, will read
> the atmospheric pressure.  Here in Phoenix, ambient pressure is 0 psig,
> 14.15 � psia.
>
> Keith Hughes
> '86 Westy Tiico (Marvin)