Garage floor with insulation?

I'm putting in a new garage floor 4" thick concrete with 2" foam insulation underneath. The foam supplier wants to know the compressive load on the foam to recommend what grade foam to use. The foam seems to be able to support 30psi, does anyone know what load is typical under the slab? It is for a residential garage and I'm going to park a full size pickup on top. The base underneath is compressed gravel. My calculations come out to about 1.5 psi (given the building code spec of 125 psf for a light truck and the weight of 95 Lbs cu ft concrete). Or do you usually just forget calculations and go for it, it seems like the foam is able to support a whole lot more than what I need?
Thanks Rob
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