Any gas used to shield an arc from the atmosphere is "shielding gas". Argon is inert, CO2 is not.
I have never tried TIG welding with the 75/25 mix, only mig.
I would guess the weld quality would go down some, thinner materials will be a little harder to weld, may have some pin- holing.
My opinion is, if you're going to TIG something you might better get the proper shielding gas for the best results. Kind of like a $200 suit and no underwear. What's a 40 minute drive in the grand scheme of things? Depending what size bottle you get, it should last a while.
In any event...good luck and have fun with it.
