FWIW:
1. My 2-1/4" diameter suction cup made by Garmin, was designed to allow the flat front face of the GPS screen to extend inwards only 2-1/2" when measured from the Model A metal dash or any other flat surface one may prefer.
2. I first tried my suction cup mounted on the passenger's in-side of the gas tank which may work extremely well in many other Model A cases; but not mine ...... as is.
3. Unfortunately, inside the car, the previous owner painted my particular exposed gas tank surface and did not sand same; hence, it has miniscule waviness in the painted surface. Coupled with the very minor curvilinear 1930 gas tank metal surface, this surface, (again in my Model A), only allows the suction cup to hold a partial vacuum for about five (5) minutes with the engine "not" running after many times trying same ..... less time when driving on normal back roads.
4. I next experimented with this Garmin suction cup and found where the suction cup worked well on a dead flat metal surface; hence, I went to a dead flat, smooth metal surface inside the cab, (per No. 2 above), which holds for hours on back roads.
5. In attaching a suction cup to the windshield, and partially blocking view, (against the Law in some States according to Garmin), it may or may not fall, especially when windshield is open ...... but each to his own because all Model A owners and no doubt conditions are different.
6. If one's gas tank does not hold the suction cup well, and one is interested, Wal-Mart, in the roofing/gutter section, has 8" long, by about 26 gauge thick factory painted light gray "shingle step flashing" metal angles with about a 4" angle leg on each side that should work well. Forty-five cents or so each. The gray color can be spray painted black, or any other color. Wal-Mart also had the upholstery material protective backing in the auto interior's section in 36" wide rolls.
7. For a Garmin suction cup, (verify dimensions of a specific Garmin Model no.), cut a piece of metal about 3-1/4" wide x 6" long and bent and drill per above no. 2. The suction cup has to fit on a "flat" 3-1/4" wide metal surface "below" the head of the dash rail screw.
8. Tip for bending metal ........ place in a vise sandwiched between (2) short pieces of Wal-Mart 1 x 2 wood strips, with rounded edges ....... bend by hand and tapping with a piece of wood ...... factory painted enameled metal will look neat in contact with wood as opposed to looking beat up and marred with metal vise jaws and beat up with a metal hammer.
9. Round all four corners point towards engine and file smooth all sharp edges if making a removable dash mounting device.
10. No matter how mounted, the next wrong turn one is forced to make at night .... in heavy traffic .... in an unknown Small Town ........ the Garmin lady will graphically guide you all through the dangerous Druggy Red Light District back of the railroad tracks ........... and graphically steer you back on the main highway in a jiffy ........ with accompanying courteous voice Commands. Not too good an idea to stop back of the tracks to ask for directions.