Note: this calculation makes many assumptions, so it's only useful in an 'average use case', not some sort of exact measurement. If you find better information, please post it and I'll update the answer.
How many pumps you would need to fill up a tire depends on many variables. First, the volume of your inner tube, which can be approximated as a torus (doughnut-shape) atop the rim size of a given diameter. You can find yours approximated on this graph.
- 26" Mountain Bike, 2.1" tire width = 4.8L
- 29" Mountain Bike, 2.1" tire width = 5.2L
- 700C Road Bike, 35mm tire width = 2L
- 20" BMX Bike, 1.85" tire width = 3L
I'm finding pump specifications quite difficult to find (I need piston diameter and stroke length), but here's a comparison of some shock pumps on an empty shock cylinder of a 2007 Fox 36 RLC 160mm Fork.
- Average Accu-Gage Pressure Reading after 100 strokes: 77.2 psi
- My best guess at RC2 air chamber specifications: 1" (25.4mm) diameter, 160mm length (max travel). If someone can find accurate specs or the true volume, I'd appreciate it.
- Fox RC2 Air Volume (estimated):
Vcylinder = pi*r^2*h = 81 mL
So then, IF pumping air into different volumes is linearly proportional (it's not, but somewhere in the ball park), and you wanted to inflate the tires listed above to ~77.2psi, it'd take about this number of pumps with the 'average' shock pump:
Tire Volume Ratio of Tire to Air Shock Number of Pumps to ~77.2 psi
26" Mountain Bike, 2.1" tire width 4.8L 59:1 5,900
29" Mountain Bike, 2.1" tire width 5.2L 64:1 6,400
700C Road Bike, 35mm tire width 2L 25:1 2,500
20" BMX Bike, 1.85" tire width 3L 37:1 3,700
So, perhaps if it was a life or death situation, you have a lot of time on your hands, or you just can't afford that gym membership, you might want to pump a few thousand times to fill your tire. You probably don't.
By contrast, if you're just looking for this functionality in one device, you could just use a dual purpose model, such as this Specialized Pump, that can inflate both high pressure, low volume containers (like your shock) and low pressure, high volume containers (like your tire).
You twist the handle to select between uses, and save yourself a few thousand pumps.