Zack's right about the 4% size difference and the effect on your speedometer (and odometer). If your speedo was exactly dead-on with your original tires, then when you are doing an actual 60MPH with the new ones, the speedo will show 62.4. Similarly, an exact 100miles covered will show 104 on the odo. And the engine will be turning 4% more RPM's than before at a given road speed.

Main thing, though, is the load carrying capacity...check the sidewall ratings for max load and compare to your original tires. It will be less, but by how much? Then keep that in mind when loading up the old brick.
--
Bob (son's 81-244GL B21F, dtr's 83-244DL B23F, 'my' 94-944 B230FD; plus grocery-getter Dodge minivan, hobbycar 77 MGB, and a few old motorcycles)

185R14 (actually 185/82R14) gives a wheel/tire diameter of 25.94 inches
185/75R14 gives a wheel/tire diameter of 24.93 inches
Circumference is 2*pi*radius, and the difference in radius is 0.505 inches
0.505 is about 4% of the radius of the original size tire, so your speedometer should read 4% higher than actual.

Zack
1980 245DL M46 275k
1985 744TD M46 251k
1988 745T+ M46 203k

My favorite tire-size calculator is at
http://www.wickedbodies.net/Tire-Size-Calculator.htm

It will let you input "R" or no middle number.
Experimentation there shows that "R" = 82.

My experimentation there also showed me I wouldn't use 185-75-14 if I want the speedometer to be halfway accurate. Would probably carry the front of the car OK, but not a fully loaded wagon. See notes later here.

Note that tire's load capacity follows almost exactly the diameter. That is, if the wheel size and overall tire diameter are the same, the load capacities will match, or will be so close that nobody will care. So, matching diameter is the thing to do.

Width is not usually an issue on 240's, lots of room in the wheel wells. I've used 205-70-14, admittedly with some slight occasional rubbing, but it may have come from meeting a curb in parallel parking, I never found out.

Also note that the sedan calls for 185-70-14 (if I recall). This tells you that that smaller size is enough to carry the (heavier) front end of the car. Just not quite enough to carry the load of a fully loaded wagon. 185-75-14 would have slightly larger diam. than the 185-70, so load cap. would be decent for the front end, I'd think.

I've found that 195-75-14 is a decent approximation for the wagon's stock tire, speedometer shows 65 when doing just about exactly 64, not too bad an error.

205-70-14 is a bit smaller than that, the error is a bit larger.
205-75-14 is a hair bigger than stock size, with only a very small error, but I'm a bit concerned about tire rubbing. Maybe OK on rear wheels, where they point straight forward.

I've also used the calculator to test sizes for 15" wheels, which you can get from 740's and supposedly fit the 240's. Some tire sizes that match diameter closely aren't in large-scale production, so are unavailable or high-priced. I think one or two sizes of 15" or 16" tires may work out.

You can find sizes that make a close match, but you then have to check that the size is in production.