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The reason your car is underreporting speed right now (I assume you checked it with a GPS or the 1/4 mile stripes and a stopwatch on the highway?) is that wagons didn't come with 185/70-14 size tires, which is for sedans. Wagons came with a larger tread diameter or circumference (not bead diameter) size (e.g., 185-14, equivalent to 185/81-14) to accommodate a higher load rating. So a choice of 185/75 is much more appropriate or closer, viz speedometer accuracy as well as carrying the loads expected of a wagon.
On page 115 of your owners manual (for '93 model year), in the lower right you'll see a table. For sedans, it shows (quoting) "185/70R", but for wagons it shows "185R14", actually a 0.81 aspect ratio as if it were really 185/81-14, as I wrote above.
The vast majority of car owners never pay attention to load rating, but that's the two-digit code (88, 89, 91, etc.) that it part of the tire's *complete* specifications, as in, e.g., 205/55ZR15-88W. Largely ignored by almost everyone, nevertheless those last three characters are also important. While the W is the maximum speed rating, the 88 is the load rating, i.e., how much weight the tire can support.
Generally, a tire with a higher aspect ratio (e.g., 75, rather than 70) carries a greater load capacity for the same cross-section width (e.g., 185). But increasing width or diameter (e.g., 14) also increases load rating.
For example:
90 = 1,323 pounds
89 = 1,279 pounds
88 = 1,235 pounds
87 = 1,201 pounds
86 = 1,168 pounds
85 = 1,135 pounds
Your owners manual also specifies a maximum load capacity in the rear for your wagon at 2,600 lbs (see page 110) -- that's 1,300 lbs per rear tire. So you want a load rating for your rear tires that's at least this capacity, meaning it should have a load rating of 90 or better. I may be wrong, but I think an original 185R14 tire had a load rating of 91 or 92 (because of the 0.81 aspect ratio).
So you should pick your tire size accordingly.
Of course, there are also other complications, such as extra plies, designations for "extra load", and letters such as C or D, all of which increase the load capacity of the tire.
Google "tire load rating", or "tire load index" to find out more.
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