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The 87 CRX is the first-generation one. The Si will have a 1.5 liter engine with MPFI (Multi-Point Fuel Injection). It will also have a moonroof. The DX or Standard version (I do not believe there is a badge on it) will have a 1.5 liter engine with a carburetor on it. The HF version has an engine whose size I cannot seem to find out, and it also has a single carburetor on it.
Spinning at 2300 RPMs at 50 MPH tells me that this is most likely an Si, or at least has an Si transmission on it.
175/70-13 would have sidewalls of roughly 175mm * 0.70 / 25.4mm/inch; 4.8 inches tall. Two sidewalls contribute to the overall diameter, so that's 13 + 4.8 + 4.8 = 22.6" overall diameter.
A 155/80-13 would have sidewalls of 155 * 0.80 / 25.4; 4.9 inches tall. The overall diameter would be 13 + 4.9 + 4.9 = 22.8". That's a difference of about 1%. Not enough to justify the change IMO. (And I actually rounded the 4.8" down from 4.82...., and rounded the 4.9 up from 4.88... so the actual change would be even less.)
It's hard to find passenger car tires in any aspect over 80.
In order to bring your 2300 RPMs down to 2000 RPM, you will need to increase the overall diameter of your tires by 15% of your original 22.6", which is just about 4". So you need to get sidewalls that are almost half-again as tall as your current ones. Which IMHO is not going to happen.
Going up to 15s means you only need sidewalls that are 3" taller than your current ones. Still very unlikely. And going with bigger wheels than that will really hurt your in-town MPG, not to mention acceleration and braking distances...
And, of course, your speedometer will be reading a whole lot lower than you are actually driving, because it assumes the tires are the stock size!
The only real practical way to drop the cruising revs would be a transmission swap, or at least a gear swap. The speedometer would even read correctly (or as correctly as it does now anyway).
The folks on the Red Pepper Racing site know a whole lot about 1st-gen CRXes and the equivalent generation of Civic. You might look for more detailed info over there.
-soD
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