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After doing a K20 swap, what adjustments would need to be made to the speedometer/odometer readings or no change? Would our stock tire sizes give the correct readings or would we need the same tire diameter of RSX? Also, the Si also has the same wheel sizes as the coupes/sedans right?

 
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u pose a good question, never really thought about that one

 
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compare the stock Si or rsx wheels to a stock civic wheel (depending on your model) or to your aftermarket wheel.

Compare this using the Tire Calculator on the left vertical toolbar on this site. That will answer your question.

 
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According to earlier gen's, you shouldn't have to change that at all. The new computer would calculate all that out and control the controls the way it should.

I guess you could say its like just putting a Civic cluster in a K20.. the computer would just take over and move the needles accordingly.

 
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Here is the better question... Is our speedometer cable driven? If it is, speedo gears need to be changed, the ecu won't be able to help you there....

 
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Also if wheel sizes are different between say the Si and RSX, they both have the a K20. Now that I'm awake, I'm gonna look at the sizes:

RSX: 205/55/16 (or 195/65/15 for Base in Canada only)
Si (SiR in Canada): 205/55/16 (for 2004 year), I swear it had 15s before. It also looks to have a 5 lug patter now too... Hmm.
Civic Coupe/Sedan: 185/70/14, 195/60/15 (or 185/65/15 for Canadian LX)

Well, it looks like the Si changed but I don't follow Si news so I dunno if that was changed this year or not. The tire calculator shows a 2.6% difference.

Does anyone have the old tire info for the Si? I'm sure it came factory with 15s before and with 4 lugs.