Speedometer correction with 16" Ford rims

[AL in NY]

I just purchased a nice set of 16" Ford wire wheels. After viewing several threads for recommended tire sizes for these rims, I have a couple of questions. The manufacturer for the Model "A" 4.47/5.00x19 tires startes the tire diameter as 29.5 inches. The recommended size I find for use on the 16" rims is 205/70R16, which have a diameter of 27.3 inches. That equates to a speedometer error of -3.7 MPH at 50 MHP. The recommended rim width for these tires is 5 to 7 inches. The 35 Ford rims are only 4 inches wide. A better tire size is 205/80R16 which measures 28.9 inches in diameter with a speedometer error of -1.0 MPH at 50 MPH. This tire also is recommended for a 5-7 inch wide rim. Sorry for the ramble, but here are the questions. Is it a big problem putting tires recommended for 5-7" rims onto 4" rims? (AND) What have folks done about the speedometer error when changing to 16" rims?