For this '86 Si I may get, I am going to be doing two kinds of driving. 90% will be long cruises of about 100 miles. The rest will be the driving required to take non-highway streets to drive the 3.5 miles between my residence and workplace.

I don't know anything about the Si transmission. Is 5th a relatively long gear or am I still going to get pretty good mpg on the highway with this car?

 

In good shape, expect upwards of 35mpg. My 2nd gen CRX w/ an Si transmission but HF 5th gets near 40mpg. You might be slightly lower. If you really want to know gear ratios and what not, ask RedPepperRacing.com. I'm a 2nd gen CRX guy.

 

So can you put just an HF 5th gear in the transmission? It would be nice to have the combination of Si acceleration for getting on the highway with the efficiency of the HF long gear for extended cruising.

I guess it would be a very noticable step from 4th to 5th but I wouldn't expect to be in 5th except on the highway anyway.