If your running 4500 at 75MPH you not only have a Del Sol engine but you have a del sol transmission as well. My B17 runs that exact RPM in that gear at that speed and my transmission ratios are the same as a B16s for every gear but first, where mine is just a hair shorter. Actually be thankful for that, the GSR trans. would actually slow you down in this due to its longer ratios = slower revving.