Mileage problems

Well considering all USDM 240’s from 89-98 all have the same tranny/diff gearing and all SR20 equipped JDM/EUR also have the same gearing. The only way it would change is with a CA18 Tranny, Diff etc… I’m 90% sure even the RB20 has the same gearing “tranny wise” not sure about the diff"

Tire size… sure.