Hi Pete
You are right that that speedometer reference number should be for a 3.31 ratio (for a 3.8). What I would highlight though is that the Speedometer would not have had the Jaguar cat's head logo on it (see photo below) which has replaced the Smith's name.
As such, it could be that your rear-diff ratio is correct but the speedometer has been altered at some point in its life. To the best of my knowledge, you would expect a 3.07 rear diff ratio to be reading around 2750 RPM at 70mph and a 3.31 ratio to be reading around 3000 RPM at the same speed. A 3.54 ratio would read correspondingly higher on the rev counter. All of these carry the caveat that your rev counter generator and rev counter are producing an accurate reading!
