If you know all the ratios from engine crank to the wheel,. And you know the circumference of the wheel, you can calculate the speed over the ground at any RPM of the engine. You can calculate it in any gear.
That's the hard way but most accurate.

You can swag it. If you assume your gauges are correct, you can determine the speed in a gear at 3000 RPM and just double the number for a somewhat accurate guess at what the speed would be at 6000 RPM.

In sixth Gear you can determine the RPMs at 3150.0 equal 90 MPH. Therefor 6300 RPM would be 180 MPH. Kinda hard to set the RPM's at 3150 but a little practice and swag and off you go.

It's simple really!

roadster with a stick