This spreadsheet based calculator allows you to calculate overdrive ratios, view various difference between two tire sizes, calculate your RPM given a speed, calculate your speed given an RPM and find out how much your speedo would be off if you changed tire sizes.
I have inputed MGB specs based off the factory repair manual, however, there are differences throughout the years. I also put in a modern tire size equivalent that is commonly used.
That being said, any of the yellow highlighted fields can be changed manually if desired. Because of this, you can use it for any type of vehicle, hence why there are 5th and 6th gear input fields.
Here is the link to the Excel spreadsheet. If clicking the link doesn't download the file, right click and select "Save As..."
The file is a Microsoft Office Excel spreadsheet and requires either Excel or another program that can open XLS files in order to open it. OpenOffice is a free program that will enable you to open the file if you do not have Microsoft Office. You can download it here: www.openoffice.org