Over the years, I’ve developed my own spreadsheet for doing axle and speedometer gearing calculations. It started in Excel, and now lives in Google Drive:
Please feel free to copy or download this calculator – like the rest of the site, it is shared under a Creative Commons license.
- Supports up to six speeds, plus two ranges
- Calculates speedometer drive gear for Fords, including speedometer error. (This will work for some other makes as well.)
- Shows the RPM after shift for each gear shift
How to Get Your Copy:
- If you’re already a Google Drive user, click the “File” menu, and “Make A Copy.” This will give you your own copy, which you can edit at will.
- If you’re not a Drive user, you can click “File” > “Download As” to get a copy in the spreadsheet program of your choice. You might have to do a little cleanup work to make it pretty again, but everything should work.
Instructions for use:
- Fill in the (yellow) blanks with your information. Vehicle and Trans fields are only there so you can keep track of your results if you choose to print this out.
- Axle field should be obvious. If you’re using a portal axle (Unimog, H1, aftermarket, etc) you’ll need to put your total axle gearing here, not just your differential gears.
- Range can be 0 for single range cars, or the ratio in your transfer case for four wheel drives or trucks with splitters.
- Tire diameter is in inches. Your equivalent tire revs per mile will show on the right for the tire diameter you enter. If you are going primarily by a manufacturer’s rev/mi figures, I’d recommend guessing your diameter until you get Rev/Mi correct as the fastest method.
- Enter your transmission’s gear ratios in rows 11-16. Unused gears can be left at “0”.
- If you need revs at a specific road speed, you can easily change the speeds in row 10. Engine revs below will change to suit the speeds you’ve entered.
- Entering your shift point in row 38 will show you your road speed for each shift, RPM before and after, and % drop in RPM.
I’ve found this spreadsheet very useful over the years, and hope you do, too! Please, feel free to leave any questions in the comments below.