msd1107 wrote:Brian,
A downloadable spreadsheet at viewtopic.php?t=16413&view=next down at the bottom, 20080411zGearRatios.xls, allows you to mathematically view various properties of any gear box ratio set.
Your configuration is about 65.9 MPH @ 4000 RPM or 3641 RPM @ 60 MPH.
Cheers,
David
1968 36/7988
Brian, I used David's brilliant spreadsheet to dial in my tach to GPS speed values. My RVI tach was adjustable with a pot inside the body of the instrument; not sure on your modified one. My speedo still has quite a bit of error, which I hope to tune up over this winter. With your tire combo you could have about 5% error in the speedo; you can easily compare directly to the GPS readings. David's sheet allows you to determine the exact 'calculated' speedo error from stock tires to your tires.
HTH and thanks again David for the sheet.