Speed/RPM

[alfert]

I have finally been driving my Elan after about 5yrs of resto. It has the 3.7 rear and has 155/70/13 tires. The ign is now electronic and the tach was modified to accept it. It seems to be about 4000 rpm at 60mph. Does this sound right?? Seems high to me.
Thanks,
Brian

[billwill]

My brain is too tired (tyred?) to do the maths now.

You work out the circumference of the tyre, divide that into a mile, that gives you wheel revs per mile, divide (or multiply) by 60 because you are looking for Revs Per Minute, Divide or multiply by 60 'cos you calibrated at 60 MPH & finally divide by the diff ration 3.77.

Have fun.

[msd1107]

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

[stugilmour]

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.

[msd1107]

Brian,

If the speedometer/odometer error (primarily odometer error, since the speedometer spring tension can be modified to bring it into compliance once the odometer is registering accurately) is greater than about 1%, changing the drive gears in the speedometer will bring it into closer calibration. See:

elan-f15/more-spreadsheets-t15049.html

to see more. This allows you to calculate the best drive gear pair in the speedometer. A shop that rebuilds Smiths instruments can often install the better set for you.

Cheers,

David
1968 36/7988