equals what RPM on the Tach - 2750? Or?
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Re: 70 MPH W 3.08
Depends on tire revs per mile, but if they're 7.75-15 or 215/70R-15,which are speced at 775 revs per mile it would be:
775 x 3.08 x (70/60) = 2785.
Duke -
Re: 70 MPH W 3.08
Depends on tire revs per mile, but if they're 7.75-15 or 215/70R-15,which are speced at 775 revs per mile it would be:
775 x 3.08 x (70/60) = 2785.
DukeComment
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Thanks, Duke *NM*
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Thanks, Duke *NM*
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Re: 70 MPH W 3.08
Measure your standing tire diameter and then go to: http://www.idavette.net/tech/ratioc.htm
You can play with any of the variables to see what happens.
GaryComment
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Re: 70 MPH W 3.08
Measure your standing tire diameter and then go to: http://www.idavette.net/tech/ratioc.htm
You can play with any of the variables to see what happens.
GaryComment
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The Web calcuators are all wrong!
If you try to compute revs at speed using the measured diameter or circumference of the tire you will typically get an error of about three percent. When pneumatic tires roll under load they deform, and the actual "rolling circumference" or revolutions per are GREATER than what the static diameter yields.
All these rev/speed calculators on the Web are WRONG! If you want accurate answers you must used the revs/mile figure provided by the tire manufacturer, and they are usually within a fraction of a percent for all manufacturers of a given size tire even though industry standards allow greater variation. Most manufacturers list the revs/mile data for all their tires on their web sites, and it is also listed on The Tire Rack site under "specs" after you have searched under a specific size and have a list of tires.
It's a very simple calculation. To obtain the revs at 60 MPH (for a car with a direct drive, 1:1 gear) just multiply the revs/mile times the axle ratio. Then you can obtain the revs at any speed or speed at any revs by linear proportioning as I did in my prior post. You can also plot it on paper by drawing a "gear chart", which will be four staight lines (for a four speed) that show the linear speed-rev relationship in any gear.
Say you have a '65 with a CR gearbox and a 3.70 axle and OEM equivalent 775 revs per mile tires. Your revs at 60 are 3.70 x 775 = 2868. Draw a line between this and the origin on a piece of graph paper with X-axis as revs and the Y-axis as speed. To get the revs at 60 in the other gears, multiply the revs at 60 in fourth by the gear ratio, which for a '65 with a CR gearbox will be 1.28, 1.64. and 2.20 or 3670, 4702, and 6308. Draw the lines between these points and the origin up to your redline, and you have a complete and accurate gear chart.
DukeComment
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The Web calcuators are all wrong!
If you try to compute revs at speed using the measured diameter or circumference of the tire you will typically get an error of about three percent. When pneumatic tires roll under load they deform, and the actual "rolling circumference" or revolutions per are GREATER than what the static diameter yields.
All these rev/speed calculators on the Web are WRONG! If you want accurate answers you must used the revs/mile figure provided by the tire manufacturer, and they are usually within a fraction of a percent for all manufacturers of a given size tire even though industry standards allow greater variation. Most manufacturers list the revs/mile data for all their tires on their web sites, and it is also listed on The Tire Rack site under "specs" after you have searched under a specific size and have a list of tires.
It's a very simple calculation. To obtain the revs at 60 MPH (for a car with a direct drive, 1:1 gear) just multiply the revs/mile times the axle ratio. Then you can obtain the revs at any speed or speed at any revs by linear proportioning as I did in my prior post. You can also plot it on paper by drawing a "gear chart", which will be four staight lines (for a four speed) that show the linear speed-rev relationship in any gear.
Say you have a '65 with a CR gearbox and a 3.70 axle and OEM equivalent 775 revs per mile tires. Your revs at 60 are 3.70 x 775 = 2868. Draw a line between this and the origin on a piece of graph paper with X-axis as revs and the Y-axis as speed. To get the revs at 60 in the other gears, multiply the revs at 60 in fourth by the gear ratio, which for a '65 with a CR gearbox will be 1.28, 1.64. and 2.20 or 3670, 4702, and 6308. Draw the lines between these points and the origin up to your redline, and you have a complete and accurate gear chart.
DukeComment
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Given the built in
error percentage of a mechanical tach, 2750 to 2850 seems close enough. Thanks again.Comment
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Given the built in
error percentage of a mechanical tach, 2750 to 2850 seems close enough. Thanks again.Comment
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I like accuracy
To paraphrase Everett Dirksen: A percent here, a percent there, pretty soon that adds up to real error.
You can literally use the tire revs per mile times axle ratio to calibrate your speedometer and tachometer. I like accurate instruments and have even taken some out of my cars to have them recalibrated so their readings are as close to dead on accurate as possible.
DukeComment
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I like accuracy
To paraphrase Everett Dirksen: A percent here, a percent there, pretty soon that adds up to real error.
You can literally use the tire revs per mile times axle ratio to calibrate your speedometer and tachometer. I like accurate instruments and have even taken some out of my cars to have them recalibrated so their readings are as close to dead on accurate as possible.
DukeComment
Comment