Daren Stone writes \>
\> final gear ratio
\> mph x ----------------------- / (.0113636) = rpm
\> drive tire dia. in feet
\>
\> For example, running a 3.91 rear end ratio and a
\> tire with a ***5.68' diameter*** (185/60HR-13), at 6000 rpm;
Michael Gendimenico writes \
\They must be BIG tires, 5.68 feet?????
Just a slight slip of the keyboard, Mike. Daren meant to type "circumference",
not diameter.
\Do you know what the tire diameter is for 175/70R13?
According to the BFG formulae, a 175/70R13 is 22.65" diameter, which would give
you 918.50 revolutions per mile including the BFG flat-spot factor. To use
Daren's formulae: 22.65" dia * pi = 71.157" dia = 5.93' circumference.
Daren's formulae run spot-on the same as the BFG formulae.
\Then I could figure out what ratio my rear is without looking.
Maybe. Are both your tachometer and speedometer perfect? Is the speedometer
calibrated for these tires? Is any gear in your transmission a
straight-through, or are they all underdrives or overdrives?
If you have a front-engine, rear-drive car, it is a simple matter to count the
turns on rear wheel and the driveshaft. Find out what ratios came on the car,
and you can zero in on one of them by counting.
Driving a steady speed on the freeway and noting the tachometer and speedometer
readings and timing the mile markers with a stopwatch will give you some data
to work with.
You can tie it all together with my spreadsheet. I can Email you instructions
to set it up in your own program, Excel, Lotus 1-2-3 or whatever. Then you can
see what's right and what's not.
Phil Ethier, THE RIGHT LINE, 672 Orleans Street, Saint Paul, MN 55107-2676
h (612) 224-3105 w (612) 298-5324 phile@pwcs.stpaul.gov
"Let me know if this gets too organized." Steve Goodman
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