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post #1 of (permalink) Old 05-16-2001, 10:13 AM Thread Starter
 
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calculating actual speed?

Does anyone have the formula for calculating actual speed handy. Something like "old tire diam. X new tire diam. / old gear ratio X new gr. blah, blah, blah" ? Can't find where I put my little list and Im pretty sure by the way I'm being passed that Im not doing 85. TIA

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post #2 of (permalink) Old 05-16-2001, 10:37 AM
 
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Re: calculating actual speed?

336 x SPEED x FINAL DRVE RATIO / TIRE DIAMETER = RPM.
SO HERES HOW TO FIGURE OUT SPEED INCREASE BY TIRE SIZE
RPM x TIRE DIAMETER / FINAL DRIVE RATIO / 336 = SPEED.
EXAMPLE: 4.10 GEARS 33" TIRES AND YOU'RE SWITHING TO 35s
2700RPM IN 4TH GEAR x 33" / 4.10 / 336 = 65MPH
2700RPM IN 4TH GEAR x 35" / 4.10 / 336 = ABOUT 70MPH

This is the formula I used to figure on my jeep and It worked out pretty well.

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post #3 of (permalink) Old 05-16-2001, 10:45 AM
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Re: calculating actual speed?

Another option is to borrow a friends GPS, that is if you don't have one, and mount it to your dashboard. Now use the mph function on the GPS and you will have an accurate speedo. Good luck.

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post #4 of (permalink) Old 05-16-2001, 08:21 PM
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Re: calculating actual speed?

[img]/wwwthreads_images/icons/tongue.gif[/img] Orrrrrrr......you can use the Turns Per Mile of the tires (available from the little book at the tire shop) and multiply THAT by the rear end ratio to get RPM at 60 MPH. [img]/wwwthreads_images/icons/crazy.gif[/img] So then Turns Per Mile X Gear Ratio (new) divided by Turns Per Mile X Gear Ratio (old) TIMES the indicated MPH of the speedo gives true MPH. [img]/wwwthreads_images/icons/crazy.gif[/img]

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post #5 of (permalink) Old 05-16-2001, 11:22 PM
 
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Re: calculating actual speed?

I have a 88YJ 258 with auto and Dana axles. Can anyone tell me what my axle drive ratio are? I copied down the numbers on the front and rear axle covers but I still don't have a clue to what the ratios are. Thanks for any help.

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