Speedometer Correction Formula

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Date: Thu, 12 Mar 1998 08:55:53 -0800
From: Barney McNamara 
Subject: Speedometer correction
To: Toy4x4@tlca.org, hanley@VNET.IBM.COM

Joe asked:
>I just upgraded tires from 225/75 (Bridgestone Duelers)
>to 31x10.5 (Dunlop Radial Rovers RV).  I recall seeing
>a formula to determine the correct speed ... I just
>can't seem to find it.  I am more interested in determining
>the correct gas mileage.

As far as speed changes caused by gears, tires, etc, I use this formula
(which I got from Jay's book.):

MPH = (Engine RPM x Tire Diameter (in inches) x 0.003 ) divided by
   (Transmission Gear Ratio x Transfer Case Ratio x Final Drive Ratio)

Example: RPM x  tire   x constant) / (5th gear x xfer hi x stock diffs)
      (3000RPM x 29.75 x   0.003)  / (0.85    x 1.00    x 4.11) = 76.6 MPH

The tire diameter must be measure carefully, as it will impact the
formula greatly. What you can see is that everything is directly
proportional. That is, a change in tire diameter from 28 inches
to 31 inches, (31-28) / 28 = 10.7%  will impact speed by 10.7%. 
And likewise, a diff gear change from 4.11 to 4.88, 
(4.88-4.11) / 4.11 = 18.7% will affect speed by the same amount.
 
Since the tire diameter is on the left side of the division sign, 
larger tires give higher speed. The gear ratios are on the right side 
of the division sign, so higher gear ratios give lower speed.

The speedometer measures the driveshaft RPM at the output of the 
transfer case, so the truck is designed to automatically correct for 
changes in gear ratio in the transmission and transfer case. The 
speedo is calibrated for the stock tire size and stock diff ratios,
so changing those will affect the speedo reading. The amount of change
in the speedometer reading can be figured out by calculating the 
amount of change the modifications have made on their component
of the equation.
______________________________________________________________
Barney McNamara              JENNY - 83 Toyota Short Bed  
( barney@flowpoint.com )     stock 22R motor ; 3" body lift
Santa Cruz, Ca.              8" alloy rims; 31" BFG A/Ts
homepage: http://www.scruz.net/~barneym/barnhome.htm
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