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Jul 21st, 2025 | | | | Joined: Nov 2008 Posts: 153 Wrench Fetcher | | Wrench Fetcher Joined: Nov 2008 Posts: 153 | Can anyone tell me what speedo gear I will need after changing from 7.50x16 tires to 8r 19.5?? (Aproximately 1,5" taller) My truck is a 64 C30, with 4 speed. Thanks!
Last edited by kerry3; 01/12/2011 9:23 PM.
| | | | | Joined: May 2005 Posts: 8,877 . | | . Joined: May 2005 Posts: 8,877 | There are also little gear change conversion boxes to put in line with the speedometer cable.
A speedometer shop will be a lot of help here.
Just for reference 8R19.5" tires are about 32-33" tall, better yet find the manufacturers specs for revolutions per mile for the exact tires you'll use.
You'll also need to know your rear axle ratio.
Grigg | | | | | Joined: Feb 2004 Posts: 28,675 Kettle Custodian (pot stirrer) | | Kettle Custodian (pot stirrer) Joined: Feb 2004 Posts: 28,675 | The simplest way to figure speedo error is a timed mile. Find a road with mile markers, and drive various speeds until you make a mile in exactly 60 seconds. That's 60 MPH. Figure the percentage error between a 60-second timed mile and the indicated speed on your speedometer, and either swap speedo drive gears in the transmission output housing or have a speedometer shop build up a speed-change box for you.
Here's an example:
Timed mile = 55 seconds
Divide 55 into 60 = 1.09
60 X 1.09 = 65.4 MPH
If your speedometer reads above or below 65 MPH, you need to correct the gear ratio of the speedo drive.
Jerry
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| | | | | Joined: Mar 2010 Posts: 843 Shop Shark | | Shop Shark Joined: Mar 2010 Posts: 843 | kerry3, your taller tires will cause the speedo cable to rotate more slowly than it did before at a given speed, yielding a lower reading on the speedo. The fix is to speed up the speedo cable with either a smaller cable drive gear in the trans tailhousing or an inline overdrive box. Beware of the speedo shop prices on the overdrive, especially in a rural area where there is no price competition. You can actually buy those overdrive boxes directly from the manufacturers.
I just went through this when I converted my '36 Chevy pickup from 4.11 to 3.55 gears. The exact overdrive ratio can be determined by driving a known mile and looking at the odometer distance measurement. In my case, the measured distance needed to be increased by 17% to give a correct reading. I gave that information to the manufacturer of the in line cable overdrive box and it was problem solved.
The gears inside the box are replaceable if you make another tire or gear ratio change in the future.
Ray
| | | | | Joined: Jan 2002 Posts: 866 'Bolter | | 'Bolter Joined: Jan 2002 Posts: 866 |
Last edited by coach-frank; 01/15/2011 4:59 PM.
| | | | | Joined: Nov 2008 Posts: 153 Wrench Fetcher | | Wrench Fetcher Joined: Nov 2008 Posts: 153 | Thanks for that link! That will come in handy for sure.....saved in my favorites! | | |
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