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Ram Man 150
1989 D-150
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100 Posts
Discussion Starter #1
I have an 89 d150 that needs a new speedo gear. The truck now has a 340 in her with a 904 tranny, I’m running 275/50/15 on it and 355 rear end gears. There is a 33 tooth gear in it now and it’s way wrong, can someone give me the correct tooth count for the gear I need, thanks
 

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Easy way to figure it out.

Grab a friend with a good stopwatch (hint: One's built into the clock app on most any Android phone).

Drive at an indicated 60MPH on the interstate.

Have a friend start a timer when you pass a mile marker; keep the indicated 60 until you pass at least one more. (Take the number of markers or number of miles and divide that into time; you want the time to travel exactly 1 mile, but it's not a bad idea to average out over a few if you can).

Now.

Take 60, and divide that by the number of seconds for a mile, then multiply that by 33, and that's the tooth count. You'll probably have to round; try to round down so it's less likely you get a ticket by some really anxious LEO.

(Charts I have don't have 275/50/15 tires on them, alas.)

RwP
 

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Then go back and do what I said.

But is it 15MPH off, or is it 25% off at 60MPH? (There IS a gigantic difference between the two!)

RwP
 

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Ram Man 150
1989 D-150
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100 Posts
Discussion Starter #7
So I found a radar speed sign and my speedo is only mph off I have a 33 tooth gear what should I get to make up the difference. I’m going faster than speedo says
 

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Also, if you do that trip down the expressway, is the odometer also reading that 14% high, or about right?

RwP
 
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