Speedometer off
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Elder User
Joined: Feb 2004
Posts: 830
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From: Indiana
Speedometer off
I've done a lot of work to my truck to get it on the road. It originally had 2.47 rear gears and now has 2.75. The original trans was a three speed on the column and now has a 4speed (the forth gear is a O/D, RTS type trans).
I've checked the speedometer for accuracy on the interstate and every mile marker I come to the speedometer only shows .9 on it. Do I need a gear with more teeth or fewer teeth on it? I'd like to get it as close to right as I can.
I've checked the speedometer for accuracy on the interstate and every mile marker I come to the speedometer only shows .9 on it. Do I need a gear with more teeth or fewer teeth on it? I'd like to get it as close to right as I can.
Elder User
Joined: Nov 2004
Posts: 666
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If your odometer shows 0.9 miles when you travel 1 mile, you would need fewer teeth on the driven gear. Mile markers on the highway are usually fairly accurate, but not always. It would be best to compare the odometer to mile markers over a 10 miles stretch for better accuracy.
Lead Driver
Joined: Dec 2002
Posts: 5,491
Likes: 16
From: Kuna ID
Thats barely off, and I think changing the gear would send it too far the other way. You might just live with it for now and next time you need new tires, get a size shorter or taller to compensate for it. I forget which way it works.
My speedometer is off by 3mph, but it doesn't bother the cops so it doesn't bother me.
My speedometer is off by 3mph, but it doesn't bother the cops so it doesn't bother me.
Postmaster
Joined: Feb 2008
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From: Out of here
I could see if you said it was .9 and 1.1, but they both can't make it off the same amount. There is no physical way for this to happen.
New User
Joined: Sep 2007
Posts: 10
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test it with a GPS to be sure how far you're off.
Mine's way off, but it's due to tire size. I don't worry about it as I'm not a fast driver.
C
Mine's way off, but it's due to tire size. I don't worry about it as I'm not a fast driver.
C
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Freshman User
Joined: Jun 2008
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yes, GPS works worked for me. the faster i go the faster my spedomerter reads off. till about 75 or 80 then it jumps all the way back to 0 
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Moderator
Joined: Jan 2001
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From: Virginia
The odometer would not be how I would check it. I would drive so the speedo said 60mph. Get someone with a watch with a second hand, and see how long it takes to go from one mile marker to the other. It it's dead on, it should take one minute. If it takes less than one minute, you know your speedo is reading fast, etc.
Posting Guru
Joined: Jun 2002
Posts: 2,091
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From: Suffolk, VA USA
Keep in mind that mileage markers aren't always accurate either. Suggest you find a measured mile to do Dave's test. You also might want to call the local PD and see if they've got one of those white speed speed measuring trailers set up around town somewhere.
Postmaster
Joined: Feb 2008
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From: Out of here
Keep in mind that if it's off, it's off by a percentage not a set amount.
That means the faster you go, the more deviation there will be.
A quick and easy test is to get behind or along side someone going a good known speed and see where you are at.
Newer vehicles with digital speedos are very accurate.
That means the faster you go, the more deviation there will be.
A quick and easy test is to get behind or along side someone going a good known speed and see where you are at.
Newer vehicles with digital speedos are very accurate.
Posting Guru
Joined: Jun 2002
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From: Suffolk, VA USA
Stosh is correct about the percentages. The general rule of thumb is 5% difference for each tooth on a speedo gear....if you're 10% off, change to a gear +/- 2 teeth. If the speedometer reads too fast, add teeth. If it spins too slow, subtract teeth.
Moderator
Joined: Jan 2001
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From: Virginia
I think I got that backwards. If you are going 60mph, and it takes less than a minute, that would mean your speedo is reading slow.
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Elder User
Joined: Feb 2004
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From: Indiana
I had a chance to check the speedometer today. There is a speedometer check course about 6 miles long. At 60 mph it took 56 seconds to go a mile.
At 56 mph it takes almost exactly 60 seconds to go a mile.
I think i have a 17 tooth gear in now, would I need one with less teeth?
At 56 mph it takes almost exactly 60 seconds to go a mile.
I think i have a 17 tooth gear in now, would I need one with less teeth?