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Author Topic: Speedo off 10mph...  (Read 5136 times)

Offline N8thegr8

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Speedo off 10mph...
« on: March 12, 2015, 01:43:22 pm »
I have an 80 C20 and I'm 10mph off at hwy speed. I have 235/85 R16s on it. Why am I so off? Thanks!
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Offline N8thegr8

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Re: Speedo off 10mph...
« Reply #1 on: March 12, 2015, 01:45:37 pm »
Would putting 15s on it fix my problem? I don't know if I can even find 8lug 15s
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Offline timthescarrd

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Re: Speedo off 10mph...
« Reply #2 on: March 12, 2015, 03:33:14 pm »
Speedometer is driven by a set of gears, one on the tailshaft of the transmission and one where the cable goes into the transmission.  the number of teeth on each gear is changed to keep the speedometer at the right speed.  235/85/R16 tires should be around 31.75" tall, the height of the tires and the gear ratio (rear end and transmission) determines speed, so if either of these have changed from factory, the speedometer will be off by some percentage at all times.

Here is a calculator and some info on drive gears
http://www.tciauto.com/tc/speedometer-gear-calculator/
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Offline Stewart G Griffin

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Re: Speedo off 10mph...
« Reply #3 on: March 12, 2015, 08:48:34 pm »
Can you check your SPID sticker to see if these were the original sized tires that came with the truck from the factory?
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Offline Irish_Alley

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Re: Speedo off 10mph...
« Reply #4 on: March 12, 2015, 11:37:44 pm »
15" rims wont make a difference.
« Last Edit: March 13, 2015, 06:30:34 am by Irish_Alley »
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Offline roundhouse

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Re: Speedo off 10mph...
« Reply #5 on: March 13, 2015, 06:17:53 am »
Someone has changed the axle gears or the speedo gear in the transmission

The speedo gear is easy to change


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Offline N8thegr8

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Re: Speedo off 10mph...
« Reply #6 on: March 14, 2015, 04:21:04 pm »
Any suggestions on a speedo gear? It's a 14 bolt rear end.
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Offline enaberif

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Re: Speedo off 10mph...
« Reply #7 on: March 14, 2015, 04:47:12 pm »
Quote from: N8thegr8 on March 14, 2015, 04:21:04 pm
Any suggestions on a speedo gear? It's a 14 bolt rear end.

Use the calculator to figure it out.
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Offline Irish_Alley

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Re: Speedo off 10mph...
« Reply #8 on: March 15, 2015, 01:12:48 am »
Made this up to help  8)
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If you can’t tell yourself the truth, who can you tell it to?~Irish_Alley

When you have eliminated the impossible, whatever remains, however improbable, must be the truth ~Sherlock Holmes

Offline roundhouse

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Re: Speedo off 10mph...
« Reply #9 on: March 15, 2015, 10:32:18 am »
Gear ratio ?
Tire size ?


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Offline Captkaos

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Re: Speedo off 10mph...
« Reply #10 on: March 15, 2015, 12:53:49 pm »
This gives a good overview of the drive and driven gears...

http://73-87.com/7387garage/drivetrain/speedo.htm
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