Speedo off 10mph...

[N8thegr8]

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!

[N8thegr8]

Would putting 15s on it fix my problem? I don't know if I can even find 8lug 15s

[timthescarrd]

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/

[Stewart G Griffin]

Can you check your SPID sticker to see if these were the original sized tires that came with the truck from the factory?

[Irish_Alley]

15" rims wont make a difference.

[roundhouse]

Someone has changed the axle gears or the speedo gear in the transmission

The speedo gear is easy to change


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[N8thegr8]

Any suggestions on a speedo gear? It's a 14 bolt rear end.

[enaberif]

Any suggestions on a speedo gear? It's a 14 bolt rear end.

Use the calculator to figure it out.

[Irish_Alley]

Made this up to help 

[roundhouse]

Gear ratio ?
Tire size ?


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[Captkaos]

This gives a good overview of the drive and driven gears...

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