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73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: N8thegr8 on March 12, 2015, 01:43:22 pm

Title: Speedo off 10mph...
Post by: N8thegr8 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!

Title: Re: Speedo off 10mph...
Post by: N8thegr8 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

Title: Re: Speedo off 10mph...
Post by: timthescarrd 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/

Title: Re: Speedo off 10mph...
Post by: Stewart G Griffin 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?

Title: Re: Speedo off 10mph...
Post by: Irish_Alley on March 12, 2015, 11:37:44 pm

15" rims wont make a difference.

Title: Re: Speedo off 10mph...
Post by: roundhouse 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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Title: Re: Speedo off 10mph...
Post by: N8thegr8 on March 14, 2015, 04:21:04 pm

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

Title: Re: Speedo off 10mph...
Post by: enaberif 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.

Title: Re: Speedo off 10mph...
Post by: Irish_Alley on March 15, 2015, 01:12:48 am

Made this up to help  8)
(http://forum.73-87chevytrucks.com/smforum/index.php?action=dlattach;topic=30546.0;attach=28703;image) (http://forum.73-87chevytrucks.com/smforum/index.php?topic=30546.msg256328#msg256328)

Title: Re: Speedo off 10mph...
Post by: roundhouse on March 15, 2015, 10:32:18 am

Gear ratio ?
Tire size ?


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Title: Re: Speedo off 10mph...
Post by: Captkaos 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