speedo reads too fast

[bvette]

speedo reads too slow

I recently purchased a 1986 C10 custom deluxe with a 305/700R4 from the original owner. The problem that I am having is the speedo reads 8mph too fast. The previous owner said the rear end was changed at some point, but they couldn't match the original gear ratio, but they got as close as they could. He stated that the speedo has been off ever since. He also stated that the engine and tranny are original to the truck. Is this something that a driven gear change could fix? If so, how many teeth do I go up or down to get at least close to where it should be? I do not know what gear is in there now, but I imagine it is the original gear. I was hoping to get some info on this before I took it apart. Any help that you can offer will be greatly appreciated. Thank You from a new member, bvette.

 

[bucket]

I think you need to know the color of the drive and driven gears, the rear ratio and the tire height to calculate what you need to do to fix it.

Or you can install a ratio box between the trans and speedo cable. But you need to know exactly the percentage your speedo is off. What speed are you 8mph off? Lower speeds will have less error, and higher speeds will be greater.

 

[74 Shortbed]

This makes things easy..

http://www.tciauto.com/tc/speedometer-gear-calculator

 

[bvette]

speedo reads too slow

Thank you for the info. The speedo is off 5mph at 55mph. At 55 I am really going 60mph I have checked at different speeds, and it is 5mph off at all speeds. I realize now that in my first post I stated that my speedo reads too fast when it is actually reading too slow. Sorry for the mixup.

 

[MikeB]

The problem that I am having is the speedo reads 8mph too fast.

5 mph off at ALL speeds is kinda weird. Does it read 5 mph when standing still?

 

[The88]

I am running 35s and my speedo is off a bit. My fix is a garmin suctioned to the windshield. That baby is right on all the time.

 

[Plane Dr.]

Mine is off 5 mph at all speeds also. When it reads 25 i'm at 20, 70 I'm at 65. I just don't worry about it. lol

 

[Camar068]

post below had the same issue.

http://www.gmsquarebody.com/forum/showthread.php?t=17460&highlight=speedo

The following links helped me understand how easy it is to change the driven gear AND that you don't have to change both the drive and driven gear.

https://shiftsst.com/articles/speedo-gears-101/

https://www.youtube.com/watch?v=wAxJ7W7WAWc&t=6s

https://www.youtube.com/watch?v=fMEe5SySuxA&t=179s

 

[vkh]

My old speedo did the same thing. I swapped it for a 100 mph speedo and it got alot closer (my tires are throwing it off a touch). If it's the same amount off at any speed I think it's the speedo, not the ratios