Speedometer Reads Fast

[shanghaipaddles]

Hi!
I just joined and have a new to me 1984 C20 suburban with a 454 and TH400. The speedometer reads 10 mph fast until about 55 mph, then it’s about 20-25 mph fast. The needle also floats around 10-15 mph, but that seems to be more at higher speeds, unless I hit a bump, then it’ll float at lower speeds too. I’m not sure if the odometer is affected. This is rather annoying, does anyone have any advice?
_Kevin

You must be registered for see images attach

 

[DoubleDingo]

Probably a size smaller tire installed. May need some cable lube in the cables, too. Reading low, my bet is tires. Bouncy, needs lube in the cable(s). Cables if there is factory overdrive

 

[shanghaipaddles]

Probably a size smaller tire installed. May need some cable lube in the cables, too. Reading low, my bet is tires. Bouncy, needs lube in the cable(s). Cables if there is factory overdrive

It has new 255/70r16 tires on it. I’m not sure what the stock size is, but wouldn’t that make it consistently off, not worse at high speed? I’ll try lubing the cable and seeing if that helps the bouncing. Thanks

 

[DoubleDingo]

If there's a tag on the door jamb it'll show tire sizes. That'll tell you what GM installed. That'll also give you something to research for tire diameter and compare against. Tire diameter is key if you're expecting the speedometer to match.

 

[shanghaipaddles]

If there's a tag on the door jamb it'll show tire sizes. That'll tell you what GM installed. That'll also give you something to research for tire diameter and compare against. Tire diameter is key if you're expecting the speedometer to match.

It was barely legible, but I did find the sticker with the tire size; 235/85r16. Which according to the tire calculator is 1.6875” smaller. That surprises me, but I’ll buy it throwing off the speedo. I still think it would be linear, not exponential. But maybe the floating needle is skewing my speed enough that I can’t tell.

 

[DoubleDingo]

It was barely legible, but I did find the sticker with the tire size; 235/85r16. Which according to the tire calculator is 1.6875” smaller. That surprises me, but I’ll buy it throwing off the speedo. I still think it would be linear, not exponential. But maybe the floating needle is skewing my speed enough that I can’t tell.

Yeah, 1.7 inch smaller diameter tire makes the drivetrain spin faster, thus, the speedometer reads higher/faster. Bigger diameter tires, opposite, lower/slower.

31.7, will be about spot on

 

[Scott91370]

It has new 255/70r16 tires on it. I’m not sure what the stock size is, but wouldn’t that make it consistently off, not worse at high speed?

It will be off by a percentage so the faster you go the more the gap will be. But 20-25mph is a lot!
I use this tire calc. Put in both sizes and it will tell you the difference at different speeds: www.dakota-truck.net/TIRECALC/tirecalc.html

You must be registered for see images attach

 

[DoubleDingo]

Fun fact: Back when I had Mean Green on the road, I had replaced the rear axle from a 4.57 to a 3.54. It was much more fun to drive, and that rear gear ratio made the truck haul @$$! The speedometer read 65 at 80. I got an rpm calculation formula from a 4x4 magazine and made a cheat sheet of various tire sizes by diameter, speeds, and then memorized the rpms at critical speeds such as 30, 45, 55, 65, basically posted speed limits. When I drove the truck as it wanted to be driven, at 80, that was 3,000 rpms. 2,600 was 70. 2,200, roughly 65. So, if I saw any POPO I just dropped the rpms down accordingly. I tried various tire sizes, 29 inch was the peppiest because of more torque; 30 wasn't bad; 31.7 was the funnest, as it got the rpms in a little later and speeds higher. I was much younger then and drove like a dink, but had a lot of fun doing so.

For your burb, 31.7 is preferred with the th400 and most likely 4.10 gears.

 

[fast 99]

Tire size wouldn't account for that big of difference. Think new tire diameter compared to a worn out one. Possibly rear gears have been changed. Odometer is gear driven by cable input, spedo needle is driven by a magnets. Relube or replace the cable. Remove the spedo driven gear count the teeth, find an appropriate replacement.

 

[Grit dog]

Probably a size smaller tire installed. May need some cable lube in the cables, too. Reading low, my bet is tires. Bouncy, needs lube in the cable(s). Cables if there is factory overdrive

No, those are pretty close to OE diameter. And even at an inch and a half shorter maybe 2-3 mph at highway speed theoretically.
It’s possible someone changed the speedo gear or put a trans in that had the gear changed for big tires. But that doesn’t explain the erratic behavior. It’s the speedo/speedo cable imo, unless like you said someone put taller rear gears in it.

 

[DoubleDingo]

.

 

[shanghaipaddles]

I’m definitely starting to think the floating is skewing the % the speedo is off. On my drive to work today, my first time driving it in 70* weather, it was more stable and closer to 10-13 mph fast. I pulled the cable at work today and it appears to be in good shape, no frays or cuts, so I lubed it with some spray lube we had at work. Now just gotta wait 6 hours to drive home after my shift and see if it helped at all or not.

 

[DoubleDingo]

I’m definitely starting to think the floating is skewing the % the speedo is off. On my drive to work today, my first time driving it in 70* weather, it was more stable and closer to 10-13 mph fast. I pulled the cable at work today and it appears to be in good shape, no frays or cuts, so I lubed it with some spray lube we had at work. Now just gotta wait 6 hours to drive home after my shift and see if it helped at all or not.

The bouncing always makes the readings higher, at least that has been my experience with it.

 

[shanghaipaddles]

Well I’m happy to report that lubing the cable has almost completely fixed the bouncing. It still bounces about 5 mph at highway speed, but much better than 20+ mph. It now reads consistently 8-10 mph high, which I could see just being the tires. Much easier to tell how fast I’m going now. Thanks for the help!

 

[ss454elco]

I have the same issue in my 84 K10, but there has been trans/transfer case swaps. at 25 I'm going 30, at 35 it's 45 and 45 is 60. I'm going to get a new adaptor/corrector to mount on the transfer case, which you could do too. Be aware they can be pricey! The needle jumping I have found can be dry cable or even a frayed cable with a strand sticking out and dragging. At least they are easy to replace.