Another Speedometer Question I know.. But couldnt find an solution

[Hunter_Michael]

1987 R10 just purchased for my 16 year old sons 1st vehicle. Ive been trying to get the speedo to show accurate. 700r4 - It has original sized tires (29") original 12bolt rear end 3:08 gears with a Red Drive gear (17 tooth). It also had a 36 tooth driven gear, which with a calculator is correct for everything . Running about 10mph faster on hte speedo then actual speed - was showing 55 on speedo when gps said 45.
Replaced the speedometer cable thinking it was the issue because it looked bad on one end. No Change.
Change the Driven gear to a 34 going the wrong way by accident and it changed to about 12-15 over.
Then put a 39 Tooth in and it still showed 8-10 over... .
What woud be next to check or replace? The drive and driven gear look fine, the cable is good and is inserted at both ends fine and turns. Kind of stumped at this point.

Vehicle Speed Sensor Possibly cause issues like this? The cruisecontrol doesnt work either?

Thanks for any help!

 

[Keith Seymore]

The speedo is purely mechanical, driven by the cable.

I'd try an external adapter next. Figure out what percentage you need and stick it on there.

Speedometer Gears - Ratio Adapters - Cables - Housings

Speedometer Gears - Ratio Adapters - Cables - Housings and more. Browse our Shop for all your Speedometer needs! Need assistance? Call Jim at 1-888-877-1008

transmissioncenter.net

Mine came with a Z shaped adapter on it from the factory.

K

 

[Keith Seymore]

They have a calculator, too, for number of teeth, rear axle ratio and (actual) tire rolling radius:

Speedometer Gear Calculator

Speedometer Gear Calculator. Fill in the blanks to find your needed gear! Need assistance? Call Jim at 1-888-877-1008! Available Mon-Fri, 8am-5pm Central

transmissioncenter.net

 

[Hunter_Michael]

I used that same calculator which said to use the 36 tooth that showed my 10mph difference. I decided to up my tooth to 39 and it didnt change anything speedo wise still 10 mph difference. Its wierd.. I ordered a new speedometer and see if thats the issue.

 

[squaredeal91]

Is your speedometer noisy or jumpy?
Might just need lubed

 

[Hunter_Michael]

No noise or jumping. It did jump prior to me replacing the speedo cable. Which I figured was the issue with the speed difference at the beginning of diagnosing the issue... However, I broke the red dial removing it by accident, so decided to get a new one. Ill post results after the new one gets here and is installed. Seems like alot of threads on speedometer issues never post the fix.

 

[Chepe]

Another option (for anybody else with speedo problems) is Speedometer Service in Colorado. My speedo pointed down when not moving and read wrong no matter what gears I put in. Rusty sent it back working perfectly.

 

[Hunter_Michael]

Just installed the new speedometer. Seems to be a smoother function then the original one, the 36 tooth gear was still 10mph faster on the speedo. Swapped the 38 tooth in and it dropped to 7-8 mph faster, then put in the 39 tooth and was 5 mph faster.

Iits working its way down with each gear change. So, I ordered the 40+ housing and now will try the 40 and or the 41 gear and see if that will get it on the spot. So far I've spent about $80, might have been easier to get the Z adapter but been fun working on the truck with the kid. If he ever decides to swap out the rims and tire size, we got it covered. lol...

Ill post an update once my parts arrive and I get a chance to install.

 

[Hunter_Michael]

Just installed the 40+ gear housing and ended with the 42 tooth gear to adjust the speedo being off by the 10mph. Its now about 1mph faster then what a gps says. Ive determined for my application, that each gear increase corrected the speed by 1.5 mph. So with the 36 tooth it was 10 mph fast, moving up to the 42, 6 tooth size higher, it corrected 9mph. I guess I could put a 43 tooth in it and bring it to just over by .5mph but Id have to order another one, So Im good with having my 16 year old sons speedo being 1mph slower then what it says. Not sure if the previous owner changed the rear gears to something other then the stock 3.08. Because the calculator for my tire size, rear gear, and drive gear should be a 36 tooth.