Howdy. I’ve got a 1990 2wd Cummins with 3 speed auto. I don’t drive it’s often but have noticed the speedo recently seems to underestimate my speed. I downloaded a speed app for my iPhone and confirmed it. It’s reading slow. What should I check? I keep it tip top so want to fix it. Thanks

 

I’ll just try the vss…you know the thread 10 below mine.

 

There are different gears for the VSS for the different gear ratios ordered on these trucks.
Different sized tires will also throw off the speedo.

How off is it at a given speed? 5 MPH off at 50.....more? less?

 

Thanks Oliver. It’s significantly off. Like 20 mph at 50. Reads low

 

And tires are very close to oem. 2wd

 

I’ve got those speed sensors located on rockauto. I’ll give one a shot

 

Does this change coincide with other work being done? Many dodges had an inline ratio adapter with the vss.

https://www.texasindustrialelectric....er_Adapter.asp

 

When I got my truck the speedometer didn't work. It is an 89 with the old cable style. After some investigation I discovered that a reverse reducing gear was missing. Luckily I was able to find one and it fixed the issue. The odometer was running backwards!

Edwin

 

Thanks u2 and all. I have to get under there and look at it. I’ll report back

 

Ok, got down here and crawled under and replaced vss sensor with no fix. Still reading about half speed. Any further thoughts?

 

this is what I replaced

 

That's the part that generates the signal.

There should be a ratio adapter ahead of it.

 

what would that look like? I will check and see if it ever had one

 

This sort of thing
https://recordtechnologies.com/products/ratio-adapter

 

My 91.5 D250 didn't have one of those either and the speedo worked fine