Speedo reading off
 

At 35mph on the gps the speedo read 30, at 75mph on gps the speedo says 65mph. Its 15% off. I havent changed any parts and its never been off before. It doesnt bounce and the cruise works fine. Im a bit perplexed as to why. Thoughts?

Usually this is caused by oversized tires.

Same 215 85 16s its had for 1.168 million miles. Im just confused..

Is it possible for the GPS to be skewed?...I ask because I often get delivery drivers on my street who can't figure out how their GPS took them somewhere completely off their route...Can you verify it another way?...For instance a cop buddy with radar?...:confused:...Ben

Or you could take it out on the interstate or a highway with mile markers. Drive 5 miles and see if the odometer agrees. If you can have somebody time how long it takes to go 1 mile at 60 mph indicated.

When I got my truck the speedometer ran backwards. It took me a while to realize the reverse/ratio adapter was missing. I found a new one online and "miracle" it agreed with the GPS.

After you check the speedo independently of the GPS you can look into whether the gear is missing/wrong at the trans pickup. I really doubt the GPS is off though.

Edwin

The gps matched the gps on my phone. 2 gps's agree. Also at 2250 rpm=75 mph.

How does the speed sensor work? Does it have zource voltage OR dors it develope its own?

2WD or 4WD?
https://cimg5.ibsrv.net/gimg/www.die...3a842599df.jpg

Its 4wd. Im more wondering how it generates a signal to the tach. Is it pulse driven or a vaiable voltage=mph?

It is pulse driven.

I remember hearing about failures where there appeared to be an error that was a constant ratio, such as yours, surprisingly due to a stripped drive tang or socket slipping at a somewhat constant rate. I would check obvious mechanical failure first, but I would then check to see if the odometer is off the same percentage as the speedometer. They are driven by separate IC outputs, and I believe it is possible for the odometer step motor to work properly, while the amplifier that puts out the voltage signal for the speedometer is failing . . .

So strange it went from bouncing to steady, but now 15% off. Eventually Ill get that speedo figured out. For now I have more mystery leaks. No cowl cracks, fixed floor vent, new cab light seals, bed light seal, new door seals, new steering shaft to firewall seal. Next is windshield gasket.

This is why I pull the carpeting / jute padding and just run some removable rubber floor mats on some of my 1st gens.
If the floor gets wet, pull the mats and let it dry out, rather than have wet smelly carpet rust out your floor boards. It is also nice if you want to wash out the rig.
Maybe it is a tiny bit louder, but trying to quiet down an old 1st gen Cummins is like telling the college kids to quite down their graduation keg party. You just get stressed out with little results.

Have you checked the vent window seals?

The vent seals seem decent unless you powerwash it. Water just seems to magically appear where the door, floorpan and kick panel meet, its just that corner. Its not coming from kickpan seal. Theres no vertical water drops etc. Wonder if water could be coming through door area and getting past the seal? Or windshield gasket.

While it seems like it's raining every other day in KS you could try sitting in the truck with a flashlight to see if you can pinpoint the leak. You guys been getting hammered lately! Watch out for hard water!

Edwin

Don't forget that there is a factory set bias on the speedos that read 10-15% over, it is a liability thing. It isn't constant through the speed range, it widens the faster you go, pretty close at slow speeds and pretty far off at speed...Mark I just did some googling and the bias is generally 4% and it varies by manufacturer...

This looks like a potential leaky spot..
https://cimg6.ibsrv.net/gimg/www.die...1c7539a501.jpg