KRB

Ah, forgot about those things.
Thanks,

KRB

Got a new VSS from NAPA. That fixed the wild, erratic needle action. Nice and steady now but still seems to read high although I haven't had a chance to check it. Maybe the gears are a little worn?

oliver foster

I rarely see a 1st gen that the speedo is dead nuts on.
Adding a size in the tires will make it closer to correct, if it is reading high right now..

KRB

It's just weird that it was like within 5mph most of it's life and now its at least 10mph high. But, at my age I pretty much drive by habit now and it's mostly at the speed limit intuitively (well, usually a little under - I'm slow...).

I do need tires before winter though. Been thinking about some tall skinnies. I like the old school pizza cutter look, and I find they work well in our terrain here. I have 7.50x16 Deestones on my '77 and love them. Of course it spends 95% of it's time off-pavement too though.

Rug_Trucker

I just do math. My '97 Lesabre was originally R75 205 15's. Swapped some Park Avenue Ultra wheels on it. 17" R55 235's. It slows the speedo down 2mph pretty much across the board when checked with the Waze gps app on my phone.

edwinsmith

The old mechanical speedo in my 89 reads within 2 mph at speed according to my GPS. Maybe the older stuff wasn't as bad as we think.

u2slow

One modern pizza cutter is the 255/85R16 at 33" tall. That should help both your ground clearance and speedo a bit.

oliver foster

X2
Great tall and skinny E rated 16 inch tire option for the stock ride height W250 guys.

KRB

Thanks, I've recorded that size in my notebook.

oliver foster

2 MPH...at what speed?
I have several trucks that read off, some 10% high, some 10% low, give or take.
I swapped 37 inch tires on Project Covid that came with 4.10 gears. I left the 4.10 gears in the axle but used a 3.54 speedo gear in the T-case. I did this to get the speedo closer. It now reads about 10% high. Meaning when it reads 75MPH I am closer to 68. 55 I am closer to 50.
I am guessing a set of 40 inch tires would get the speedo almost dead nuts.

On Poncho with 35 inch tires and 3.5 gears in the axles and for the speedo gear in the T-case. I read low. Say, I 'll be doing 50 when the speedo reads 46/47.

edwinsmith

When I got my truck the speedo didn't work at all. in fact the odometer ran backwards. After a bit of research I found that the reverse ratio gear was missing. Luckily I found a company that was selling them and I bought it and installed it. I went driving with my GPS and at 60 mph the speedo reads 59-60 depending on where you catch the jiggle. Since the digital GPS is always off +/- 1 digit I figured my max difference could be 2 mph. I checked the odometer and it shows 1 mile going from one mile marker to the next. When I bought new tires I got the same size and the speedo didn't change.

Edwin

KRB

Well, I reckon I celebrated too soon. On my next drive it was back to wildly erratic. I was going 90mph down my gravel drive yesterday morning...

Guess I need to see about rebuilding the gears in the ratio adaptor. FIRST I'll trace back the VSS wires to make sure they are compromised.

Oh well, always something.

KRB

I think I've discovered what's causing my erratic speedo and it's not mechanical.

A couple weeks back I decided to take a few minutes and do something for myself. Dad gave me a replacement stereo for Christmas like about 5 years ago. Haven't had a radio for longer than that. Couldn't find a working OEM replacement or even a new one - which I wanted. SO I settled for the most basically simplest one from Crutchfield. I finally installed it and I tolerate the look and functions but enjoy the radio. Anyhow, after unhooking the big flat ground strap off the old one and attaching to the new stereo I realized I not only had sounds from the speakers but also a more normal speedo. Not fixed but behaving more normal.

Yesterday, after putting a new deck and other odds & ends on my trailer I hooked to it for a 60 mile round trip to the stockyard and back with a load of calves. The speedo was back to normal (5mph high and consistant).

I suspect it's a bad ground now. Between messing with that big stereo ground strap and the trailer lights being hooked up something changed. I'll trace the wiring from the VSS to see what I find. Years (maybe decades now?) ago I added some ground straps to the engine-frame-body up in the engine bay. Guess I should clean them all up and see what else is needed.

I'll try and drive it again soon sans trailer and see what happens.

oliver foster

You are onto something.
Checking and cleaning up all grounds you can find is the next step.

KRB

I spent some time yesterday taking apart and cleaning all the ground connections that I could find, including the extras I have added. Didn't look bad but shined them up & added dielectric grease. Drove down the road about two miles and back to feed the cattle and the speedometer functioned perfectly. Not even the slight flutter when it did work. I'll drive it to town tomorrow and put some more miles on it to see how it behaves. My lights were starting to pulse again at low idle, we'll see if that changed any too.