Speedo Off/High RPM/ABS Light On

Good evening folks,
Decently long time reader, first time poster here as I've been looking for a GMT-400 for years, and finally the stars aligned and was able to buy the truck I've wanted since I was a kid.

Some background, I drive pretty much exclusively 90's trucks, have quite a few but this is my first GM 90's truck (previously had a great '09 Silverado when I was younger), and am good with a wrench. Have done almost all my own wrench turning and builds since I was younger, and am relatively decent at it.

Here's the rundown of what has me puzzled.

Truck Information:
-1996 Silverado Extended Cab. 1 "true" previous owner.
-140,000 original miles
-Everything appears to be pretty much stock, well maintained and in relatively good order. It's a nice, tight truck. Thrilled with my purchase.
-Tires are stock 16" rims on the recommended passenger tires

What's going on:
-My speedometer is drastically off - exactly 25mph based on using Speedbox GPS along side. It's 25 MPH fast
-It feels like either I'm not getting all the way to Overdrive, or if I am getting there, it's running what feels to be pretty high RPMs. At 65 MPH I'm turning 2500 RPM which seems steep? I tried to count the shifts on the highway, and counted through 5 but it just didn't feel like it grabbed OD?
-ABS light is on. I only mention this because I have no idea if there's any possibility this is tied in and might spark some thought for someone? I don't think they're correlated but didn't want to leave it out.
- I've made it about 100 miles on 1/4 of a tank, about 50/50 highway and some stop and go. Doesn't seem like this is too terribly off.

My initial thought was did the PO stick bigger tires on at a point and re-gear but that re-gear still seems like it'd be way off for this kind of speed difference. Also logically, my brain doesn't THINK that the speedo being off would correlate to the transmission's shift points/OD running high, but again it's been kicking around in my mind and I wanted to say it out loud for piece of mind here.

Does anyone have any thoughts here?

All in all the truck drives wonderfully and very comfortable. Just don't want it working too hard, and 2500 RPM at 65MPH seemed...odd. And the speedo being +25...when I changed tires in my old Jeep I changed out the mechanical gear, in my Bronco I was able to calibrate the speedo on the dash. No idea what's going on here.

And I know I have to pay tax, so here's the photo tax. The Silverado parked at home next to my '95 Bronco. I'll take better photos this weekend and do a proper intro.

SlowcountrySilverado said:

Good evening folks,


Does anyone have any thoughts here?

Click to expand...

Has it behaved like this since you bought it? What did prev owner say, if you asked him about it?

plug in a scan tool capable of rendering live data including speedo and tach info to confirm/deny your dash readings as well as your transmission's auto forward shift pattern (1-2-3-4) is happening when going down the road. Im wondering if you dont have 3-4 upshift... Use the app WAZE to independently check vehicle speed

For comparison, Im doing ~2750 rpms at ~70mph in 3rd gear, 2200 or so in 4th ('03 tahoe, 3.73 gears, stock dia tires, 4L60E)

NickTransmissions said:

Has it behaved like this since you bought it? What did prev owner say, if you asked him about it?

plug in a scan tool capable of rendering live data including speedo and tach info to confirm/deny your dash readings as well as your transmission's auto forward shift pattern (1-2-3-4) is happening when going down the road. Im wondering if you dont have 3-4 upshift... Use the app WAZE to independently check vehicle speed

For comparison, Im doing ~2750 rpms at ~70mph in 3rd gear, 2200 or so in 4th ('03 tahoe, 3.73 gears, stock dia tires, 4L60E)

Click to expand...

Hey Nick, thanks for popping in quickly.

NickTransmissions said:

Has it behaved like this since you bought it? What did prev owner say, if you asked him about it? - Yes, I've only had it for 3 days and immediately noticed it.

plug in a scan tool capable of rendering live data including speedo and tach info to confirm/deny your dash readings as well as your transmission's auto forward shift pattern (1-2-3-4) is happening when going down the road. Im wondering if you dont have 3-4 upshift... Use the app WAZE to independently check vehicle speed - Just dug out my old scan tool and will hook it up this afternoon and get out on the highway and see what it says. Using Waze/Speedbox, both confirm I'm showing exactly 25mph FAST on the speedo. i.e. showing 85mph when doing 60.

For comparison, Im doing ~2750 rpms at ~70mph in 3rd gear, 2200 or so in 4th ('03 tahoe, 3.73 gears, stock dia tires, 4L60E) - Yeah, this seems more realistic. I'm getting through 1-2-3-4 shifting at aroudn 2400-2500, then once I clear into what should be OD, it never finds it, or OD is just really tall.

Click to expand...

Okay, some new discoveries tonight and definitely want yalls take on it. Might should move this but here's what I learned.

Plugged in my live data OBD tool, and looks like the Tach was off.

-Live Data was reading 1900ish RPM, in 5th (OD? this is where I was able to count to), at 65-70mph cruising, so that feels about right.
- The data read 1500 RPM at 50 MPH with the speedo reading 75 mph
-1974 at 70 mph with the speedo reading 95 mph and the tach showing 2600
-At idle, showing 600ish RPM on the reader, and over 1k on the dash.

At least this confirmed that I was shifting right and getting into the right gears. Do the numbers from my live data seem to be pretty accurate with where they should on this truck?

So I thought maybe the needles were off. I pulled the cluster apart, and tried to adjust both the tach and speedo needles. Took awhile of playing with it, but finally got the tach to line up with my reader at idle. Roughly 600rpm, and tried to get he speedo lined up too, then test drove.

-Speedo was still off. But it's "proportionally" off if that makes sense. at 70MPH I'm now showing 90MPH on the dash, at 60MPG it's like 75 on the dash, and at 50MPH it's showing 40ish on the dash. Thought this was odd. At 28mph, i'm showing 37 on the dash.

-Tach is a similar situation

At idle, tach on the dash matches RPM speed on my reader.
At 960 RPM on my reader, I show 1,100 on the dash
At cruising speed, I show 1980ish on the reader, and 2100ish on the dash.

So something changed, the gap closed a little bit, but sometihng is still wrong. Seen anything like this? Do i just have a fried cluster?

Thanks - hoping to help resolve this weird one for anyone in the future!

JJ

SlowcountrySilverado said:

Okay, some new discoveries tonight and definitely want yalls take on it. Might should move this but here's what I learned.

Plugged in my live data OBD tool, and looks like the Tach was off.

-Live Data was reading 1900ish RPM, in 5th (OD? this is where I was able to count to), at 65-70mph

Click to expand...

- What is your axle ratio?

- What transmission do you have installed in the truck?

- What is your tire diameter (stock is 27 or 28 inches, I believe)?

No way for anyone to tell if those numbers are what they should be unless we know answers to the above.

Hey Nick, not sure what the axle ratio is but don't have any reason to believe it's anything other than stock.

Has the stock transmission that came with the truck, that's original.

Tires are 28".

The numbers that the reader are spitting out feels like things are about where they should be and it does drive pretty well. Under 2,000 rpm in overdrive.

Starting to believe the issue is elsewhere? Any thoughts?

SlowcountrySilverado said:

Hey Nick, not sure what the axle ratio is but don't have any reason to believe it's anything other than stock.

Has the stock transmission that came with the truck, that's original.

Tires are 28".

The numbers that the reader are spitting out feels like things are about where they should be and it does drive pretty well. Under 2,000 rpm in overdrive.

Starting to believe the issue is elsewhere? Any thoughts?

Click to expand...

What transmission?

"Stock transmission" doesnt tell us anything. You either have a 4 speed auto (4L60E or 4L80E) or perhpas a 5-speed manual (in your prev post you mentioned being in 5th gear/OD so that indicates a stick shift)

RPO codes in either the glove box or door panel can tell you what engine, transmission and axle/differential the truck has. Alternatively, you can get the truck on jack stands, put the trans in neutral and make a paint mark on the rear tire. Then count the number of times you can spin the driveshaft for every one complete revolution of that wheel.

My thoughts are that I need to know the above; perhaps others may not and can give you better guidance from experience based on what you provided.

Regardless, the problem (if one actually exists) does not seem to be related to the transmission itself (that's a good thing!).

Nice looking trucks, btw.