Speedometer higher than actual.

[mooreATL]

I drove a new to me truck back from 4 states away yesterday, and noticed the mph listed were different than my fathers who was driving behind me, as well as my GPS stated mph.

Its a 95 3500HD sierra. The wheels are 19.5", and when getting air in the dually tires, I noticed it said "low profile" on it. Ot doesn't llok terribly different in size than the front wheels, excepthe typr of tread

The tires are 225/70r19.5 on the front and back, but only the bak say low profile (they don't look like low profile that are on cars).

Could this make my speedometer say 80mph while im actually driving 71-72mph?

I have won't be able to take it to. Mechanic until next week for a complete look over, so I am trying to diagnose upcoming costs as best I can until then.

 

[mooreATL]

Bump, have looked into this more, and I can't figure out what I need to update in the computer to match the existing tires that I have, and what I need to update it with. Only that I need to do it! A link from another post is broken that may of given me more information .Local mechanics have no clue either, or won't even look at a 1995, they say go to the dealer, the dealer says they wont look at trucks that old and try to sell me a new truck

Tire size 225/70r19.5

1995 Gmc sierra 3500hd manual 7.4l 454, dually.

Any information or direction on how I can fix would be appreciated!

 

[someotherguy]

The 225/70/19.5 tires are what came on that truck and the speedometer should already be calibrated for that size. The "low profile" markings on the tires are just markings.. the size is what matters. They're "low profile" compared to big rig tires, which BTW some of those big trucks share that tire size depending on the configuration (car hauler trailers, lowboys, stuff like that.)

Your 1995 is the last year that you can semi-easily change the speedometer calibration without having to do any reprogramming in the PCM. (on the GMT400 trucks the 1992-1995 years are easily DIY-modifiable for the speedo calibration.) There's a small module under your glove compartment.. on a 1995 it's actually clipped underneath the plastic tray the PCM sits on, so it's a little tricky fishing it out of there, but not difficult.

We call this module the DRAC. There are jumpers soldered in at the factory depending on rear axle ratio and tire size. You can look up the tables/do a little math and change the positions the jumpers are in, or you can remove them and install a DIP switch, which is my preference because then it's cake to fine-tune it by trying a couple different settings and compare to GPS.

Now, I'll repeat that your tire size is the factory size and your speedo *should* already be correct. It's possible your speed sensor in the transmission may be faulty, but if you're seeing normal speedo behavior - a smooth increase/decrease matching your driving speed, no jumps or spikes, it's probably not the sensor.

More info on the DRAC module: https://harristuning.com/Tbi/vssb-drac-calibration/

Dude with the site advises to just move the jumpers. Take it from me, go ahead and put DIP switches in there. You might need to try a couple different settings to get the speedometer spot-on. It's easy (if you can solder) and cheap.

You must be registered for see images attach

However, it's easier to use 6 position DIP switches as the position #1 doesn't change across any of the settings. No need to shave the switch bodies when you use 6 position. Dunno why the forum software insists on rotating this picture for me tho..

You must be registered for see images attach

Richard

 

[mooreATL]

The 225/70/19.5 tires are what came on that truck and the speedometer should already be calibrated for that size. The "low profile" markings on the tires are just markings.. the size is what matters. They're "low profile" compared to big rig tires, which BTW some of those big trucks share that tire size depending on the configuration (car hauler trailers, lowboys, stuff like that.)

Your 1995 is the last year that you can semi-easily change the speedometer calibration without having to do any reprogramming in the PCM. (on the GMT400 trucks the 1992-1995 years are easily DIY-modifiable for the speedo calibration.) There's a small module under your glove compartment.. on a 1995 it's actually clipped underneath the plastic tray the PCM sits on, so it's a little tricky fishing it out of there, but not difficult.

We call this module the DRAC. There are jumpers soldered in at the factory depending on rear axle ratio and tire size. You can look up the tables/do a little math and change the positions the jumpers are in, or you can remove them and install a DIP switch, which is my preference because then it's cake to fine-tune it by trying a couple different settings and compare to GPS.

Now, I'll repeat that your tire size is the factory size and your speedo *should* already be correct. It's possible your speed sensor in the transmission may be faulty, but if you're seeing normal speedo behavior - a smooth increase/decrease matching your driving speed, no jumps or spikes, it's probably not the sensor.

More info on the DRAC module: https://harristuning.com/Tbi/vssb-drac-calibration/

Dude with the site advises to just move the jumpers. Take it from me, go ahead and put DIP switches in there. You might need to try a couple different settings to get the speedometer spot-on. It's easy (if you can solder) and cheap.

You must be registered for see images attach

However, it's easier to use 6 position DIP switches as the position #1 doesn't change across any of the settings. No need to shave the switch bodies when you use 6 position. Dunno why the forum software insists on rotating this picture for me tho..

You must be registered for see images attach

Richa

 

[mooreATL]

Edited to remove question already answered

Will any soldered 6 pin work? The only part of this I presently have any experience with is the actual soldering lol.

Presently the speedometer doesn't jump around at all, just a flat % higher than actual speed vs gps (hence it being father from accurate the faster I go).

Lastly I am having difficulty locating the speed sensor for my nv4500, I cant find it in my manual or in YT videos, what resource should I use to locate it?