Speedo Inaccurate

[SUBURBAN5]

I appreciate your help and attentiveness, will give them a look.

No problem. Goodluck

 

[drag sgt]

Before jumping to a very expensive cluster purchase, I'd look at putting in some adjustable DRAC switches into your VSSB (or buying one online if you're uncomfortable with circuit soldering). I just went through this exact issue a couple months ago on my diesel. It was already progressively inaccurate on the OEM 245/75R16 tires, and even more so once I installed 235/85s. Here's my own experiences thread with links to resources: https://www.thetruckstop.us/forum/threads/speedo-trip-accuracy-calibration.51294/

In summary, even with the specific digit combo in the VSSB that the DRAC chart said I should have, I still needed to keep adjusting it bit by bit until the speedometer matched my GPS--which matched the experience of another user.

 

[scott2093]

In summary, even with the specific digit combo in the VSSB that the DRAC chart said I should have, I still needed to keep adjusting it bit by bit until the speedometer matched my GPS--which matched the experience of another user.

Only checking gps to speedo seems like it could throw off all kinds of things. Best, if not pretty important to focus on what the computer is saying the speed is. For all you know, your truck may be thinking it's doing 80 when the "broken" speedo and gps shows 50 for example.....

 

[drag sgt]

Only checking gps to speedo seems like it could throw off all kinds of things. Best, if not pretty important to focus on what the computer is saying the speed is. For all you know, your truck may be thinking it's doing 80 when the "broken" speedo and gps shows 50 for example.....

I'm not an expert by any means in the 6.5 PCM, but I'm pretty sure the VSSB "translates" everything prior to the outputs being sent to the PCM and cluster speedometer. I believe the MPH that shows up in most OBD2 readers is the uncorrected VSS input signal, and isn't used by the engine.

None of this is to say that it also couldn't be a cluster issue but OEM sized A/T tires were enough to throw my truck speedometer off by 7-8mph @ 60, so it's not out of the question.

 

[scott2093]

I'm not an expert by any means in the 6.5 PCM, but I'm pretty sure the VSSB "translates" everything prior to the outputs being sent to the PCM and cluster speedometer. I believe the MPH that shows up in most OBD2 readers is the uncorrected VSS input signal, and isn't used by the engine.

Makes sense sorta. I maybe didn't consider the newer make of the truck.
With my 93 gear and tire change, I chased my tail with the charts (closer to inputting 4.10 gears when I had 3.73 to get speedo closer but never was happy with the results) .... I put jumpers back where they should be with my changes according to charts,decided to put a scanner on and checked the computer speed to gps... the charts lined up pretty much dead on .My speedo was the only thing off. Replaced it and everything played nice together . But that's obd1 obviously. Just curious about all the related stuff that transmission uses .

 

[dannyk11135]

Before jumping to a very expensive cluster purchase, I'd look at putting in some adjustable DRAC switches into your VSSB (or buying one online if you're uncomfortable with circuit soldering). I just went through this exact issue a couple months ago on my diesel. It was already progressively inaccurate on the OEM 245/75R16 tires, and even more so once I installed 235/85s. Here's my own experiences thread with links to resources: https://www.thetruckstop.us/forum/threads/speedo-trip-accuracy-calibration.51294/

In summary, even with the specific digit combo in the VSSB that the DRAC chart said I should have, I still needed to keep adjusting it bit by bit until the speedometer matched my GPS--which matched the experience of another user.

just installed a new drac so I rather not mess with it. Besides, the rest of the gauges are wonky too lol