Speedo & Trip Accuracy/Calibration

[drag sgt]

I've got 235/85s on my Suburban with 3.73s and need some opinion as to whether this is within normal error range for these trucks. I just did a 155 mile trip after fill-up and the trip meter is at 149.5 so roughly 3.5% under. Unfortunately, I forgot to take a pic of my regular odometer to see if was the same.

Second, my speed from various tools is a bit all over the place. At 70 on the speedometer, Google Maps registers 65/66 and Torque Pro through the OBD2 port registers just 63. I assume Pro is getting directly from the ECU and the speedo is pulling from the VSSB? If I re calibrate the VSSB should I expect the direct OBD2 readings to adjust as well, or will it just bring the gauge more in line with my actual speed?

 

[dbrannon79]

Adjusting the VSSB will also show on the ECM. I had to adjust mine when I changed from 4.10 gears to 3.73's There is a writeup somewhere here on how to adjust it. when you pull it out the pins for the adjustments are soldered in place, but you can order some dip switches to solder in place and make it fully adjustable.

once I got all the dip switches in on mine, I left it hanging out and had to drive down the road with my GPS on my phone at various speeds to make sure I had it set correctly. i will look for the writeup and see if I can post a link to it.

 

[dbrannon79]

Here is the link.

How to Recalibrate/adjust your speedometer

This thread will provide detailed instructions for those with a 1992-1998 Gm pickup truck, with either 2wd or 4wd who wish to recalibrate there VSSB(vehicle speed sensor Buffer). This should be done when gearing ratio or tire sizes are changed to keep both your speedometer and Odometer correct...

www.thetruckstop.us

Calibrate TBI Speedometer VSSB DRAC | TBI

Calibrate TBI Speedometer VSSB DRAC

harristuning.com

https://web.archive.org/web/20120217134050/http://www.tbichips.com/drac/

Here are the switches I used to do mine.

https://www.amazon.com/gp/product/B07JLHCXKF/

 

[drag sgt]

once I got all the dip switches in on mine, I left it hanging out and had to drive down the road with my GPS on my phone at various speeds to make sure I had it set correctly. i will look for the writeup and see if I can post a link to it.

Got my dip switches soldered in last night and this is exactly what I did this afternoon; take it out on a country road and test the speedometer at various speeds. It's within 1 mph of the Google GPS now and I could probably fiddle a little more but it's accurate enough for me and I like the tiny 1MPH buffer to prevent me from speeding.

What actually baffles me though is I had go with a higher gear ratio to get the correct dip number combo. When I did 3.73 gears and the taller tire size, the accuracy actually got worse (GPS said 63 when speedo was at 70), so I started working the other way; trying 3.83, 3.95, and eventually landing at 4.10. My RPO code is GT4, and the engine RPMs in Torque Pro and on the physical gauge both matched so I know it doesn't have 4.10s. It was only the speedometer in Torque Pro which remained consistently low. This makes me wonder if maybe when the previous owner ordered the E-Torq tune that they said they had 4.10 gears (not that I know if it even matters). The math seems to add up though with the speed reported in Torque Pro of 55mph at around 1800 RPMs matching the math of a 4.10 gear ratio whereas a 3.73 would be 60mph.

 

[dbrannon79]

I basically had the same issue. mine had 4.10 gears and I had 3.73's installed. I first set the switches to match for 3.73 and it wasn't right. I had to keep trying different settings before I got it right. it's not perfect but close, I can't remember what setting I left it at as long as it works I'm happy with it.

Glad the info help you get yours working where you wanted it.