1995 speedometer reading fast

[Darrell]

My '95 C1500 (stock) speedometer is reading 12 mph faster than my actual speed. This has been verified with the GPS and also against 2 other vehicles which have the correct reading. So I know it's this truck only.

I never really paid much attention to it until as of lately. So I can't really say how long it's been like this. There's is no jumping around of the speedo needle at all, a smooth movement always from a dead start to what ever "X" mph I'm driving at it's always 12 mph's off.

I've
1) Rebuilt the orig 5.7 eng (stock with mild cam 2 yrs ago)
2) Installed a new 4L60E trans as well (same time as eng).
3) Tires (stock 275/60R15 which came stock on the SS models)

And just last week I installed a new speedometer sensor which bolts onto the tail shaft of trans with no luck. The sensor seemed to be straight forward and a plug and play type sensor. It was running 12 mph faster before and still after the new sensor.

Am I missing something else or is there some calibration I need to do and if so how/what?

Thanks

 

[littlemlittle]

Did you replace the speedo "gear" in the transmission? Or just the sensor? I bet your transmission came out of a car or something like that with a different speedo gear.



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[Darrell]

Did you replace the speedo "gear" in the transmission? Or just the sensor? I bet your transmission came out of a car or something like that with a different speedo gear.



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I just replaced the sensor only. I ordered the new transmission from a vendor and it was shipped to me. I just ordered a standard 4L60E trans and never gave any more thought to it.

I'm not a transmission guy, and it sound like it's easier to live with it than fix...lol

 

[someotherguy]

Your tires are shorter than what came stock on your truck, which likely came with 235/70/15's. Each truck is calibrated for the tire size and axle ratio that were original equipment.

You can fix your speedo by simple modification to your DRAC unit which is under the PCM on your '95. Last year model you can do this mod.

If you can solder and do simple math, you can fix it for the cost of a dip switch and a little time/effort.

Richard

 

[PlayingWithTBI]

Did you replace the speedo "gear" in the transmission? Or just the sensor? I bet your transmission came out of a car or something like that with a different speedo gear.

These trucks didn't come with a "speedo gear", it was a reluctor and sensor which gave you 2000 PPM (pulses per mile) more or less. Like @someotherguy said, you can calibrate it in your DRAC or VSSB.

 

[someotherguy]

Didn't have the link handy, since TBIChips was taken over by a new owner they've redesigned their site. Here's the article on picking the settings for your DRAC; I haven't used their built-in calculator, but it's probably fine...used to just have the simple formula posted for determining the input value. Then you just pick that setting off the chart. Also write down the ones before and after so you have them handy to fine-tune if the one it "should be" isn't close enough.

A DIP switch soldered in place of the jumpers makes the DRAC easily configurable. While there's seven positions for the jumpers, the first position never changes so you only need a six position switch. Using a six also saves you room on the double-bank DRAC where you need to solder in two DIP switches instead of just one - the sevens are a very tight fit crammed next to each other; where a pair of sixes fit fine with room to spare. Example; had to shave the switch body for the sevens to fit, which made them kind of flimsy:

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Six position DIPs in the same arrangement, though in the double bank setup it makes more sense to flip one upside-down like I did in the previous pic, since the second bank is inverse from the first. Don't let that trip your brain up - just look at the settings..

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*In general*, 1992-1993 will have a single bank, 1994-1995 have the dual.

1988-1991 have to go inside the cluster to modify 'em...whole different setup.

Richard

 

[Blue Turd]

I just recalibrated my 95 using the DIP method. I never really noticed it until recently when I was running a GPS app that has speed listed in the corner.

Apparently my truck had 4.10 gears put into it at some point before I bought it, hence the inconsistency of what should have been 3.73 according to my glovebox code.

You can get to the module right underneath the ECU behind the glove box.

http://tbichips.com/?page_id=504

 

[someotherguy]

I just recalibrated my 95 using the DIP method. I never really noticed it until recently when I was running a GPS app that has speed listed in the corner.

Apparently my truck had 4.10 gears put into it at some point before I bought it, hence the inconsistency of what should have been 3.73 according to my glovebox code.

You can get to the module right underneath the ECU behind the glove box.

http://tbichips.com/?page_id=504

Haha after all that crap I typed and dug up the pics of my old DRACs and I forgot to include the article I mentioned. Thanks for filling in the necessary info!

Richard

 

[Darrell]

Thanks, I was a little confused at first on what was being conveyed..lol, but I found the recalibration link on TBIchips

Thanks again guys

 

[Mildphil]

I am having a similar issue, except it starts at around 3mph fast at lower speeds (~10 on the speedo, 6-7 real) and gets up to 6mph fast at higher speeds (70mph on speedo, 64 mph real) I have the stock tire size (245,75R16) and my speed sensors on the hubs match the GPS speed perfectly in torque. The only variance from stock is I have P rated tires instead of LT, but online the overall diameter is the same. Any ideas on what can be causing this?