Speedo Calibration Question

[Alaska Burban]

I just got done creating a one ton burban, yay! got a problem though... I put in 4.88's and am running 35" tires on the street and 38's for the trail. not too worried about the speedo being right with the 38's, but it shows me going about 75 or so when I am actually doing 65 with the 35's. I found info on recalibrating my DRAC, if it had one, but I either cant find it or it doesnt exist as a seperate entity. also it is electric, and cant find gears for it if they even exist, so I cant figure out how to recalibrate it the cheap and easy route. (GM wants $500 to fiddle, and say they prob. cant get it right anyway) any ideas?
1991 GMC Suburban 3/4 ton with a 4L80E, Full Float 14 rear, Dana 60 front, 4.88's and Lockers... thanks in advance!

 

[K5Chris]

look up behind the dash. some are behind the glove box by the pcm, mine was near the steering column.
it should be in a white plastic box like this:

i wrote this by the way: www.tbichips.com/drac

 

[Alaska Burban]

the TBI thing was yours? cool! is what I was using to hunt mine down. looked behind the glove box and found the PCM, didnt find that white box thou. will look near the steering column now. I know there is a black box under the column with a harness coming in both sides, but dont think that is it...

 

[HarryH3]

look up behind the dash. some are behind the glove box by the pcm, mine was near the steering column.
it should be in a white plastic box like this:

i wrote this by the way: www.tbichips.com/drac

Great article! I used it to calibrate the speedo in our '94 Sub and it's now within 0.5 MPH of what my GPS shows for speed.

But shouldn't this "Digital Ration Adapter Controller" be this: "Digital Ratio Adapter Controller"

 

[dyeager535]

Check around your cigarette lighter. I'm fairly certain thats where it was tucked on the '91 1 ton I pulled apart.

 

[bdozeraz]

That was a great write up. I just used it to calibrate the speedo in my motorhome. I would highly recomend putting the switches in rather than just moving the jumpers. I did the switches and I am glad because my calculations were a little off and I had to reset it. I just left the box hanging out, drove a little to check it then pulled over and readjusted it. It is only .5mph off now.

Brent

 

[Alaska Burban]

Found it! it was located directly below my stereo deck unit clipped to the sheet metal. throwing on the dip switches now, thanks for the write up!

 

[WKC]

I've also been doing some research on the DRAC recalibration, and that was a great article. Here is a calculator I found that you guys might have seen too where you can simply input your Tire Circumference, Gear Ratio, and Pulses in the Speed Sensor and it will tell you your Input Ratio and what pins to connect or disconnect.

DRAC Calculator