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Re-Calibrating VSSB due to tire upsize

Matt Bachand

Matt Bachand

Depends on the 6.5
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Worcester, MA
I'm using this link I found here at DTR

http://www.tbichips.com/drac/

Using a tire calculator website, my 235/85 r 16's are roughly 655.7 Revolutions Per mile.

I devide that into 63,360 (inches per mile) and I get

96.6295556

I am then confused as what to do to get my final ratio.

I have 4.10 gears.

Also, do i need to buy a dip switch or can I just solder a coupld pins across the board like in the picture?

Thanks as always.
 
My problem is the equation they give is not in a clear mathematical standards that make it easy to decipher. Perhaps its me.
 
(63360)(Gear Ratio)(40)
____________________

(Tire Circumference in inch)(128,000)




(63360)(4.10)(40)
_______________

(96.6295556)(128,000)


If i'm reading that correctly that comes out to .8961233
 
0.884277 1 0 0 1 0 1 1
0.894043 1 0 0 1 0 1 0
0.904297 1 0 0 1 0 0 1


The middle one is the closest, so I need to remove previous connectors, and connect jumpers 1, 4, 6

That should do it me thinks.
 
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I think i lucked out, 1 and 4 are already closed, that means i just need to jump out 6 on one side and remove the 6 jump from the other side!!

This will make things easy!!
 
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The dip switch is needed to make life easier so you don't have to solder and unsolder every time you change tires.

I was going to do this but when I check my speedo compare to the GPS, it is right on. I used 265/75R16 in the Burb compare to 245/75R16. My guess is the difference is insignificant within 1 mph.
 
I would make your circumference 100" which takes into account average diameter for tread wear. Thats with a fully inflated tire with no deflection, just advertised tire height. So with it on truck and the load on your truck, it might make them more like 97" effective circumference with the deflection of the tire with the load.

That is 235mm wide divided by 24.5 to make inches, multiplied by .85 (which is the ratio of the wall height) multiplied by 2 (for above and below wheel) then add 16" for the wheel and thats the diameter or tire heaight. Multiply by Pi to get circumference. So if you have a spec that says what your tire height/diameter is then just multiply by Pi to get circumference.

Im thinking your tire height is 32.3" tall. Or just go measure it on the truck.

You could average it for tread wear. Tread is probably 3/4" thick and youd change it at like 1/4" so subtract 1/2" from diameter as average diameter over time.
 
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JMJNet said:
The dip switch is needed to make life easier so you don't have to solder and unsolder every time you change tires.

I was going to do this but when I check my speedo compare to the GPS, it is right on. I used 265/75R16 in the Burb compare to 245/75R16. My guess is the difference is insignificant within 1 mph.
Click to expand...

With that difference in tire height (1.2") it would only be a 2MPH difference at 65MPH from original tire height. And with tires loaded down and with tread wear it probably does make it closer to reality.

However when I went from 225/75R15 (28.8") to 245/70R17 (31") I had a 5MPH difference at 65MPH with a 2.2" difference.

And my old tires were actually only like 27.5" tall after tread wear and age.
 
I guess that explains it. It is still within the range of the tread and treadwear. One less thing to do.

Actually when I bought the burb, it had 235/85R16 which has insignificant difference to the 265 tires. I am not sure if the calibration was done on this or the stock 245 tires.
 
switching to 265s put my speedo right on but my odometer is off now. I haven't decided which is more important.
 
Matt Bachand said:
0.884277 1 0 0 1 0 1 1
0.894043 1 0 0 1 0 1 0
0.904297 1 0 0 1 0 0 1


The middle one is the closest, so I need to remove previous connectors, and connect jumpers 1, 4, 7

That should do it me thinks.
Click to expand...
Yup, that'll do it. When I mounted my 235/85's at Christmas, I reset my DRAC according to those instructions. Like you, I found that my 'answer' was sorta between two choices. I picked the nearest one and set up the jumpers per instructions. Result: My speedo/odo are less than 1 mile off. Not spot-on, but close enuff for me! :smile5:

When I regear to 3.73's next month, I'll go in and do it again. This time, I'll use the dip switch idea. I purchased two last Thanksgiving, in anticipation of performing the adjustment procedure. When I pulled the little white box out from behind the dash, I found that I had the earlier single-bank DRAC instead of the later dual-bank VSSB. Weird. :confused:
 
I gues the dip switch is better because if you're off, you can easily try the 'next closest' to see if its better. Since I just have to move one pin from one bank to the other, i'm just going to try that instead of having to solder in 2 entire dip banks.

Maybe I'll round up to the closest instead of round down. SHould give me a little better fuel mileage, lol.
 
Well, I layed a paperclip (gold one??) across and soldered it on, not easy for me at all. Would have been easier had I bent it to come through on the back side I think ?? and solder from back not from top.

MPH at 50 read on truck was 53 on GPS. Not much different from before....

Went to radioshack to buy a 7dip, and they only had 8's. Not enough room on the sides to use 2 8's, and they didn't have 6's, so no luck.

I measured my back tires, and they are 30 3/16's tall measured with a level flat over both rears (dually) ground up. The 2 outer tires I have are almost brand new tread wise, but old, with a tad of dryrot in between a few of the treads.

Not sure what to do now. Thinking about just dealing with it, but I would rather my actual speed be less than speedo speed not more, for speeding ticket reasons. (not that I ever drive fast in this pig but still...)
 
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Matt Bachand said:
Well, I layed a paperclip (gold one??) across and soldered it on, not easy for me at all. Would have been easier had I bent it to come through on the back side I think ?? and solder from back not from top.
Click to expand...
Soldering from behind is definitely easier to do. Also makes a cleaner joint, with less chance of shorting across an adjacent contact.

Went to radio shack to buy a 7dip, and they only had 8's. Not enough room on the sides to use 2 8's, and they didn't have 6's, so no luck.Soldering from behind is definitely easier to do.
Click to expand...
I bought my 7 pin dippies from an electrical supply store. You can also find them online. I was in a hurry and didn't feel like waiting, so I started calling locally. Found two sources - both commercial supply places.

I measured my back tires, and they are 30 3/16's tall measured with a level flat over both rears (dually) ground up. The 2 outer tires I have are almost brand new tread wise.
Click to expand...
Yeah, I tried that at first. Too much math and too much room for error. I simply marked a chalk mark on one tire where it contacted the floor and then marked the floor. Then rolled the truck backwards till the mark faced down again. And marked the floor. Measured the distance between marks. Seems like it was 98" - over 8 feet. :eek:

Not sure what to do now. Thinking about just dealing with it, but I would rather my actual speed be less than speedo speed not more, for speeding ticket reasons. (not that I ever drive fast in this pig but still...)
Click to expand...
Yup. Mine reads over, which is perfect for me. I'll always look faster than I am. :D:D
 
94.836828230242 Seems to be my circumference, not 96.something I was using before.

The thing that bugs me now, is that if I calculated it larger than actual, i would think my speedo would be reading faster, not slower.

This is making me think that my paperclip solder/joint isn't making contact?? although its stuck there good with solder, so it must be. Unless its a non conductive material....
 
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Perhaps the makers of the DRAC/VSSB took into account the deflection of the tires and theoretical values for tires, because you should use a higher circumference to make the speedo read faster.

So perhaps you should use 101.5" which is the theoretical circumference of a 235/85R16= 32.2" tall, or at least something close to that, like 100" because most specs I have looked at show somewhere around 31.5" tall which would sill only be 99" circumference.
 
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