My sons 2006 shows about 5 mph faster than actual speed comparing it too GPS reading . The tire size is stock LT 265/70 R17 that came on truck .
He is running a Bully Dog an it has a tire size funtion but it ask for the size diameter an we did not know which numbers on the side of the tire indicated
that . Anyway can anyone help us with this proublem , the truck has a automatic with the 5.9 in it .

 

Those tires are 31.6 inches

http://www.onlineconversion.com/tire...conversion.htm

 

He must have messed with the BullyDog setting for tire sizes, otherwise it wouldn't be that far off. Set it back to 31" dia for 265s. That's what read right with my 265's with about 50% tread.
The Bullydog went haywire in my truck when I upped the tire size to 33" for my 285s and then wouldn't go back to 31" when I put the summer 265s on.
Ended up throwing codes, larger size wouldn't erase from the ECM and the BD wouldn't unload.
Tech guy at BD walked me through resetting/reprogramming the BD box like they would if I sent it in. Then the BD tech said our trucks do not like the tire size being programmed above 34" (I only went to 33 I thought). It made the brakes stop working (like ABS was controlling it on dry pavement!).
My suggestion is set it to 31" and leave it, even with taller tires.

 

it depends on how accurate your ECM was originally. Mine was 3MPH off with stock tires and no programmer at all.

 

Thanks guys been out of town a couple of days i,ll try putting that in for him
He,s shipped out to England an left the truck an camper with us to use he,s in the Air Force . He told me before he left that he did not enter a tire size in the BD . So what ever the factory had it set at is whats in it. Do I need to enter it as 31.6 or is just 31 OK ?

 

The BD goes in 1/2" increments, so 31.5" would be closest. Personally I'd go a bit lower than actual. Hate to see the odometer putting on more miles than I'm actually driving!
If the speedo's showing faster than actual now, definately adjust it. The odo is racking up miles faster than the rest of the truck.

 

My Smarty sets my speedo just fine. I had mine set at 34" then down to 33.75" , and I think I'll drop it down to 33.62" .

My GPS is off. I check mine with a chronometer and a radar gun.

 

Finally got time to set it with the Bully Dog , it did not have a setting choice
of 31.6 . The choices were 31.50 or 31.75 so I went with th 50, an checking
it against the GPS got within 1 MPH sure beats 5 MPH fast . But who knows
when I get a chance to check it by radar it may be spot on . There may be a way to set it to 31.60 with a OEM type device plugged in, but that would require me to reinstall the factory settings for a visit to the dealer . I,m surprized BD did not have the standard OEM tire size as a choice ? Its ashame the odometer has registered the additional mileage
because of this error .

 

My '06 also indicated faster than actual travel speed with the 265s. Mine was about 3-4mph off.

I have now installed 285/70 and the truck is traveling faster than indicated by ~1-2mph. I have considered getting the ECM flashed by the dealer but I thought I'd drive around this way for a while to make up for lost miles

 

Drove the truck about 100 miles today an checked it a different speeds .
At 65mph its off by 2 at 55 seems about 1 to 1.50 . I am thinking I,ll do
the 31,75 I guess thats oneway of getting the miles back . But I am wondering am I not correct in thinking that I would need to reinstall the
factory settings before visiting the dealer to have them flash it ? If for no other reason
just to prevent drawing attention to the fact it has a BD program installed .
Or would the BD program prevent there factory device from communicating
with the truck ECM ?

 

Find a friend with a Smarty to set it for you, or better yet, buy a Smarty and love it!

 

Well, at this point I'm not real interested in programmers. Then I need to get gauges, which I probably should get regardless. I don't want to start fartin' around with it and then start having reliability issues. Maybe in time, right now not so much. Trying to keep from modding it to bits.

 

Mine is way more reliable now with those simple and effective upgrades. What? You got no willpower?

 

Got no money And I usually break things.

And if the truck is running every time I turn it on, I can't get any more reliable than that.

 

Smarty Jr is cheap, adds power and mileage, lowers exhaust temps, helps your transmission, eliminates the idle lope, clears your complexion, increases your attractiveness to the opposite sex, keeps IRS auditors away, and you can read and clear codes, and adjust your speedometer.

You don't need gauges with a Smarty Jr.