I've had my HPTuners since Nov. '07. I never really did much except scans & clear DTCs. Awhile back, I put on staggered tire sizes. Grabber HTS 255/70/16 in front, & 275/70/16 in rear. So now my speedo would be off.

I went on HPT forum, looked up pertinent info, & did rollout method so I'd have a way to plug in numbers to make it all work. My bro helped me & the first 2x we weren't flashing, only reading. We confirmed w/. TomTom, & it still showed a speedo difference.

After figuring it out, that we needed to flash, & simply hitting confirm wasn't enuff, we got it squared away.
We rounded pie to 3.14. The program goes out further than that. I got my VSS very, very close. My math & HPT program varies only due to how many decimal spaces you round off.
My math said 90,760 VSS pulses per mile; HPT calculates to 90,766. That's an accuracy of .166%( that's .166 of 1%).

We read what Chris @ HPTuners had to say. His replies & those of others helped.

AKlowrider & a few others had some worthwhile advice on rollout when I originally did staggered tire size thread.

 

so....it works then :phew:

 

I havent played with HPT much but i just put in my displayed speed and my actual speed and press apply with EFILive. and it worked first try when i checked it with my GPS..
i have 275/60/17s front and 285/60/17s in the rear.. altho yours are an inch difference where as mine are only a .5" difference.

But whats it matter if they are staggered?

 

Jonathan, it matters cause my rear diameters are 31" & the fronts are 30", even tho both are 16'' tires.
It creates a variance of 3%, which I was told by TireRack is the limit they like to see on variance, & when problems (may) occur.

Ya, I woulda got mine the 1st time, but again I forgot to flash, since I didn't know any better. No I'm not griping at you Jonathan, just tellin u how it went down. My brother left me a note last night he could help today w/. measuring tire rollout & dinking w/. HPT.

I didn't have time when I came home to even learn what it took to calibrate thru a read & rewrite (or flash might be a better term.) He kinda sprung it on me. I definitely appreciated the help.

It's no secret I'm not good w/. computers. I get intimidated by them rather easily. So the 1st time I really jumped into a reflash, I wanted it to be small. Like a speedo calib., a fan turn on/off or a rear gear change.

I sure wasn't gonna tune an engine since 1 thing affects another & another...

Baby steps, baby steps.

 

I almost forgot bout u Bill. Who tunes your truck?