My Speedo is off by 4 mph according to my new GPS unit. I have the 3815 tuner set to 33.00" for tire size, which direction should I go, lower or higher and how much? I have 285/75/17 Firestone Destination M/T's that have a massive 22/32" tread depth, so I think the deep tread is throwing the tire size off ???? Thanks.

 

Can you change your tire size with the 3815?
If it's slower, than make your tire size bigger, if it's fast, then smaller. But since you have 285s, I think that the 275 is 33", and 305 or 315 is 35". So I'd try a 33.5" tire or so, see what that does.

 

my bad I guess I should have said.....its reading 4 mph faster. Truck says 64, when GPS says 60. I also noticed the signs that have radar in them that flash your speed also says 60 mph. So I need to raise the number in the tuner?

 

Do what the Superchips manual says to do. Measure the ACTUAL diameter of the tire on a flat surface with the actual amount of air in them that you drive with. Then put THAT number in the programmer for your tire height.
1) most tires are never the height their specs say. Could be off up to 3/4" either way, they are usually smaller, though.
2) even if they are, that is assumed with 100% of their rated cold air pressure.

 

With my 285/70/17's I'm set at 32.5". As Hammer pointed out you need to measure their actual height with proper tire pressure, but since you have a GPS I'd just adjust it a little and see if it matches the GPS, if it doesn't adjust it again and see if it matches the GPS.

 

gps couple be off by acouple anyways. just do what they said and measure the actual height of the tire

 

My Nittos were 3/4" smaller than there advertised size when I measured them so my speedo was off. Also GPS devices can be slightly off. I checked my Garmin with the Calibrated Radar Unit in my Patrol car and it was approx. 1mph to 1 1/2 mph off.

 

No dumba$$, that would take some sort of machine like the ones in "Honey, I Shrunk the Kids" & "Honey, I Blew Up the Kid"! That's Hollywood, the movies and science fiction stuff!

But you can RECALIBRATE your speedometer to give an accurate reading for tire sizes that are not the stock diameter though.

Oh yeah, I am a wise a$$, btw...

 

Well if you convert the diameter of your tires to inches, in theory they would be 33.8" You could always try increasing it and see if that helps, or decrease and see what that does too. I don't think trial and error would hurt you other than the time it takes to do it.

 

I was wondering which direction because I planned on tuning in the morning before I headed home from X-mas. Was hopin' I guessed right for my trip home......I raised the number and it was the wrong way....LOL! Anyways since returning I tried several settings, Looks like 31.25" got me right on. Weird......The tires measured 32.5 " with the straightedge/tape measure trick, the tires are roughly 33.5" from factory, yet I have to have it set @ 31.25"? Oh well, maybe it's my 3.55 gears? Anyways, thanks for the help, I passed about 4 of the radar signs in my travels this week all reading the exact same as my GPS.