After upsizing my tires from 265-75-r16 to 285-75-r16 which is a 2inch increase in diameter and 18bs per tire, Im seeing more light throttle knock retard. Ive adjusted the speed and shift points to compensate for the larger diameter tire. However Im still encountering more light throttle knock retard. The additional weight and inertia appears to be causing this. There must be something I can do to fix this perhaps VE, spark or fueling.

Please advise!!!!

 

You should not have increased diameter with that tire all you did was make it wider. The first number has to do with width not diameter. That is not a big difference either. Are you showing any codes? Get a tune from Jesse that might help!

 

going from a 265/75 to a 285/75, you will increase about .5 inch and width. It does have an effect on the height. Go to a tire conversion and just mess around with the first number and you will see what i mean.

 

Sounds like either riching it up a little or taking a couple degrees of timing out will solve the problem. But if it is less than 1 degree kr, I wouldn't worry about it. Try another tank of gas and just make sure that isn't it.
The tire diameter will change with the width if the second # stays the same.
1st # = width in mm
2nd #= side wall height referenced in percentage of width
in other words the sidewall is 75% of the width - 285mm
brands differ in size slightly due to differences way of measuring and construction.

 

From 265/75-16 to 285/75-16 is exactly a 1.2" increase in diameter, from 31.6" to 32.8". However, like mjhoward already said, sizes vary between manufacturers, and even between tires within a manfacturer, so you should measure to make sure.

Can't speak to the knock issue.

 

when you say you've "adjusted the speed and shift points to compensate" for the change, what did you do exactly?

 

The rolling diameter has something of an affect as well.
16" diameter wheel numbers+ tire size.
103.14" vs 99.43"

 

The Tuning was adjusted for the larger tires at 32.8 inches. The speedometer was adjusted to accommodate fewer tire rotations to match the same speed. The software automatically scaled the WOT shift points back to match the shift points based on vehicle speed. I went from a 265-75-R16 Firestone Wilderness AT to a 285-75-R16 Yokohama Geolander. The new tires are 57lbs compared to 37lbs. If I knew how these new power pigs would slow the truck down slightly I would have replaced with the existing size. Basically Im getting some off idle KR. At 2000rpm Im getting 8lbs of boost in 4th gear. The boost starts to build at 1500rpm. There doesnt seem to be much I can do about this. Resizing bypass valve? I tried reducing timing, optumizing AFR without much success. Im starting to believe that this could be resonance coming from the twin screw. The strange thing is that at higher RPM knock retard goes away. I normally try to power through it.

Any thoughts?