Over the last few weeks I've gone through a number of tests to try and determine where this vibration is coming from. First I started with my original 15x6 wheels/tires on the front with the 15x10s on the rear. Same vibration, so I redirected attention to the rear tires/wheels.
Next, I ran the new 15x8 245/60 setup on the front with the original 15x6 on the rear. It looks goofy, but the truck ran smooth without the 15x10s on it. With the 15x8s on the front the steering has a weird point where it feels like the tires grab more and turn in if I hit any kind of bump in the road. For the current experiment, this is an aside and something I will think about later.
Last test was to run the original 15x6 on the front with the 15x8 and 245/60 on the rear. This configuration gave me a very slight wobble/vibration at the same RPM as the 15x10 295/50 combo. At this point I was really fed up with these tires. It was also at this point that my daughter figured out she could turn the garage into a skating rink! This was a good reminder to look at the positives.
One thing that concerned me was the insane amount of weights they had to add to my 15x10s to get them balanced. They had 6 oz of stick on weights on one side and 2.5 oz on the other. This, combined with my tests, got me thinking that I may have screwed up these wheels when I changed the backspace. I rechecked the runout and got 0.034" radially and 0.055" laterally. From what I can find this is still within the upper limits of spec, but it's still more than my original 15x6 wheel which was less than 0.030" in both measurements.
I also measured runout on the tires which was about 0.060". Again, the tire shop told me the runout was within spec when they checked it. After all this investigation, I decided I needed to abandon this setup for now and go back to what I know was working. Problem was, I now had these Cooper tires that I didn't want anymore and I didn't feel right selling them to someone else when I knew they might be out of spec. Luckily, I purchased these from Summit and they were more than willing to honor Coopers 45 day satisfaction guarantee exchange policy. All I had to do was send these tires back and they would let me exchange them for another tire within Cooper's catalogue. So, the truck went up on stands for a few weeks while I waited on new tires.
