I originally dynoed around 307 whp 330 wtq peak on heavier 235 snow tires, but when I put my lighter 255 RA1 race setup I noticed a decrease of about 20whp (not to mention it was a bit wet outside). Same exact hp & tq curves, just everything was shifted on the y-axis by minus 20whp/10wtq to about 285 whp 320 wtq. Now I've tried doing a search on tire size vs dyno and all I can find is this from google:

"When you increase tire size, you effectively raise the final drive gear ratio, effectively reducing the amount of torque that you put to the ground. Since Dynos measure HP at the rear wheels, you would see an impact."

Anyone care to shed more light into this?

I forgot to mention that this was done at the same dyno dynamics. I would like to thank www.theshopct.com for the free dyno day!

Thanks.