That number is way too high. If you allow 300rpm's for slippage, which would be about 6800 rpm's, you would still be running about 163 mph. For the 143, you should only be turning about 6200 rpm's with about 300 of slippage.
If the transmission isn't slipping in high gear (burnt fluid or clutch material) then it has to be losing it in the converter.
Bill M