Here in the states, for many years, 300,000 lbs was the seperation limit for heavies.
That included the 707's and the DC-8's, that went up to 355,000 lbs. That was the way it was for years, until the B-757 poked its ugly head out and turned a few aircraft up side down. When the federalies realised this, they lowered the "heavy" catagory down to 225,000 lbs to include the 757.
The company I worked for at the time, had one very odd DC-8 that had a MGTO weight of 293,000 lbs. Guess what? We had a few nutcases that insisted that we tell ATC that we were not a "heavy". I pitied the poor sod behind us that had to encounter our wake over a pidly 7000 lbs. I'm sure that was not in their daily program!
Bedder Believe it; I should be back in HKG soon. Put on your suds shoes!
P