venting tub through waste drain?

I have a toilet that has its own vent that comes off about 8" below the top of the toilet drain. it comes out with a 3" diameter and then reduces to 2" as it goes up the wall. can I tye the tub drain in down stream and use the toilet vent to vent the tub.would that beallowed?

thanks for any feed back.

tub

If you stated the problem correctly, and we have to assume you did, then no you cannot do that. The flushing water from the toilet would create a venturi effect and pull the trap seal from the tub's "P" trap.