Allowing for the conical shape of ultrarelativistic blazar jets with opening angles of a few degrees on parsec-scales we show that their bulk Lorentz factors and viewing angles can be much larger than the values usually inferred by combining their flux variability and proper motion measurements .
This is in accord with our earlier finding that such ultrarelativistic ( Lorentz factor , \Gamma > 30 ) conical jets can reconcile the relatively slow apparent motions of VLBI knots in TeV blazars with the extremely fast flows implied by their rapid \gamma -ray variability .
This jet geometry also implies that de-projected jet opening angles will typically be significantly underestimated from VLBI measurements .
In addition , de-projected jet lengths will be considerably overestimated if high Lorentz factors and significant opening angles are not taken into account .