Speedway tyres dont work so good on modern MX bikes, unless you riding it as a slider and upright and i have seen very few people do it and go fast, only tyre that seems to work on a heavy MX bike is the Michilen X10 and the new dunlop, everything else seems to go pear shaped on the wall. You watch a trials tyre on a MX bike and its almost rolling off the rim, the speedway tyre doesnt work the same way in the wall. And they dont have the tread, there control tyre for a reason.
IF you just going for a spin on DT then i can send you some used 19' speedways cheap, but your going to wan to keep you eye out for a cheap 18' rim if your serious and then go down that path at a later date if you like DT.