If you're riding your bike that fast, you should first and foremost be wearing safety gear.
Secondly, if your tire is expanding at 40mph, then it's probably not rated for that speed. I suggest finding a tire that is, or it could blow out -- and I'd imagine that bike would be difficult to control on a flat rear tire, and impossible to control if you lost the front.
I learned this the hard way years ago on a go kart with mower tires on the front. Luckily, I escaped unhurt but it could have easily been catastrophic.