“We didn't have pace all weekend, and that's the way it is,” he said.
“That's the way it has been all year, and unfortunately, we haven't been able to improve it.
“We need to change the way the car feels, and we haven't been able to push that hard at all. I'm not sure what's going on. It's just a very hard car to drive.
“It hurts its tyres. You have to drive it to conserve the tyre. That's not fast, and it's not just myself who is saying that – Allan was say that all weekend too.
“I thought that this year was going to be something special, but for whatever reason, it hasn't been.”
While Murphy said he has “no idea” if he'd be back as a full-time driver next year, he was adamant that he was not finished in the sport.
“This is what I do. It's frustrating. I don't do this to say I race V8 Supercars and just make up the numbers,” he said.
“We've got great sponsors here. Castrol is absolutely fantastic. They deserve better, and everyone in the team works damn hard for it.
“I guess this has all been unexpected, our lack of performance. I was 100 percent sure that we'd be able to make this package work.”