Error loading tensorflow - Could not find "cudart64_80.dll"
I had a similar issue, but with the version 9.1 which I had on my machine.
The one which was missing  'cudart64_90.dll', whereas there was  'cudart64_91.dll'. So I did a 'downgrade' from CUDA 9.1 to 9.0 and it solved my problem. Hope it helps.
I am trying to import tensorflow (with GPU) and keep getting the following error:

ImportError: Could not find 'cudart64_80.dll'. TensorFlow requires that this DLL be installed in a directory that is named in your %PATH% environment variable

Setup:

NVIDIA GTX 1080
CUDA Development Tool v8.0
cuDNN 6.0
tensorflow-gpu 1.4

Environment variables:

CUDA_HOME: C:\Program Files\NVIDIA GPU Computing Toolkit\CUDA\v8.0
CUDA_PATH: C:\Program Files\NVIDIA GPU Computing Toolkit\CUDA\v8.0
CUDA_PATH_V8.0: C:\Program Files\NVIDIA GPU Computing Toolkit\CUDA\v8.0

I have also added the following to the %PATH% variable:

C:\Program Files\NVIDIA GPU Computing Toolkit\CUDA\v8.0\bin
C:\Program Files\NVIDIA GPU Computing Toolkit\CUDA\v8.0\libnvvp
C:\Program Files\NVIDIA GPU Computing Toolkit\CUDA\v8.0\extras\CUPTI\libx64
C:\Program Files\NVIDIA GPU Computing Toolkit\CUDA\v8.0\lib\x64

What am I missing? Why it can't find cudart64_80.dll despite its location is explicitly specified in %PATH%?
Any help would be much appreciated.