install cuda toolkit 6.5 in Ubuntu 16.04

I already have CUDA 7.5 tool kit installed, can I also have the 6.5 tool kit installed.

As I understand the tool kit is just a collection of libraries and the you should be able to specify which one to use using the makefile

Sorry I realise the cuda toolkit also contains the nvcc compiler, I thought that was downloaded in the cuda dev package.

So is it possible to have cuda 6.5 installed in a ubuntu 16.04 where cuda 7.5 is already installed. I want to compile something using the older cuda version.

I do not think that it is possible to have nvidia-cuda-toolkit 6.5 and 7.5 installed at the same time (on Ubuntu 16.04)

The best chance that I see is a manual installation of the packages with 6.5 in their name from
https://launchpad.net/ubuntu/+source/nvidia-cuda-toolkit/6.5.19-2/+build/8298413 (for 64bit amd64) respectively https://launchpad.net/ubuntu/+source/nvidia-cuda-toolkit/6.5.19-2/+build/8298414 (for 32bit i386 architecture)

Hi I have the following packages installed
ii cuda-repo-ubuntu1404-6-5-prod 6.5-19 amd64 CUDA repo configuration files.
ii libcuda1-361 361.42-0ubuntu2 amd64 NVIDIA CUDA runtime library
ii libcudart7.5:amd64 7.5.18-0ubuntu1 amd64 NVIDIA CUDA Runtime Library
ii nvidia-cuda-dev 7.5.18-0ubuntu1 amd64 NVIDIA CUDA development files
ii nvidia-cuda-doc 7.5.18-0ubuntu1 all NVIDIA CUDA and OpenCL documentation
ii nvidia-cuda-gdb 7.5.18-0ubuntu1 amd64 NVIDIA CUDA Debugger (GDB)
ii nvidia-cuda-toolkit 7.5.18-0ubuntu1 amd64 NVIDIA CUDA development toolkit

I downloaded cuda-repo-ubuntu1404-6-5-prod 6.5-19 from the Nividia site. It mentioned it had support for 980 cards.
I suppose the package didn't actually install the cuda toolkit because it just says repo configuration files.

so I run a sudo apt-get update and get the following error
W: file:///var/cuda-repo-6-5-prod/Release.gpg: Signature by key 889BEE522DA690103C4B085ED88C3D385C37D3BE uses weak digest algorithm (SHA1)
E: Failed to fetch file:/var/cuda-repo-6-5-prod/Release No Hash entry in Release file /var/lib/apt/lists/partial/_var_cuda-repo-6-5-prod_Release which is considered strong enough for security purposes
E: Some index files failed to download. They have been ignored, or old ones used instead.

I would prefer to install cuda using apt-get because I think you need to install a lot of dependencies

Sorry, but we cannot help when you have problems with software from external sources (like cuda from nvidia.com)
You have to decide whether you stick to the packages provided in the Ubuntu repositories (that is the area where we provide support), or if you use programs from foreign sources (we can give only very limited help with that).

I suggest that you post your questions in a NVidia forum, maybe https://devtalk.nvidia.com/default/board/53/ is the right address for such questions.