Attach one of these to the fender, and learn to estimate speed by how fast its spinning....
....more seriously...
I had a similar problem with our Burg 944. It initially had an LSD tranny from an earlier car, whose speedo was mechanical off the front wheel. The later trannys have an electronic pickup. I eventually bought a later non-LSD tranny with the proper pickup that the car could plug into. The earlier LSD tranny went into the race car, which is using a GPS based speedo.
Not sure how the 911's compare speedo-wise. But an option for you might be a GPS based speedo. Is your car using a mechanical or electronic speedo? If electronic, there are some adaptors I've seen online, that can use a bolt or some other spinning reference object, but getting the electronic signal to match the speedo is not necessarily easy.