I am just trying to call the ping:

curl --request POST \ --url https://api.fortellis.io/vehicles/reference/v4/vehicle-specifications/ping \ --header "authorization: Bearer eyJhbGcijkiJSUzI1..." \ --header "Subscription-Id: test" \  

I get: {"fault":{"faultstring":"Content-Length is missing","detail":{"errorcode":"messaging.adaptors.http.flow.LengthRequired"}}}

Now there is a saleforce error that is similar and had a solution:

adding Content-Length: 0, which kinda makes sense....

so adding --header "Content-Length: 0" to the original call then I get: 

{ "message": "Invalid SubscriptionId or Implementation provider is not configured for the requested API." }

so doesn't test doesn't work on this call?