boat.rb

class Boat
  driveable
end

boat_spec.rb

describe Boat
  it { should be_driveable }
end

car.rb

class Car
  driveable
end

car_spec.rb

describe Car
  it { should be_driveable }
end

driveable_specs.rb

# how do I define this and mix it into rspec?
def be_driveable
  it 'should turn left' do
    @it.turn_left
    @it.angle.should == 270
  end

  it 'should turn right' do
    @it.turn_right
    @it.angle.should == 90
  end
end

Maybe you should use RSpec shared examples ? (I didn't test those)

shared_examples_for "a vehicle" do
  let(:vehicle) { described_class }

  it 'should turn left' do
    vehicle.turn_left
    vehicle.angle.should == 270
  end

  it 'should turn right' do
    vehicle.turn_right
    vehicle.angle.should == 90
  end
end

describe Boat do
  it_behaves_like "a vehicle"
end

describe Car do
  it_behaves_like "a vehicle"
end

More info here: http://relishapp.com/rspec/rspec-core/v/2-0/dir/example-groups/shared-example-group

Thats what I ended up doing, but It ended up being longer than I wanted because I couldn't interpolate the model name. I had to set three separate subject with three separate shared examples, which was not ideal.

In Test::Unit I would have just make a test plugin the did some meta-programming to used the class names to generate the appropriate objects.

Maybe like this:
[Boat, Car].each do |vehicle|
describe vehicle do
it_behaves_like "a vehicle"
end
end