Using the current method of locking the ratio is giving poor results on any car with a lot of torque and/or soft tires.

The problem is that you are setting the ratio with zero braking force.  The tire tread block is not compressed at all, such as it is when under load.

I suggest a different method.  Instead of inputting your tacho RPM, and then locking it when that RPM is reached, the dyno should hold a certain speed and then you input the RPM at that speed.  

As an example:

1. Set roller RPM.  This would be different for different roller diameters, but lets say 3000rpm.

2. Drive the car to 3000 roller RPM and the brake will activate under PID control.  The more throttle the more load.

3. Read off the vehicle tacho and input this into an "Tacho RPM at hold speed" 

I've used this method on other control systems and it works very well, especially for winter tires or anything that easily deforms as you can set the ratio in the same conditions that the power run is performed in.

Thanks!