Can you mix a half tank of e85 with a half tank of regular unleaded in a 2005 ford ranger?

Most all cars/trucks were designed to run fuels with ethanol in them in the mid 80's.

BUT not necessarily run safely. The fuel system will take a fair % of ethanol without modifications, but the engine's fuel system cannot deliver enough fuel for running high % of E85.

To run it, I would highly recommend datalogging, monitoring the A/F ratio or O2 sensor readings to see how the engine is handling it. Probably the best way is to monitor the injector duty cycle and do not let it exceed 80%.

Can it be done safely? Yes, but not without careful monitoring the ECM to see how it is handling it.

To run it 100% you will need larger injectors, approx 50% larger, and the ECM will need to be calibrated for that by a tuner. Dealers will not be able to do that tuning.