If you do so (add ethanol to gas) you'll be doing it at the risk of increasing NOx and perhaps failing NOx instead of HC. Our recommendation is to smog check your Geo Tracker with the fuel currently being sold (based on summer or winter blend) and if your Geo fails the smog check, having it diagnosed and properly repaired. Your Geo Tracker should pass the smog check with non-ethanol gasoline or gasoline with 6% ethanol.
Ethanol does burn cleaner and only produces CO2 and water. The extra oxygen helps remove excess hydrocarbons changing the hydro part to water and completes the carbon reaction so CO transforms to CO2. In smog tests NOx has been know to increase with the use of Ethanol when it is involved at mixture rates in vehicles not designed for E85 (Ethanol blend) gasoline.
Also for vehicles not designed to run on Ethanol blend fuels, 10% running through the system will leave a thick coating of varnish and sludge in the tank and fuel lines and that might end up clogging the fuel filter.
In terms of adding the incorrect amount of Ethanol to gasoline, your Geo Tracker's oxygen sensor might detect a lean condition and cause the fuel trims to increase fuel. If the mixture is too diluted it will max the fuel trims and set a high fuel trim trouble code; although you vehicle will not be affected by this since it is equipped with an OBD I system. gas in California contains accurately measured amounts of ethanol, only to limits allowing a vehicle's ECU to adjust for fuel trim efficiently.