Affect of Gasoline Additives on Gas Mileage

My Toyota Prius used to get 50 mpg, now at 55000 miles it gets only about 45 mpg. Today I had it at the dealers and asked them to check it out. No problems. The service man said the reduction in mileage was due to the increase in additives to gas. Is this true?

Your service “advisor” is a self-important nitwit who thinks he can pass this nonsense off on anyone. In other words, he does not have a clue, but is too stupid or lazy to have the problem investigated further.

Gasoline has alway had additives, and there has been some increase in detergent to keep fuel injection systems clean, additives usually keep your fuel mileage from DECREASING, by keeping the system funtioning properly.

Your mileage could decrease for a number of reasons:

  1. Your driving style and pattern has changed; very often the cause of mileage change. Are any of your relatives driving the car?
  2. Your fuel injectors are wearing out, putting more gas in the engine (unlikely)
  3. You have a dirty air filter, impeding air flow to the engine, and running a rich mixture
  4. You have a small gasoline leak in the line between the tank and the engine compartment.
  5. The regenerative braking of the Prius or the electric motor is not assisting as before. This takes an expert to check out.
  6. You thermostat is stuck in the OPEN position and the car is running too cold.
  7. You have changed the way you measure your gas mileage; or you are relying on the dashboard computer, which may be malfunctioning.

There may be other possible causes. I would take it to an expert independent mechanic or another dealer. These guys are giving you the “sunshine treatment” unless your driving style has changed, as noted.

In order not to look silly, make sure you ACCURATELY perform ACTUAL gas mileage measurements to verify that you indeed have a problem.

Reason #358 not to go to the dealer. At least he did not want to charge you $$$$$ to check it out, I guess it is still under warranty?

My guess is the summer temperatures and the use of A/C is driving it down a little. However that is nothing more than a guess. The items listed by Docnick could well include something that may be fixed. He has listed several good ones.

Now I will add one. If your local stations are adding alcohol or adding more now, then that could well be at least part of the cause. Alcohol has less energy per gallon than gasoline. You will get less mileage, but what you are recording seems like it is too much for just what they normally add. In any case alcohol is not usually considered an additive, it is a fuel.

Try buying fuel from different gas stations. Some pumps are not perfectly calibrated.