From Exxon, after filling w Shell for the first time and a bottle of Gumout, my '07 Prius went from 47.5 to 43.5 mpg. So I went back to Exxon. Guess what? It then went even further down to 41.2 ! What to do! What to do? Is there a chance that the Gumout screwed things up? It has been getting colder here in Northern NJ, around 32F. I'm staying w Exxon and see what happens. Maybe the Gumout will have gotten out.
The Prius doesn't like cold weather. So, it might not the type of gas you buy. I stay around the upper 40's it helps that I live in Calif.
You don't need Gumout or any other additive when you use a good brand of 87 octane gas. In fact, with the bladder in the tank, I don't recommend putting anything into the tank but gas. If you damage the bladder, you'll have a major repair bill on your hands.
I suspect it'e the temperatures not the gas unless the local stations have switched over to methanol mixed gas which does seem to affect milage. Have you looked into grill blocking ?
The colder it gets, the less mileage I get. But I expected that. also, if there is snow on the roads it affects your rolling resistance further hurting mileage. We are just more sensitive to that as we constantly are reminded of our mileage due to the on screen display. Not all that much differences in the gasolines anymore................
As others have said, it's the colder weather, not the gas or snake-oil additive. You might want to check your tire pressure too, cold weather can cause that to drop.
I live in new york and when the weather gets real cold and drive short distances i only get in the upper 30s . When i drive on pkwys i get around 45. IN the warmer weather i average 45 to 53. Tires 42 front 40 rear
Search Grill Blocking using the search link in the top right of the page. It is blocking the grill to retain engine heat, mostly done in extreme cold but can be used to advantage in warmer areas if you have a means to monitor coolant temperatures. (Scan Gauge or similar)
