Gas mileage

I know winter is hard on gas mileage, but I just filled up my wife’s 99 Bonneville (3800 series 2)- which used to be real easy on gas - and it came out to about 12 MPG - that was mostly short trip driving - this car used to get 30 MPG HWY and about 20 in town - it has new plugs, new air filter and fuel filter - what else could be making it suck so much gas? It only has 48K on it - so it’s not been run hard.

Short trip driving is the WORST thing for gas mileage, especially in the winter. Add in the possibility of ethanol in the gas and you can easily drop to 12 mpg. The mileage from one tankful of gas is not enough to tell you anything. You need to average the mileage over time.

Check the air pressure in the tires. Underinflated tires reduce fuel mileage, and tires lose pressure when the temperature falls. See how all these little things add up?

If the engine is not reaching correct operating temp it will use more gas. Does it warm up quickly and reach the correct temperature. If not you may need a new thermostat.

used premium (92) gas last week - this week it averaged closer to 16 MPG - I think maybe spending .20 per gallon may make up for the cost per gallon in using less gas overall

wird thing is that my 97 GMC with a 350 Vortec (best engine ever built) gets same mileage winter/summer no matter what type gas you use very little variance

Mathematical error. You are basing your 12 mpg figure on a single fill-up. If the gas pump shut off late you will arrive at a calculation several mpg below actual. Return to your normal filling stations and you will see that the Bonneville’s fuel economy is back to normal.

Cold winter mornings - how much time is it warming up before she gets in and drives away. That’s all I could figure out when my mileage dropped from 27 to 18. Does anyone know if this seems realistic.
Does idling to warm up a car somehow really eat gas?

does the manual say premium gas required or recommended? If not, don’t use it, you will get Zero improvement in mpg. If premium is recommended then yes, you will get probably better milage with premium (depends on car).

Premium has no effecton gas mileage unless your Bonneville is supercharged and requires it. It’s just a coincidence.

or course idling uses gas!

Then I take it the non-superviced Bonnevilles don’t come with the “recommendation” for premium? I have no reason to question it, and if it is true, then premium will have zero chance of improving mileage and in some cases premium has caused problems in cars designed for regular.

A few easy things . . . parking brake working properly? How about the brakes generally? Any dragging/binding? Jack it up and see if your wheels turn free and easy. Tire pressure? Check it when you’re checking the brakes . . . I usually look at the sticker on the door jam and go up 2 psi. Don’t idle the car in the driveway. This is a waste. Even in cold weather . . . start it up, wait 20-30 seconds, and then start and drive easy for the first 5 miles or so. Short trips? Try and combine them, walk once in a while. Jackrabbit starts? Although you didn’t mention it . . . it is worth mentioning. This wastes gas. Also . . . McP said it all about using only one tankful as your measuring stick . . . try three or four tankfuls. Rocketman