Gas Milage went down? What's up with that?

[Just4Fun]

Well, I have gotten 14.5mpg for about 5 tanks, then I switch about 100 miles into a take of gas to mobil 1 5-30, I figure it should go up from there.. So I fill up about 100 miles after the switch and I'm at 14.9, I think great.. Now I've been in the high 13 range and I don't know what's going on with that..

This is a 2000, stock everything and 120,000 miles.. What should I change? o2 Maybe? Plugs / Wires? I don't have much info on it and just got it so I couldn't say if the 90K was done or not but any ideas would help.. Don't wanna spend $100 on something I don't need to..

Thanks a lot guys!

 

[DMX84]

Lets start with the important stuff first. The 90k service, you need to find out if this was done (you don’t want to break you I belt and leave your & your family stranded somewhere).
There should be a sticker on your engine cover saying timing belt replaced bla,bla,bla.
If it’s not there doesn’t necessarily mean it wasn’t done, you’ll need to start tracking through the dealerships to find the one the PO may have used.

Mileage, So you are saying you’ve done nothing different other than change oil brands? Same gas, same driving patterns for the most part etc.?
I doubt that your plug wires are bad. Plugs maybe, it they didn’t get done at the 90k service.
I’m thinking maybe a lazy o2 sensor, but not sure. It seems weird that this is just starting after an oil change.
DMX

 

[Cruiser_Nerd]

Tire pressure? One low tire will easily account for that drop. Plugs might be worth a replacement anyhow if you suspect they're original. Only use the factory plug.

Wires?

Do you get reformulated gas there for the winter season? First tank of reformulated gas in October-November my mileage would drop 10%, return to normal in the spring. That's past tense. Since they banned MTBE here, we get an ethanol blend that sucks (but not as bad) year round.

 

[Augie]

Mine went down from 15 to about 14 MPG the last couple of tanks. I live in Ohio and we do get winter mix gas. I hate winter gas.

 

[cary]

How many tanks is this averaged over? One tank tells you nothing.

 

[Tinkerer]

Dropping 1 mpg (high 14s down to high 13s) can also result from different driving pattern - more stop & go, more acceleration & braking, higher speeds on the highway.

Especially with a vehicle like the 100 series: Lots of mass overall, lots of rolling mass, lots of aerodynamic drag at high speeds (especially if you have oversized tires, lift, etc.)

My mpg varies by more than 2 mpg, from tank to tank - just due to varying driving pattern, I reckon. I wouldn't think much about it unless it is permanently reduced, on average.

 

[spressomon]

Dropping 1 mpg (high 14s down to high 13s) can also result from different driving pattern - more stop & go, more acceleration & braking, higher speeds on the highway.

Especially with a vehicle like the 100 series: Lots of mass overall, lots of rolling mass, lots of aerodynamic drag at high speeds (especially if you have oversized tires, lift, etc.)

My mpg varies by more than 2 mpg, from tank to tank - just due to varying driving pattern, I reckon. I wouldn't think much about it unless it is permanently reduced, on average.

Ditto.

 

[Just4Fun]

Naw, I did this over about 8 tanks of gas.. I check it every tank and I've put about 3k on the cruiser.. Tires are fine, first thing I checked when I got the cruiser, first thing I checked when the MPG dropped.. I will replace the plugs and see about the 90k sevice.. Driving pattern pretty much always stays the same so I'm not sure on that front.. Due to the location we live in it really never changes much..

 

[LXPearl]

Make sure you are not getting blended gas - it drops mileage significantly. Ethanol does not have as much energy as petrol, so the blend requires more fuel use. Consumer Reports just had an article on it.