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Thicker weight oil -> better gas mileage?

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Throckmorton

Throckmorton

Lifer
I used 10W-30 Mobil 1 in my 1999 Jeep XJ 4.0 until about a month ago. For the whole 5000mi life of the Mobil 1 I was getting 15 or 16mpg. Before that I was living in Houston so the driving conditions were completely different, but I got the same mileage.

With the last change I put in Chevron Supreme 10W-40. Since then I've noticed that I get over 17mpg. The fillup just did came out to 17.75mpg!!

Conventional wisdom says that thinner oil boosts gas mileage, but could it be that my 130k 4.0L does better with thicker oil due to the lower tolerances than modern engines?
 
1mpg difference means nothing. That's low enough that you could have taken your foot off the accelerator sooner for like one block.
 
You're right... I need more data. Damnit, I was going to change my air filter but now I have to leave the old one for a few more tanks at least.
 
Also did you fill up at the same station? Some pumps cut off sooner than others. So it may only be half a gallon diff but that adds up in terms of gas milage and other factors.
 
I thought it was the same station but now I'm not sure. There's one other station I use sometimes. Making note of the station and pump I used today.
 
The only possible way the 10W40 is getting better MPG is since your car is very old and likely high mileage, the oil that is getting into the cylinders may be helping restore a bit more compression, better hp and efficiency
 
Throckmorton said:
I thought it was the same station but now I'm not sure. There's one other station I use sometimes. Making note of the station and pump I used today.
Click to expand...

When I record my MPG data in Excel, I always keep the location + pump. Unfortunately, I've been switching gas stations a bit down here trying to find one that I like and is a decent price. I usually try to keep using the same pump too... I'm one of those weird people that would wait for a pump with every other one open 😛.
 
There shouldn't a significant different between oil weight. If anything a heavier weight would reduce efficiency and increase consumption if the parts aren't over heating on light weight oil.

It could be that the route that you take have less lights and corners than the old route. Air filter and fuel blend (winter or summer) also play a roll.
 
1 tank means nothing. You need a trend. Heck before winter set in, I was doing 20-21 mpg in my Trailblazer, but one tank I got 23mpg. Just one.
 
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