Why did I pick up 1.8mpg going to a higher viscosity oil ?

[Camperguy]

So my on my fifth fill-up and I picked up 1.8mpg going from 5w-20 to 5w-40 in my 14 JK. I've run PUP 5w-20 and PP Euro 5w-40 since. I know this isn't a scientific long term experiment but I honestly thought it was going to go down at least a mpg.

 

[kschachn]

Simple answer being that the observed fuel economy change isn’t because of the oil.

It never is actually. That change is deep in the noise of everyday driving. You’ll never be able to single out that one variable. It’s there, otherwise nobody would be chasing lower viscosity oils. But you’ll never be able to isolate it.

 

[camrydriver111]

Sometimes I see a temporary increase in mpg after an oil change.

 

[kschachn]

Here is a good post by someone who knows what he’s talking about. This was posted in reference to a slightly different question but the answer is the same:

Best Brand of Gasoline

Get all my Premium at Sam's Club. Other places around here are 40-50 cents more expensive than Sam's Club premium, making it well worth driving a couple extra miles to fill it up. Get my 87 wherever it's convenient.

bobistheoilguy.com

 

[eljefino]

The weather got warmer. That's it. A little bit counts a lot.

Also, nice avatar.

 

[Lubener]

The weather got warmer. That's it. A little bit counts a lot.

Also, nice avatar.

Summer blend gasolines are now being dispensed.

 

[Jetronic]

I went to 20W-50 redline in my diesel, from 0w-30 and didn't notice any negative change in MPG That's 50% more HTHS and KV.

 

[vavavroom]

Too many factors affect mpg to attribute this change to the lubricant.

 

[wwillson]

I see great MPG increases when I drive with a tailwind. The headwind going the other direction doesn't make me feel as good about the MPG.

 

[Rand]

Short answer: You didn't.

 

[Tkonrr]

My wife experiences the same thing as you with the same engine. Her 2015 Wrangler calls for 5w-20. I change to 10w-40 during the Summers. She reports better mpg's with it also. Over 190,000 hard miles (high loads, high rpm's) and still works well.

 

[Passport1]

You spent more time driving downhill?

 

[xmanjeff]

my mpg was always best just before an oil change

 

[facility1]

outside air temp can and does play a factor in mpg change to increase.

 

[nascarnation]

my mpg was always best just before an oil change

Since sheardown is a real thing, that seems to jibe with the physics.

 

[4WD]

outside air temp can and does play a factor in mpg change to increase.

Just from butt dyno - my Jeep loves 45°F air temps …

 

[ripcord]

Highly likely you were thinking about the MPG after changing oil weights and subconsciously adjusted your driving style.

 

[dumkid]

I go from 11 to 15+ mpg just depending on whether i go offroading or not. Something like this could happen just because you didn't have to stop at as many lights or something

 

[Lubener]

You spent more time driving downhill?

Not likely....You eventually have to drive uphill to drive downhill..

 

[TiGeo]

My wife experiences the same thing as you with the same engine. Her 2015 Wrangler calls for 5w-20. I change to 10w-40 during the Summers. She reports better mpg's with it also. Over 190,000 hard miles (high loads, high rpm's) and still works well.

Because of the summer fuel and warmer temps...