RonLNY

Hi everyone,

So I know there are a bunch of threads about mpg, but I have a weird situation here. My 2011 RX450h has about 3300 miles and I have only ever used 87 octane gas. (My salesman told me this is fine, and I should only go higher if I hear knocking, which I never have.) I've recently been getting 27.5-28 mpg pretty consistently, and even 29.5 on long stretches of highway.

After reading some of the other threads, I decided to try a tank of 89 octane. In my area, it's about 5% more expensive than 87, so I figured if I could top 29 mpg in city driving, it would be worth it. I was nearly empty at my last fill-up, so I loaded it up with 89.

Well wouldn't you know, I've gone through about half a tank, and done only my normal drive that I do every single day (almost all city stop & go, with some highway, moreso on the weekends) and have plunged to 23.5 mpg. When I give it the gas, the engine seems to make a much throatier sound than before, like the difference between being in a 4cyl and a V6.

I can think of a few possible explanations: I got some "bad gas"; my (h) was perfectly happy with 87 octane and is not adjusting well to the change; or the fill-up happened to coincide with some other kind of problem.

Not sure if it's related, but soon after I got it, I tried Eco Mode for a while, and noticed that my mpg was actually much worse than usual. It seemed to stay on battery power for longer when at low speed on flat surfaces, but needed to work much harder to get up hills, and I think the decreased horsepower was negated by the extra fuel required to move with the less-powerful engine. This sounds contrary to what other people on the forums describe of their experience in Eco Mode, so it could be more evidence for an engine problem, or it could be completely unrelated.

Anyone seen anything like this before?

JimH2

14 pages over 8 years on the similar topic.

https://www.clublexus.com/forums/ls4...um-merged.html

RonLNY

Thanks for the reply. I'm not driving any differently though since the last fill-up, and doing only my regular daily commute, same as usual. Developing a mysterious lead foot, accounting for a difference of more than 4mpg, at the exact same time as filling up with a new type of gas, would be an awfully big coincidence.

Doing the calculation manually is a good suggestion though. I did this for the first several fill-ups after getting the car and it was always spot on (well, assuming you trust the odometer, of course) so I didn't think to question it. When this tank runs out, I'll reset the trip meter, give it another tank of 89 and see what happens.

Interesting, there doesn't seem to be much of a consensus. I didn't see any evidence that higher-grade gas could actually lead to WORSE mpg though! For completeness, I should probably try a few tanks of both 89 and 91 and see which comes out best. At 28mpg with 87 octane though, I really don't have anything to complain about.

Really, I was just trying to figure out how it's possible that switching to a higher-octane gas would reduce the mpg by that much, and if it's an indication of some kind of engine trouble. The car is only 3 months old at this point and I'm still learning the ins and outs.

wyau01

It is summer time, if you are running with the AC on, that could be a factor.
Also, if you fill up during a hot day, the fuel will be less dense, another factor.
Maybe something is wrong with that gas station.
Many things to consider, can't really give you a clear answer.
From my experience, driving in NYC uses more gas than outside of NYC.

wa3cuj

A local news crew did an expose on ethanol levels using lab tests. Even though the stickers on the pumps here say "up to 10%" their tests showed results all over the map (many wat over 10%). When I can find ethanol free fuel I get better mileage. So maybe we are also dealing with an ethanol variation in this thread also.

billy44bo

"if you are running with the AC on" I never turn it off in the Winter you keep it on to get all of the mositure out of the car to keep the fog off the windows.

ilovemaui

I notice about a 10% improvement with non ethanol gas

wa3cuj

I agree. However, it has become increasingly difficult to find its as gas prices rise.

currybob

I'm lucky to have many locations where I can get 100% gas and always run 87 and steadly average 32-34 during summer and 31-33 winter (we don't have winter gas here). I'm over 10,000 miles and very pleased with this vehicle.

Bob

billy44bo

must be nice I have to get the crap with Ethanol in it.

currybob

Yea, one of the advantages of living in the SE in a rural area I guess. You should know right Billy.

Bob

billy44bo

In Mobile on the coast the only place you can get it are places that are selling for boats and they do it at a premium price that might be .50 gallon more than 89 with Ethanol.

currybob

Oh, forgot you are in Mobile. At least fuel should start dropping due to recent market drops in unleaded gas. If it holds for awhile we should see about a 0.40 drop.
Bob

Cruiter

I see it mostly at 'Pure' brand stations in central Georgia. Usually it's about 12cents more per gal. I've done a google search and can't find it anywhere near me.

currybob

Once again I feel even more lucky now for I usually only pay about 5 cents more for it. There are about a dozen stations near me that have 100% gas and some of them do ask to much for it but there is one right near where I live and he keeps his just a little above 87 and sometimes it's the same.

Bob