Little higher rpm = better gas mileage

[Duffyjr]

For the last few weeks I've been driving my truck (see sig) in D instead of OD around town and it appears I'm getting better gas mileage when comparing my trip meter to the gas gauge. Usually when my trip meter hits 150 I have half a tank, but today I noticed the trip meter at 155 and the gas gauge is half way between 1/2 and 3/4. That's a lot when you figure it has a 30 gallon gas tank. In OD around 37 mph it runs 1050 rpm while in D at 37 its 1350 rpm. So lugging the engine in OD at lower speeds must use more gas.

 

[CT8]

O.K.

 

[exranger06]

Lower RPM doesn't necessarily mean better gas mileage. Lugging an engine can absolutely use more gas than letting it stay in its powerband at a higher RPM.

 

[Trav]

BMW did testing years ago and claimed 2800 RPM was optimal. If that's correct or applicable today I have no idea this was before the internet and google, it was a paper document.
I do know German manufacturers geared their US export cars lower to raise the RPM due to the 55 MPH speed limit at the time.

 

[hatt]

I wouldn't be surprised. It seems vehicles today are built around .gov tests vs real world. My truck seems to usually be in a higher gear than it should be.

 

[HTSS_TR]

Best gas mileage is at top gear and lowest RPM without lugging the engine.

According to Ultra gauge, my S2000 gets 56-60 MPG at 6th gear and 1800-1900 RPM on flat surface, the speed is about 33-34 MPH.

Normal hwy speed of 80 MPH it gets only 24-26 MPG.

 

[tomcat27]

optimal rpm is very engine-specific;

 

[abycat]

Originally Posted By: tomcat27
optimal rpm is very engine-specific;


I strongly agree. my truck obviously 2800 is just screaming and my car at 3000 the cam profile changes and it has way more power but uses a ton of fuel. I'd say under 2800 for most engines though.

 

[Fsharp]

At low speeds it may be more efficient in third (D) with the torque converter locked than in OD with it open.

 

[Trav]

Originally Posted By: tomcat27
optimal rpm is very engine-specific;

That was a BMW document for their 4 cylinder engines at that time, naturally its not applicable to every engine.
That being said it is not uncommon to find some VW 4 cyl engines with a stick turning 2800-3K in top gear at 65 mph.

 

[bvance554]

Thats the least scientific fuel mileage study i have ever seen. Its hard to come to a real conclusion between driving habits when people actually check their mpg, but you're just eyeballing the guage.

 

[Superflop]

So oddly enough quite a few years ago I was moving my sister up to Washington from CA. Her ranger pickup got fairly terrible mileage as we were driving. The od kept it when we were at the speed limit at a relatively low rpm. When we got further north a couple sporty cars blew past us and when they were a bit ahead I decided to kick it up and shadow them so they could draw out the ticket givers. Once we got above 80 or 85 the fuel mileage increased dramatically.. all I could think was wow. I can't entirely recall all the specifics but we were well into the 3k numbers for rpm and this thing just sipped. Couldn't believe it and although she still has her pickup I've never been able to nor had the inclination to repeat the trip to really get some hard numbers. That dang truck went from slugging it down to being dainty. She even was surprised.

 

[chevrofreak]

I've noticed the same thing in my 96 Crown Vic. Driving in D instead of OD I got my highest tank average with the most miles out of the tank.

 

[surfstar]

Originally Posted By: Trav
That being said it is not uncommon to find some VW 4 cyl engines with a stick turning 2800-3K in top gear at 65 mph.


Pretty much all small 4cyl are geared like this. My 2.0 Mazda was, my 2.2 Vue is, the Civic I have is slightly lower b/c it is the high efficiency model, but all other versions of that era match 3k @ 65. Very common from what Ive seen of low output, small 4-bangers.



eyeballing your gauge at 1/2 tank? Sure you didn't [censored] and provide a little free gas boost?

 

[Trav]

The same VW 2.0 slow German spec turns 2800 at about 85 MPH, 20 MPH faster than the US spec.

 

[JHZR2]

Op it's also getting warmer and fuel blends may be changing. 5 miles in 150 is too far in the noise. You may be on to something, but I'd suspect that if it were so simple, it would be programmed in to help meet cafe.

 

[19jacobob93]

Had this conversation with a friend the other day and he noticed the same thing. He has the same car and engine as me, but he just fitted a new ratio diff that made his engine go from 1,700 rpm to 2,100 rpm on the highway yet his fuel usage hasn't gone up and may have dropped slightly.
Mine has the economy diff that sits around 1,600-1,800 rpm on the hwy, they are extremely low revving engines, but once I knocked the shifter into 3 instead of D for 30 mins and didn't notice an increase in fuel usage while sitting at 2,700 rpm!

 

[yvon_la]

usually there is a minimum rpm recommended so ask you manufacturer or maybe it s in the book ,in big rig its around 1200 rpm minimum ( but this is for a diesel big rig usually car or pickup engine value are double that so you would be talking a minimum of 2000 rpm

 

[Charlie1935]

Back in the day my 59 Austin Healey Sprite turned 4000 RPM at 60 MPH and got 35+ MPG.

 

[IndyIan]

I think most of your savings is that the trans isn't shifting all the time and unlocking the TC every time you touch the gas.

In a manual car, you will get better mileage at lower rpms, but you don't want to lug too much. The shift light on my Focus will tell me to shift down too ~1300 rpm in the next gear if I'm just rolling through town, and if its on the flat 5th gear at 30mph definitely gives better mileage than 4th.

Around town I've started just using D3(stays in bottom 3 gears of 5) in the CRV, its top gear is too high for 30-35mph in town so its shifting all the time. Honda for some reason decided not to give a D4 option so you can use all but 5th.