Interesting MPG comparison

[nickw]

So I have been kicking around the idea to throw in some lower gears. I have been debating 5.29's if/when I went with the 255/85's but had no idea how it may impact my mileage so I thought I would do a quick comparison.

I had to make the trip up to Olympia from Portland a couple time this last week and decided to take advantage of the flat I5 route. Mostly flat with only a few slight inclines, no hills to speak of.

I have done the calcs and running the 5.29's with the 33.2" 255/85's is 'roughly' similiar to my current 4.10, 31.8" 265/75's in 4th gear. I set the cruise to 100kph which netted an overal RPM of 2700 and kept it pretty steady for the 112 or so miles.

I drove back to Portland then headed back up to Olympia just to make sure I was driving the exact same route with the same hills. This time I dropped it down into OD which put the RPMs in the 2250 range. I even filled up at the exact same station both times.

Results as follows:

Trip up @ 2700 rpm - 17.75 mpg

Trip up @ 2250 rpm - 20.75 mpg

This may not surprise some, but I was quite shocked at the results. I was always under the assumption that the 3.4 would be more effecient higher in the RPM range since it makes max torque above 3K.

 

[Schattenjager]

That is very interesting - did you notice anything unusual as far as weather - windy? or did you run into traffic that required a bit of slowing and accelerating? That is quite a gap in MPG related to RPM - esp if the engine makes its peak torque even higher up the dial. I'll be curious to see what some of our more sage members have to say.

Good post!

 

[DaveInDenver]

Was the payload the same both ways? Basically spinning a lower RPM will always use less fuel if you don't need the power. If you had more stuff on board and you needed the engine to work harder, the test could be different. Efficiency measures how the engine produces the peak power per fuel consumed, not necessarily the point where the absolute least fuel is consumed. I thought there was a thread about this somewhere. If you find that with your truck a 4.56 or 4.88 gearing allows you to spin the engine lower and still make sufficient power to move you, then you can save some fuel. Others of us find we ask our engines to work so hard that they must be make the most amount of power and gearing them to be at the most efficient point yields the best mileage, even if that mileage is relatively bad.

 

[Nullifier]

Exactly me thoughts. If you were empty that is one thing loaded for ong trip is different. Also flat land and hills have huge variables just like putting along in the lower gears do which is where most of us are when we are covering long distance offroad.

With my 4cyl my gas went to crap when my 255/85 went on once the 4.88 gears went in my mileage came back up to 20mpg highway on relatively flat ground (loaded extra heavy).

If you are running amanual which sounds like you are I would run 4.88 gears not 5.29. That maybe a little steep for the manual tranny. I know scott runs them but he is using an auto which has a lower final ratio then the manual set up.

 

[DaveInDenver]

Exactly me thoughts. If you were empty that is one thing loaded for ong trip is different. Also flat land and hills have huge variables just like putting along in the lower gears do which is where most of us are when we are covering long distance offroad.

With my 4cyl my gas went to crap when my 255/85 went on once the 4.88 gears went in my mileage came back up to 20mpg highway on relatively flat ground (loaded extra heavy).

If you are running amanual which sounds like you are I would run 4.88 gears not 5.29. That maybe a little steep for the manual tranny. I know scott runs them but he is using an auto which has a lower final ratio then the manual set up.

I run 5.29 gears with a 5 speed on 33" tires, but I have a 22R-E and still top the 5,300 lbs mark loaded. I would rather have a slightly taller gear, like a 5.08, but there's nothing between 4.88 and 5.29. In the end, I'm not unhappy with 5.29 gears other than being stuck with 33" or taller tires (I'd like to go back to a 32" since my lift plans have now been derailed for a few years).

 

[Martinjmpr]

I'm surprised that you're surprised. The engine was turning more at 2700rpm than at 2250rpm so of course it's going to burn more gas.

Yes, the engine makes max torque at a higher engine speed, but so what? Once you get up to cruising speed your vehicle doesn't require anything close to max torque. That's the whole reason for OD anyway - because you don't really need all that power once you get the vehicle moving.

When I'm on the highway I try to keep the engine speed as close to 2000rpm as possible. Obviously, driving through the Rocky Mountains that isn't easy, but I do what I can.

BTW, when factoring mileage comparisons with bigger tires, you also have to consider the additional weight of the bigger tires, and if the tires give your vehicle a higher profile, there will be more wind resistance.

 

[DaveInDenver]

Once you get up to cruising speed your vehicle doesn't require anything close to max torque. That's the whole reason for OD anyway - because you don't really need all that power once you get the vehicle moving.

Been a while since you've driven a 22R-E, then? ;-) I keep my right foot pretty much planted all the time on the interstate. All you guys with lots of HP, I'm afraid I'm gonna get creamed and if I'm not braking, I'm accelerating trying to keep up!

 

[nickw]

Variables

Traffic patterns were 'similiar', I was in cruise 90% of the time, with the other 10% being slow increases/decreases with a steady foot to get around traffic. I never went under 55 or over 65. Both times I did stop once to buy my 'co driver' something at the store for putting up with driving 62 mph the entire way, getting passed by semi's and little old ladies

The load was very similiar both times, the misses, the dog and maybe 100 lbs of stuff.

Temperature/time of day was the big variable, which I did not take into consideration. The first trip was on a dry early afternoon while the second trip was on a partially wet evening.

 

[Martinjmpr]

Been a while since you've driven a 22R-E, then? ;-) I keep my right foot pretty much planted all the time on the interstate. All you guys with lots of HP, I'm afraid I'm gonna get creamed and if I'm not braking, I'm accelerating trying to keep up!

I had an 85 with the 22r, reg cab with a shell, and I don't remember having to keep it hammered down most of the time.

How fast are you driving anyway? Back in 85 the speed limit was still 55 and I rarely went above 60.

 

[DaveInDenver]

I had an 85 with the 22r, reg cab with a shell, and I don't remember having to keep it hammered down most of the time.

How fast are you driving anyway? Back in 85 the speed limit was still 55 and I rarely went above 60.

'Bout 65 is my top practical operating speed, which is roughly my sweet spot of 2700 RPM, give or take (mind you a lot of this was experimentation with my old engine, I don't have enough hours on the new engine to say for sure that the characteristics are identical, I may find in time that this engine is happier higher or lower). Anyway, I find my truck has been happiest between 2500 and 3000 RPM, below that and I can't get anywhere and above that my mileage drops like crazy. With the new engine, if I keep the hammer down and there's no significant headwind, I can tickle 80MPH. I'm turning about 4500 RPM at that point, but it will officially break the speed limit!

 

[tacollie]

I have 255/85's on my 02 with the manual and on I-25 here in colorado at 70 on my speedo I get about 20mpg. Around town I get 16-20 depending on how I drive. Before I put them on I NEVER got over 18 on the interstate. I have a bull bar, winch, 3" lift, bike rack, sliders, and ultra heavy sleeping platform in my topper. I do drive a little slower and I have lost acceleration. Even in the mountains my mpg are the same as before, I just down shift more. It is a little harder to pass, but still better than my 90 4Runner. Eventually I will probably get 4.88s because most of my driving is at 60 or less.

 

[erin]

Do you have the v6 or the 4cyl? Are you accounting for the percentage your speedo is reading off due to the tires?

 

[nickw]

Nope

Do you have the v6 or the 4cyl? Are you accounting for the percentage your speedo is reading off due to the tires?

I didnt bother with speedo re-calcs since I was just shooting to get a comparison between the two scenarios. I have the 3.4l engine-

Since the circ. of the the tires (30.7" stock vs. 31.8" current) varies by 3.5% wouldnt I have to actually add 3.5% on to my numbers to net actual mpg?

 

[erin]

Sorry, the question was meant for tacollie, due to his tire and gear changes.

 

[tacollie]

Sorry, I have the V6. Also, before I would do 75 according to the speedo. Now I do 70 according to the speedo. I don't drive to fast these days.