chrisbnaz

oh yea, I didn't think about what stock tires would be, I have 265s too.

snap-on

stock 4.56 with 31's 3vze and 5spd.

70mph (on GPS) is about 2800 in my truck.... 3000rpm 75 ish....

drew303

If you lower the differential gearing your RPMS to maintain the same speed are going to increase. Not the other way around. The axles are turning slower with lower dif gears...... So unless you don't change your tires (ie go bigger) your rpms will need to be higher to maintain the same speed as before a gear change.

having said that... 3000 rpm with WOT and 3000rpm with a slight depression of the pedal are two very different things. =) Its much better to run 3k with some throttle than run 2500rpm with the pedal to the floor. This is where gears do their magic =)

drew303

oh and I run 70 in 5th at around 3000rpm on level ground.

4.10's on worn 31's. But our transmissions varie so this isn't very accurate to compare to everyone who has 4.10's and 31's. I can't run 4th below 40mph but my bro's 85 (whatever tranny he has i dunno) but his gears are much closer together (or shorter). you can shift to 4th comfortable around 33mph...

I dont know what his does on the freeway, never driven it on the highway

Tool Pants

What I remember from 100 years ago is that engine is most efficient when operated at the peak torque.

Per google my 1988 22R pickup has a peak torque of 142 at 3,400 rpms. Horse power is 112 at 4,600 rpms. My pickup is stock with a 5 speed manual tansmission and stock sized 14" tires. 4th is direct drive and 5th is overdrive.

In theory to get the best gas mileage I would keep the engine at a steady 3,400 rpms. Not hard to do on the freeway.

At 3.400 rpms in 5th I go 75 mph. At 3,400 rpms in 4th I go 65 mph. Per the theory I should get the best gas mileage at 75 mph in 5th, or 65 mph in 4th.

So the hype to drive 55 mph to save gas does not fit the theory, if the theory is true. The thing that comes to mind however is air resistance. The faster you go the more fuel you need to overcome it.

Any physics majors in the audience?

abecedarian

it fits the theory but you're forgetting something very important related to vehicle speed: wind resistance
as speed increases, so does drag, and as drag increases it takes a proportionate increase in power to increase speed
so if you factored in the drag, you'd find peak economy occurs at a lower engine/road speed than peak torque.

chrisbnaz

I don't see how that would make sense. I understand the theory, but in freeway driving, the lower rpm the better. You're usually cruising at a constant speed, therefore the less gas pedal the better.

Take my camaro for instance. It's a 6 speed, 4.10 gears 500+ hp on spray..Doing mainly freeway driving, I can get nearly 25mpg out of it if I'm nice to it.

The 2 overdrive gears of the 6speed aren't there to match peak torque with driving speed, they're there to get the rpms as low as possible while still maintaining driveability. At 75 in 6th gear, I'm at a little over 2k rpms.

I don't think you have to be physics professor to find the flaws in your theory.

More rpm = more gas going through engine.

abecedarian

with the lower drag coefficient of the camaro, it takes less power to maintain a constant velocity, therefore the engine does not have to make peak torque to maintain highway speed.
but any time you can maintain speed in a particular gear at lower rpms with the lowest throttle angle, you will get better mileage

put that v8 in a c2500 and tell me what kind of mileage you get.

Kiwipushrod

Kiwi

drew303

Not true.

You're using more gas with the pedal mashed and 2500rpm lugging up a hill in 5th compared to less pedal (less gas) doing the same hill in 4th (or 3rd depending).

I drove to Portland from Olympia which is about a hour half to 2 hour drive. Used up over half a tank of gas getting down there trying to keep 75+ compared to lil over a qtr tank getting home doing 65 and 4th. Granted winds are out of the south so i had a tailwind but I dont see a qtr tank of gas missing because of a 5 knot headwind =p

Typically if you can get the rpms low on the freeway and use as lil skinny as possible, or atleast less than you would in the next gear down to maintain the desired speed, your gonna get the best efficiency.

Overall, 65 seems to be the sweet speed with our pickups.

abecedarian

so... you think kiwipushrod just grinned because he could?

iamsuperbleeder

I think so, at least it appears that way, lol

abecedarian

wow... just a few more images and we're on our way to making a yotatech-invaders game.

Kiwipushrod

https://www.yotatech.com/forums/f116...ileage-136748/

Kiwi

chrisbnaz

I wasn't talking about going up mountain grades at 65mph...Here where I live in AZ, 99% of the freeways are almost completely flat..Once you get to the desired speed (ie: 65mph) all thats required a sliiiiiight slight foot resting on the pedal to maintain the speed.

Obviously, driving completely flat freeways compared to the same speeds going up a 6/7% mountain grade is going to require a two different driving techniques. Leaving it in 5th while going up a grade at 2500 rpm would be horribly ineffecient in a 4cyl truck that makes 11.2hp and 9.2324 ft lbs of torque

eatmorechicken

Also, the rpm of the motor does not necessarily determine the amount of fuel it is consuming. If the motor is struggling in 5th gear at 65, it is using more air/fuel then it would in 4th gear at 65 not struggling. For the most part the less your foot is on the throttle, the less air the motor is drawing in, therefore the less gas it is mixing in with the air.

khoopes

Each engine has a peak efficiency point, but each vehicle/gearing/windloading/weight has its efficiency point as well. In aircraft this makes a huge difference, it is more suttle in auto until you get into 100mph+ sports cars I guess. For instance, my airplane crusies at 150mph and burns 10gph, (15mpg) but you could back it down to say, 100mph and burn like 5gph. (20mpg). But the difference is 150 is WOT, 2600rpm, and probably 24"mp and the other would obviously be say 21" manifold pressure and 2300 RPM. However, at 70 miles per hour I doubt you would get much better economy than at 100 because the curve is not linear.

If you want to track your engine fuel consumption, all things being equal, find a power curve that gives you the manifold pressure vs rpm and you can estimate fuel consumption based on horsepower alone. The engine takes so much gas to make so much horsepower. Then, examine gear loading and determine what RPM gives you the best efficiency. The catch is the curve changes with manifold pressure which is directly effected by altitude and temperature.

This is only skimming the surface you could write a book on this subject and I am not qualified to write it!

Oh, I indicate 71 mph at 300rpm with 4.10gears and 235/75/15 tires. Haven't checked it with the GPS but I will soon.