MPG Question (point me in the right direction)

[TylerZ281500]

Today in the shower i had a thought about MPG in relevance to RPMs's

my initial though was that fewer RPM's means the engine rotates less which means less gas consumed correct? if that theory is semi correct then wouldnt being light on the gas pedal and being stuck in 1st and 2nd gear maybe third for some time increase the amount of fuel consumed, unlike it would in 4th.

my thoughts are that my truck (88 K1500, 350, 700R4) has some very odd gears, even my friend noticed. i dont know the difference between gears and rpms in a 700R4 vs 4L60E but my friend mentioned there was noticeably fewer rpms in each gear than his 4L60E. i dont have nuumbers as i didnt ask but he noticed a couple hundred or so difference in some gears, which i found odd but believable because at 4th gear im not far from idle.

now back to the MPG question, based on my driving habits, im a light foot.before i hit the gas i take my foot of the brake let it creep then hit the gas, and i drive slow, not horribly slow but im not stomping on it at every light. would it make more sense for me to be a little heavier on the pedal if i knew i would hit 4th and keep it there for a while? do 3rd and 4th gear or just 4th outweigh the amount of MPG's than 1st or 2nd? (that made sense in my head, not sure if it does in writing)
i Know rear end gears also play a big part of this but lets just keep that as an invisible factor and not worry about it for the moment.

Im not looking to squeeze more miles outta my truck i get a steady 13 on a normal day, im just wondering how much play there is in RPM's with MPG's. just something id like to know, dont just tell me im wrong or im dumb for thinking this, actually prove it. if you prove my whole theory wrong i wont be mad if you have some supporting wisdom behind it. Live and Learn thats all im trying to do. also what i said might not make a ton of sense, ii had a difficult time putting what was in my head down onto the page.

 

[ChrisAU]

I've always been in the school of thought of get to driving like granny quickly to save gas. If that makes senses. 2000-2500 RPM for 30 seconds or so getting to 60 mph uses more gas than 3500 RPM for 15 seconds to get to 1500 RPM cruising.

I have no idea if I am right, just seems correct.

 

[DieselPower]

Check out some of the hypermiling forums for driving styles that are economically advantageous. Most of them are very tough to follow without annoying everyone else on the road around you. Typically it is far better to accelerate lightly and use downhill runs to increase your speed. Don't try to accelerate on hills as it requires the most amount of power to make happen. Coast as much as possible. Think of your brakes as a big cost - every time you have to use them you are bleeding off speed that cost you to acquire. If you have to step on the brakes much at all, that means your fuel economy is getting trashed. Open windows and A/C cause drag - and that means less efficiency.

Rpms really do matter. A gas engine keeps the air/fuel mix at a fairly consistent ratio of 13.7:1 (roughly). Every time the 5.7 liter engine turns over twice (4 stroke cycle...), the engine is roughly sucking in a new volume of air and fuel. The volumetric efficiency is ~85%, so .85 * 5.7 = ~4.8 liters of air&fuel coming through. So - driving at 2600 rpm instead of 2000 rpm means 600 revolutions, or 300 4 stroke cycles. Multiply that by 4.8 and you have pushed through 1440 liters of air/fuel extra through the engine - per minute. That's going to add up. Also - it takes a lot of power to push something as aerodynamically efficient as a truck through the air. The faster you try to push it, the harder it is to move the air out of the way. A 350 puts out plenty of power to move that air, but there is a fuel cost to making that power. Keeping your speed down helps drastically. I remember reading something about 45 mph being a fairly efficient speed to maintain, but that really is unrealistic on most freeways. 70 mph is massively inefficient though!

 

[TylerZ281500]

i agree with all that, and i have taken that into consideration. im just saying are you better off getting a good start and not lolligagging around in your lower gears to get up to speed or is it better to take it slow. say a car in first is at 3000 rpm then it drops to 2000 in second then 1500 in 3rd then 900 in 4th (not realistic rpm ranges but its an example). would it be better to get out of first and second and get to third and 4th and keep cruising or should you take all the gears slower so your stuck at that 3000-2000 mark. because more rotation means more fuel and as the trans switches gears the rpm's drop, so in my head i keep thinking that if you get into you lower gears quicker youd almost get better mileage. does that makes sense or does it sound absurd?

 

[Joesgarage]

Go slower and keep the RPM below 2400 in all gears.
you'd be suprised how many miles you get from a tank full

 

[Problem2420]

im only getting about 200 miles per tank and there are some slight stops but other then that moslty regualer driving and somtimes granny driving i think im not running right

 

[ChrisAU]

im only getting about 200 miles per tank and there are some slight stops but other then that moslty regualer driving and somtimes granny driving i think im not running right

Yeah that is TERRIBLE!

 

[Blue95]

A tbi truck likes about 2100-2300 at 70 for cruising.

 

[DRAGGIN95]

Yeah something is wrong to only be able to get 200 per tank, this is also where people that have deleted their EGR valve start to suffer, I have seen multiple people get rid of their EGR valve on these TBI motor's and loose about 2 mpg out on the highway.

 

[jcloud]

With 35s and stock gears I hit 188 miles at 1/4th of a tank. But I'm getting more mileage than what its showing because i havent programmed my computer with the size tires I have. I want new gears soon though! If you drive with a light foot your gonna end up with better mpg