Gearing Question...

[LS5DodgeRam]

hey guys... I'm sorry I know it is another gearing question... but I just cant wrap my head around the logic behind them...

Background: I have a 4.7l with 3.55s and just got a 6 inch lift and already had 20x10 inch rims with 33x12.50 inch tires... I will be upgrading to 35x12.50 tires at the end of summer and I may be doing a 1 inch coil spacer on top of the 6 inch lift and a 4 inch block instead of the 3 inch for the rear... should give me an extra inch of lift. I do a lot of highway driving (180 miles at a time or more) to my college and home. I have about 11.1 mpg average and I do not like that at all(on the defending end of this i have not collaborated my computer to run with the tires I have). when my truck shifts it takes a little to get going... nothing to complain about because I like the way it cruises through the highway.

my next step is definitely an intake that will help a little with mpgs and then I want a tuner that around town I can tune it to run performance mode but on the highway I can put it on eco for the trips home or to school.

now for my problem/questions:
I have head numerous times that gears help with mpgs... however I still cant wrap my head around this logic... it seems to me it has to do with the amount of stress the engine is put through with massive tires. however on the highway I cruise at 70 with around 2,100 rpms but with higher gear ratio it would only go up... since the gears spin less per revolution of the driveshaft.

how is it that with higher rpms to achieve the same mpgs save gas?
How is it that gears help mpg? is it in city driving mainly or highway driving?
is it truly a worthwhile investment for lifted truck with 35s?
what gears should be done?


thanks in advance,
Christopher

 

[Brakelate]

Well, the idea being that at some magical point, the RPM's and Speed hit a sweet spot that is a nice balance between pulling power and freeway overdrive.

In other words, RPM's are not a set in stone equation to mpg, as it is a matter of how hard the motor is working at the most used target speed.

For example, you could keep your stock "freeway" gears, but will be running much deeper in the throttle to maintain that speed, than if you had lower "crawling" gears, where despite running a higher rpm, you will be at only partial throttle, rather than wide open or closer to it.

Same reason some V-8 rigs get a better mpg at cruise than a 4 cyl rig, which is screaming along, or deeper in the throttle to maintain the same speed.

It is all a complex mathematical game where torque, horsepower, throttle position and speed all collide.

Naturally this is extremely hard to nail down, as driving style, elevation, the speed you run, the grade of the hills you climb and all of this come into play.

So, first step is to look at the Dyno chart for your motor. Where is peak power made? Then figure out your priorities. What do you do most and where is the vehicle spending most of it's time? Optimize that, and just accept the compromise in the other ends, whether it be acceleration or top speed, as they naturally are in conflict.

You could be running 80% throttle lugging along at 2500 Rpm and using more fuel as opposed to 30% throttle cruising at 3800 rpm, thus using more fuel, despite how hard and fast the engine sounds like it is working.

Hope that makes sense.

 

[LS5DodgeRam]

this definitely helped i appreciate the time to respond. i don't have to worry much about altitude since i'm in Florida and the only inclines my truck really sees are the over passes. the absolute most gear ratio i would even do is 4.10 but i just don't know if i should do 3.92s or 4.10s since i do highway driving a lot more than city

 

[jethro 03]

Excellent explanation!

Sent from my SAMSUNG-SGH-I747 using Tapatalk

 

[Redtruck-VA]

That was a very good explanation. However I feel you still are not getting it... You are going to taller heavier tires killing your overall gear ratio. Not counting the higher you raise the truck the more air being trapped under it, increasing drag and making the engine work off of it's sweet spot. Being a 4.7 this is even more important than with larger engines. If you want to play, then IMO there is no choice than 4:56. The rule being 33" and taller 4:56, shorter than 33" then 4:10. You already state the engine is working taking off and that work load will only increase with taller tires. Now when you are pushing that added weight and drag down the road at a given speed the only choice of decreasing the engine work load is with gears. Or more horsepower. If you still fear the gear, all I can say is I use a 4:10 gear with 28" tires, turn 2150 @ 65mph and generally get 17 mpg. Good luck...

 

[ST-8]

4.7 w/35s you need 4.56s