Originally Posted by Jim Hef
You want to explain how higher revs on the highway will result in better fuel mileage???
Originally Posted by HEMInator
Exactly! Higher ratios will DECREASE fuel economy
on the highway and INCREASE in the city. On the RAMs, the standard ration on the 2009s was 3.5X whereas the option existed for around $375 I think to add 3.92 gears. I added 3.92 gears which actually sees about a 1mpg increase in city and 1 mpg decrease on highway so the overall fuel economy is the same but the low end acceleration is definitely better and it increases towing capability.
Guys... Lugging an engine will NEVER increase fuel economey! NEVER.
Simple Math is in favor of CORRECT gear ratios... My point is they are absolutely WRONG in our vehicles from the factory.
I have very specific experience with the combination of rear gear ratios, tire hights and 45rfe/545rfe transmission overdrives... When I went from 3.55 gears to 4.56 gears in my 2002 4.7L QC Ram with the 45RFE trans (.75 overdrive) only raised my RPM by [email protected]
... So I was just over [email protected]
. I gained 3-4MPG at 60MPH! .... Simple math and physics back this up. Dont forget this is with a 45RFE .75 overdrive... the 545RFE offers a dual .75/.67 overdrive.
(they are both the EXACT same transmissions with the only difference being the programing)
With that said... beyond popular beleif, RPMs alone do NOT dictate fuel economy. Its the load on the engine at ANY given RPM that has EVERYTHING to do with fuel economy. Lower RPMs do NOT always mean better fuel economy as most think. By going too low of an RPM you increase the load on the engine... Chose a gear that will get you slightly deeper into your torque curve and you will see improvements on fuel economy & overall performance.
Here is some simple math to help you best decide on what gear ratio to chose...
Find overall gear ratio-
OVER DRIVE GEAR x REAR GEAR = OVERALL GEAR RATIO (OGR)
FIND YOUR RPMS at ANY SPEED-
SPEED x OGR x 336 / TIRE HEIGHT = RPMs
1) OVERALL GEAR RATIO (Factory Gear Ratios 3.42)
.75x 3.42 = 2.56 OGR
.67x 3.42 = 2.29 OGR <--- OMG!
2) FIND RPM @ SPEED
60MPH x 2.56 x 336 / 30" = 1720rpms (.75 overdrive)
60MPH x 2.29 x 336 / 30" = 1539rpms (.67 overdrive)
Does anyone here think there engine makes enough torque at 1539RPMs to hold a 2.29 OGR? Let me help... Perhaps down hill but not on nornal roads. So if your lucky your engine will be spending most of its time in the 2.56 OGR (.75 overdrive)
NOW Same math... BUT gears that put us closer to the desired [email protected]
. I will use 4.56's because experience has already proven them to be the most mathamatically correct gear ratio for our combinations.
.75x 4.56 = 3.42 OGR
.67x 4.56 = 3.05 OGR
2) Find [email protected]
60MPH x 3.42 x 336 / 30" = 2298rpms
60MPH x 3.05 x 336 / 30" = 2049rpms <--- Bingo Money maker!
In my experience I have found that a good rule of thumb is to aim for [email protected] This gives us the best all round performance gains to include fuel economy.
With the .75/.67 545RFE, 30" tall tires and 4.56 gears you will see increases in MPG from 1MPH to 70MPH! This is not an opinion... this has been proven over the years with the same combinations on the Rams, Dakotas & Durangos!
Just do the math... You just may learn somthing new.
For all those "LOW RPM" Guys... Try a little excersise.
Next time your on the freeway... set your cruz to 50-60MPH... With out touching anything else watch your actual MPG gauge. You will see that on flat ground youll get say 20MPG... But as soon as you hit an incline your MPG drops to 13-14MPG... and hit a decline and it shoots up to 30+MPG... nothing changes except the load on the engine... Lower the load on the engine and your MPG increase. This entire time the RPMs have not changed... This backs up the fact that by simply moving the load from the engine to the gears, we are able to increase the fuel economey. The CORRECT gear ratio in the rear can and will increase MPG and performance.
Who would not want that?
Now the best thing about 4.56's and the new OGR... as with MDS cylinder deactiveation... we now have MORE mechanical advantage to hold the MDS to 4 cylinders under higher loads... meaning your MPG will increase even more!
SPEED SAFE, NICK