Originally Posted by Kev-
The auto industry is just catching up to the mountain bike industry, that's all
My first bike was a ten speed. Later, my first mountain bike had 18 gears. Later on, 21. A few years ago, 24 was the norm. Now they're at 27, and 30 is coming soon. I wonder if they'll add a fourth crank gear to bump it up to 40
There is more involved in fuel economy increases than the final drive ratio. Some cars get better mileage at 75 mph than at 60 mph, for example, even in the same gear. This is because the motor's maximum efficiency is reached at a particular RPM range, along with a drag coefficient which is low enough that the 15 mph increase isn't defeating the engine's efficiency gains. (That doesn't necessarily mean that only sleek cars meet this description though) It's not
generally "lower RPM = more efficient".
For highway mileage, it's all about the final gear matching up the current travel speed with the engine's efficient RPM range. A lower OD gear isn't always better, if they already have the ideal OD gear. Having more gears in the tranny can help ensure the final gear is
the ideally efficient gear for typical highway speeds, rather than a compromised range, as well as keeping the engine nearer to it's most efficient RPM range while in the varying speeds of city driving.
So, how much mileage increase can be had from a new tranny largely depends on how poorly matched the current tranny's gears are to the engine's max efficiency range.
Very well said... not too many people understand this fact yet. They think RPMs alone dictate fuel economey... when it fact its the load on the engine at any given RPM that has more to do with fuel efficiency then RPM's alone.
Overall gear ratios should be taken into account. Currently my 2002 4.7L Quad Cab has 4.56's with an Overall Gear ratio of 3.42 in overdrive. It sits right at 2000RPM@60MPH... And it gets 18-20MPH at that same RPM/SPEED... it does NOT downshift for hills.... It hardly ever downshifts.
Now when I first purchased it, it came with 3.55's and the overall gear ratio was 2.66... With this 2.66 gear ratio the best I could average was 15-17MPG... and it would downshift about a mile before a hill... There was not an incline where this thing did not drop out of overdrive to rev the RPMs up to pull the slightest incline... Think about it... thats with a .75 overdrive! With the current 545RFE and the .67 overdrive it has an OGR of 2.37... So I know without a doubt the engine does not have the low end grunt at 1700rpms to even hold that ratio. So even though its a dual .75/.67 overdrive... (2.66/2.32 OGR) it will spend most its time in the .75OD / 2.66OGR.... so that .67 will hardly ever be used.
BUT add 4.56s to the over driven combination and you get a very pleasent benefit. You end up with the .75/.67OD and the OGR changes to 3.42/3.05... This means that your engine will have more mechanical advantage to hold the .67 overdrive and actually get more use out of it.
So if they can actually make an 8 speed with a .60 overdrive and then add a 4.88 rear gear ratio you could have a 2.98 overall gear ratio... This would be a great ratio for anything over 4500lbs and shaped like a brick!
Also this is considering the vehicle has 30" tall tires or taller... with today's SUV's and trucks 30" seems to be common.
SPEED SAFE, NICK