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Old 10-31-2013, 03:09 PM   #31
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Originally Posted by Al Fresco View Post
EarlyRetirement:

I agree with everything you say. Indeed, a higher gear ratio does equate to more torque. Nonetheless, this does not explain (at least to me) why higher engine rpm's would be required to maintain the same driving speed if the gear ratio is lowered.
The gear ratio is increased for more torque not decreased. As in 3.42:1 increased to 3.93:1will result in higher engine speeds. The are saying by decreasing the overdrive ratio will decrease engine speed . It can be over driven or under driven. It results in a positive or negative ratio
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Old 10-31-2013, 03:09 PM   #32
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Originally Posted by Badlands View Post
As the other respondents said, there are many variables, but changing to a lower gear ratio won't be aiding your mileage. Think of it this way; the lower your gear ratio, the greater the RPM's from your engine to maintain an equal driving speed. The higher the RPM's, the more the engine is working, and down goes the MPG. I go back to my muscle car days for an example, too. My Dodge Charger Daytona 500 had a 440" engine, and I commonly ran 3.91 gears, though the car came with 4.10's in it's Trak Pak format. For Saturday night bragging rights, I'd change the gear set over to 4.56's -- huge torque and extreme acceleration capabilities. But (and I don't remember the exact details -- it was a few years back), at 55 mph, driving normal and not abusing the accelerator, I lost 2-3 MPG with the 4.56's in the Dana axle, as opposed to the 3.91's. And, as you note, the effect of a headwind can be mighty humbling!
Memories. I had a 68 road runner... 440 souped up some..850 double pumper...bla bla it got 2 MPG on the highway before I installed 4:30 gears. Lol
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Old 10-31-2013, 03:10 PM   #33
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Originally Posted by garbonz View Post
Slow down to 60. Thats the only way to appreciably increase MPG, regardless of what else you do.
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Old 10-31-2013, 04:43 PM   #34
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Originally Posted by EarlyRetirement View Post
4X4 is more expensive because you have a set of gears in the front axle also (twice as much??). My F250 has 3.73, this gearing came standard with the tow prep package. I’m of the opinion that towing a TT is towing a TT regardless of what snake-oil/voodoo tricks you may try. The only thing that may work is lowering your speed (55mph), and lets face it, you’re camping, what’s the rush anyway
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great advice everyone. so maybe I'll go for the 3.92 ratio as that is as low as I can go. There is only like $10.00 difference in price between the 3.55 and the 3.92.
Well that goes without saying. I think his statement meant the price from one ratio to the next and that the labor was the same for either gear set. If he is going change ratios a ratio within a jump of one or two the over cost of the job should be the same or should be negligible he is coming from 3:20 something to 3.42 or 3.92 the cost won't matter enough due to which one he chooses.
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Old 10-31-2013, 04:59 PM   #35
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JFM-jr:

Thanks for your attempt to allay my confusion on this subject.

Per your description, the drive train sequence is basically as follows: engine>transmission>pinion>ring- gear>>wheels. To a first approximation, the gear ratio is the number of teeth on the driven gear (ring-gear) to the number of teeth on the driving gear (pinion). Per your example, with a 3.73 gear ratio, the pinion must turn 3.73 times to cause the ring gear to turn one time. If, as in your example, the gear ratio is increased to 4.75, I understand that to mean that, for a constant engine>transmission>pinion rpm, the ring-gear>>wheels will make fewer revolutions than is the case with a 3.73 ratio. In other words, all other factors being equal, the speed will decrease at constant rpm going from a 3.73 to a 4.75 gear ratio. Say 2,500 rpm are required to maintain 60 mph. Again, all other factors being equal, with the lower gear ratio, would it not be necessary to slow (not increase) the engine rpm to maintain the same speed compared to the rpm required with the higher ratio?

Consider a bicycle traveling at constant speed on a flat road. All other factors being equal, if the gear ratio as defined above is lowered (i.e., the number of teeth on the driven gear effectively decrease relative to those on the driving gear) the rider must decrease the pedaling speed in order to maintain that constant speed. Otherwise the ground speed will increase.

To reiterate, I am responding to the claim that increased engine rpm are required to maintain constant speed with a lower gear ratio.
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Old 10-31-2013, 05:18 PM   #36
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Ok...I from Arkansas. I talk slow, understand nothing and skip stuff when I read....so if I got anything correct then there are a few factors for fuel consumption basics. Friction:tires and weight maybe. Wind resistance: faster is more. Torque:horse power to weight/gear ratio. Somehow this math has to work for your hp/weight/fuel use.

Is this even close?
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Old 10-31-2013, 05:25 PM   #37
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Originally Posted by JFM-jr View Post
This OD off feature was not available until 2006 on the gas Rams you could only eliminate 5th gear for the measley 250 rpms it was worth
i agree but most trucks run around empty and the EPA has pushed manufacturers to squeeze out every single bit they can..not to mention bragging rights among manufacturers for the best mileage on a pickup. the complexity, repair cost increase are definitely not worth it for a truck used like a truck.
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Old 10-31-2013, 05:27 PM   #38
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Gee, you guys are smart. Me , I just get in the truck and drive, The only difference between towing and not towing, the Nissan has a right/left position on the shifter in drive. The left side lowers my rpms and I shift to the left when I get up to highway speed(usually 60). I guess I'm using the gear that provides torque to get going , then the other to maintain highway speed. I usually drive between 55 and 60 to get the best mileage. Gear ratio is 3.73.
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Old 10-31-2013, 05:28 PM   #39
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Originally Posted by EarlyRetirement View Post
4X4 is more expensive because you have a set of gears in the front axle also (twice as much??).

more than twice as much, that front diff is a whole lot more involved than the rear. on say a K-5 or more current IFS the front labor is quite a bit more than the rear.
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Old 10-31-2013, 06:14 PM   #40
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If, as in your example, the gear ratio is increased to 4.75, I understand that to mean that, for a constant engine>transmission>pinion rpm, the ring-gear>>wheels will make fewer revolutions .
Above is where your logic is flawed it is not the wheels making fewer revolutions but instead the pinion making more revolutions. The wheels/axles at 60mph is a constant this will never change regardless of gear ratio. What does change is the pinion revolutions needed to turn the wheels to that same 60mph. A 4.56 will need alot more revolutions to maintain 60mph than a 3.73 will the only way to increase pinion revolutions is with your gas pedal
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