Educate me on the science of gears....

[97WillowXLT]

alright, ive be wondering lots about gears...

on cars, i know the reason for putting in higher # gears is to accelerate through gears faster thus reaching top speeds faster.

i was under the impression, the higher number of the gears the faster acceleration, but the lower the top end speed in any given gear.....and lower gears were vice-versa.

now, that ive went with 32x11.50's the explorer is going to be required to get better gas mileage.....im going to do the basic mods...

as far as the gears, 4.10 gears would probably be better for city driving where there's lots of accelerating after stopping.......but im going to be driving 60 miles a day highway, and less than 10 miles a day "city". would my 3.73 gears yield better gas mileage on the highway versus the 4.10's?

discuss.

 

[CDW6212R]

Weigh your wheel/tire. If the weight is under about 60 pounds, I'd keep the 3.73's for a while. Most big tire problems come from the extra weight, not the actual diameter.

Some wheels are very heavy, some are not. I lost a bunch of performance, and efficiency, when I changed to 30.5" tires. My problem isn't the 29/30.5" diameter change. My problem is that I went from a 58.5 pound combo, to a 72 pound wheel and tire. I have 3.73's, and 4.10's would probably be best for mine now.

The difference between 3.73/4.10 isn't much. Do some before and after testing. If you can't find a huge difference, the weight change isn't much, then stay with what you have. For raw acceleration, at the expense of fuel mileage, you might end up wanting around 4.30:1, higher gears. Good luck,

 

[Cali' Explorer]

Another thing to keep in mind isn't just the speed you are travelling but how you drive. For example, if you sit at 60 MPH with cruise control and never go up any hills or accelerate around people, definitely stick with 3.73's. If, on the other hand, you are constantly speeding up, passing people and slowing down or climbing lots of steep hills, the 3.73's will require your X to do a lot of shifting back and forth to keep in the power band. If I'm doing a lot of minor passes, I would rather run the 4.10's and not have to downshift as much as downshifting wastes a lot of power, plus, your engine typically revs way up before it shifts.

If you drive like many of us do (IE: More or less lead footed), I'd say you're better of with 4.10's either way... keeping the engine more in the power band. As a matter of note, in my 5.0 AT X, I get better mileage most of the time driving with O/D off on the freeways because the Transmission isn't hunting all day. Plus, while it is in a higher RPM band, the engine isn't actually working as hard to keep me at speed because the flywheel and driveline is running at higher speeds. The only time I drive with O/D on is if I'm on a long drive (2 hours+) and I plan to put it in Cruise Control and leave it there the whole trip.

Just some other things to add onto CDW's post.

 

[davidmmm69]

goo d advice. even for me.
thank

 

[unclemeat]

First it is called kenimatics, and it is really engineering more than science.

What you refer to as gears is the axle ratio. The axle ratio is the number of revolution the pinion gear must rotate to rotate the ring gear on time. Or the ratio of the driveshaft to the tire. The purpose of this gear reduction is to provided a mechanical advantage for the engine, very similar to using a longer wrench to free a stuck nut.

Changing axle ratio gears is typically done to compensate for larger diameter tires. Fuel econemy and acceleration do not go together well. Our engines have different RPM ranges that offer the most torque, horsepower and effiency. Axle ratio gearing is typically intended to maximize one of these RPM ranges.

A lower gear ratio (higher numerically) will provide greater torque and acceleration, however the engine must spin faster, fuel econemy is lost and top speed is reduced.

With a 32-inch tire a 4.10 axle ratio is likely the most effecient. There is not a lot of difference betweem 3.73 and 4.10. But if you are increasing tire size fuel econemy will suffer due to additional rolling resistance and additional power being required to spin the larger tires.

 

[rookieshooter]

I think if you do the math ............ effective ratio/ equivelant ratio that a 32" tire and a 4:10 go hand in hand and will give you about the same equivelant ratio that you had with the 3:73 and stock tires. Everyone here gave some good advice. About the weight of the tire combo. I do like the idea of the lighter vs. heavier. All weight above the springs is "Sprung weight everything below springs is Unsprung weight" In most cases, especially racing, try to keep all unsprung weight on the light side. Generally all unsprung weight is dead weight. Your shocks and suspension can handle all the above sprung weight but can't do much for anything below.

 

[Cali' Explorer]

Unclemeat-
I do have one slight difference of opinion in terms of efficiency. I do agree that if you are totally cruising (IE: Not using the pedal at all), that a lower axle ratio (in this case 3.73) gives you better mileage. BUT, at the same time, isn't your engine most efficient at the point where it reaches the highest HP rating? I mean, assuming that you do not have it floored, keeping the engine in its peak RPM range means it is putting out the most power and thus requiring you to "nudge" the engine less to keep you at speed.

I could be totally wrong here, I deal mostly with efficiency in electric motors which generate their highest torque at stall and most efficiency/HP at mid range RPMs, but from everything I've ever read, peak efficientcy and peak HP go hand in hand. That's why you get the peak HP at that point, the motor is running the most efficiently it can, using as much of what is available to it as possible to produce the maximum HP.

Again, I could be totally wrong, and in reality, keeping the engine in its peak HP range by pedal is very difficult (especially with the urge to give it just a little more). Maybe Aldive can chime in here since he is the mileage king?

 

[CDW6212R]

Steven, you are closer here to being able to understand the real trade offs(compromise/efficiency) than the others.

The most efficient RPM in an engine is where you want the engine to run most of the time. That's is usually somewhere in the 2200-3200 RPM range. Before overdrive transmissions, engines were always run faster, or above the ideal RPM's. The more high speed, low throttle cruising that is done, the higher the ratio can be. Some of the Corvettes run in the 1600-1800 RPM range on the highway.

With newer overdrive transmissions, it is very possible to get the RPM's too low, which is inefficient. That's the subject here. The compromise is between the best RPM, the gearing to achieve lower cruising RPM's, and the actual RPM usage. If the RPM's never changed, or were the same for every person, then the answer would be easy.

The best gear ratio will be different for everybody, given identical vehicles, assuming different driving styles. These 3.73:1 and 4.10:1 ratios are close to best for most people. The difference needs to come down to driving habits, and rotating mass.

Much heavier wheels/tires will kill gas mileage, the acceleration to any speed will take more gas. For a cruising vehicle, a highway gearing is more important. For a bunch of accelerating from a stop, to any speed, higher gear ratios are better(4.10, 4.30, etc.) Regards,

 

[TheFox88]

Does anyone know which RPM will yield the best power or mileage? Also I've heard that the lower (numerically) the gears are, the better the gas mileage. By putting on larger tires you are effectivley changing the gear ratio to a lower (numerically) gear. Now, if the weight of the new tires/wheels is not much different from stock, wouldn't that allow you to get an increase in mileage? Or am i completley wrong here?

 

[CDW6212R]

My first post was a good general idea. As you raise the rotaing mass of the wheel/tire, you will need a higher ratio diff. If you keep the weight down, then the stock range ratios will be better for fuel mileage.

As you drive harder, or accelerate more often from a stop, you will need a higher ratio diff. If you drive cruising much more often, then the stock range ratios will be better for fuel mileage.

Some people who do a great deal of their driving on highways, cruising, can gain mileage by lowering their diff. gear ratio(3.27:1, or 3.55:1).

My 91 has 3.27's and my 93 has 3.73's, the fuel mileage is about the same on the highway. On my mail route, the 3.27's are netting me less than 9mpg. The 3.73's have given me right at 10 mpg for years.

Consider your actual driving style, and usage. Combine that with these recommendations, and choose one, it's not set in stone. Good luck,

 

[unclemeat]

An internal combustion engine has some different physics than an electric motor. Air and oil flow (fluid mechanics) and heat transfer (thermodynamics) are the goverening equations. More air flow is only more effecient with laminar air flow. Turbulent air flow is less effecient. I do not have a factory spec horsepower verses fuel consumption graph but CDW's info is good. Individual driver habits and road conditions have a big effect. Balencing all of these conditions is engineering. Each change changes the entire system. When modifing driver prefernce and performance objective must be considered and that isthe best way to make you decision.

 

[rookieshooter]

What CDW said about not being set in stone. The best example I can think of is NASCAR Take the same track which is the main factor, put 42 of the best stock car drivers in the world and the best crew cheifs. Do they all run the same gear set NO! Part of the crew cheifs job is to match HIS driver based on his style of driving to the gears. Some driver may go into the turn deeper before letting off so they may have a lower gear set so the engine rpm's can help decelaerate the car and then give it more gas thru the apex to help pull it out of the turn hopeing to pass some one along the way. some may ride the high side while other stay low. Some may want more top end to draft so they would want a higher numerically gear and try to pass on the long straightaway. (hate those restrictor plates) My style of driving and the terain that i live is a mix of valleys and hills up to 3500 feet with winding roads. I personally can not stand to be behind some one doing 5 or slower mph's then the speed limit on these winding hilly roads so I would pick the 4:10 gear over the 3:73 givine the same tires,just so I can pass in the short distance that I have before the next turn. But that's me

 

[Cali' Explorer]

Well, here's the real question, Zero, do you already have the tires or are you "Planning" on getting them? I couldn't tell by your post whether or not you actually had them yet or not.

My suggestion would be, if you don't, get them and put them on and see how it drives. If you find you are constantly bogging the engine and having to downshift (whether manually via the stick or by giving it more gas), you should look into gears. If you find that only every once in a great whiel you are having to really mash the throttle to get the X to where you want to, you would probably be fine with what you have.

I know I need at least 4.10's in my X because, even with stock tires, I find myself having to mash the pedal too much (of course, a complete overhaul will help most of that too), but that's ultimately the point. You really shouldn't go spend the money on the gears because CDW or I think you need them, you should drive it yourself and decide what you like.

Heck, you may want to go ene steeper than 4.10:1 after getting the 32's on, who knows (although, for mileage sakes, I doubt this would be the case).