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cramerica -> RE: Gears (4/24/2008 4:50:31 PM)
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Short Answer - If you have an automatic, check out 4.10's or 4.56's. If you have a manual, 3.92's or 4.10's.
LOOONG Answer:
Adjusting the gearing "a little" is tough, because you have discreet ratio choices. For example...the Chrysler 9.25" axle allows a 3.21, 3.55, 3.92...4.10...4.56 ratio, maybe a few under 3.21 for older cars. It boils down to the constraints imposed by the housing size, input location, and the nature of hypoid gearing.
Higher numerical ratios (eg. 4.10, 4.56) mean the driveshaft revolves a more per each revolution of the wheels.Therefore you get greater torque multiplication, which equals better acceleration and towing ability. Higher ratios also allow you to overcome the negative effects that larger tires have on acceleration and towing (see below).
However, high axle ratios also mean that the engine spins faster for a given ground speed. This can hurt MPG, and in extreme cases, limit the usability of the vehicle. Many off-road-only or military trucks are geared so low they can barely manage 55MPH on the highway.
Larger tires decrease the force exerted against the pavement for a given amount of axle torque. Think of the radius of a tire like a lever or wrench attached to the axle. Now think of a trailer, or a steep hill, or the rolling / aerodynamic resistance of your truck, as a hand pulling on the end of the wrench. Even if the force of the hand remains constant, a longer wrench will create greater torque on the axle. The engine has to fight this torque. Increasing tire size lengthens the lever, and weakens your acceleration and towing/hill climbing performance. Remember F=ma (Force = Mass X Acceleration)...it can be rewritten Acceleration=Force/Mass. Decrease the Force with larger tires, and your acceleration will shrink accordingly.
MPG's are trickier, but generally, higher ratios (within reason) help city MPG a bit and hurt highway MPG a bit. Remember F=ma. Since you spend alot of time in the city accelerating (stop-n-go), low gearing lets the engine produce less torque to get the truck moving. That equals less gas burnt. On the highway though, higher cruising RPM can push the engine outside its area of ideal volumetric efficiency, decreasing MPG. Again, some guys have experienced MPG gains in both cases; you can never make a guarantee.
A little multiplication and a pad of paper (or even better an Excel sheet) can help you peg the best gear ratio for your big tires, or the biggest tire you can run with your new gears. Below is an example for my truck:
2007 Dodge Dakota
3.21:1 Stock Axle Ratio
245/70 R16 Tires
1) Tire Radius : 245= Tread Width in millimeters ; 70 = (Side wall height/Tread Width)*100 ; 16=Rim Diameter in inches ; 25.4 mm per inch.
So, ((245*0.70*2)/25.4)+16 = 29.5" diameter ; 14.75" Radius ; Remember to *2 the sidewall height.
Say I wanted to run 275/75 R17 (not sure if the make those or if they fit, just an example)
((275*.75*2)/25.4)+17 =33.2" diameter ; 16.6" Radius.
So my tire size percent difference, or the approximate effect on acceleration / towing, is:
33.2 / 29.5 = about 1.125
So if I increase my axle ratio by that amount, I should feel little difference in acceleration and towing:
3.21 * 1.125 = 3.61
Well, they don't make 3.61 gears. But...3.55's are close. Maybe I should look into a slightly smaller tire, or jump to 3.92's.
Figure out the percentage difference in your tire / gear size , and apply it in the opposite direction to the other component.
There's alot of cool stuff you can do with ratios...determine your RPMS at any speed in any gear, figure out what gear you'll hit 60 MPH in, figure out a theoretical top speed, the steepest hill you could pull a trailer up without loosing speed...etc. It's all multiplication. I'll be glad to help out with other calculations or Excel sheets...but I've already written 3 pages and I'm running out of beer. Hit me up and I'll help where I can. I'm a mechanical engineer so this stuff is like crack to me. Pathetic? Possibly.
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