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md
Dodge Dakota
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10/07/2003
09:39:24
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 Subject: Using RPM/Gear Ratio Calculators
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Before installing my 4.10 gears I read posts from many on this site recommending the use of calculators to determine effects of tire sizes and/or gear ratios on RPM - calculators such as the one found at Randy's Ring and Pinion web site.
For some who tow and/or go offroad changes of 100+ RPM can be significant.
I've noticed that every calculator I have seen asks for "tire diameter" as one of the values (gear ratio and speed beeing the others) to be plugged into the formula to determine engine RPM.
The question/comment I have concerns which tire diameter figure to use - that which is supplied by the tire manufacturers spec sheet, or the measured diameter often referred to as ride highth (diameter as measured on the truck and under the actual weight of the truck). Depending on the construction of the tire and sidewall strength and vehicle weight, the diferrence in these two diameter measurements is most often AT LEAST an inch, sometimes more.
The reason I raise this point is twofold:
First, I am getting ready to buy a larger tire for my 2000 Dak, 4.7 and I tow a boat into the mountains of northern Idaho at least twice per year - given the torque curve of the 4.7, RPM's is an important issue.
Second, I have yet to see any dialogue on tire diameter at any of the sites offering calculators.
For my application, using the manufactuer's listed diameter for my stock tires VS using the "ride highth" diameter, the calculator renders about a 110 RPM diferrence - as I said, for some of us that is significant. (note: using the "ride highth" diameter comes much closer to duplicating my actual tach reading).
I'm wondering if any of you have run into a similar situation using these calculators? Have you experienced the same outcome as I have? Since there is a significant diferrence in the outcomes of the calculation depending on which diameter is used, why don't these sites address this issue?
Finally, which diameter do you guys use in making your calculations?
Thanks,
MD
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JZ
Dodge Dakota
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10/07/2003
11:56:33
| RE: Using RPM/Gear Ratio Calculators
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I would guess that measuring from the center of your axle to the ground with your tires properly inflated and your truck loaded as it would be for your trip would give you the most accurate calculations. Just double that measurement and use for the tire diameter in the calculator.
Good luck
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md
Dodge Dakota
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10/08/2003
10:42:38
| RE: Using RPM/Gear Ratio Calculators
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Well, I know how to get the ride hight measurement - the issue is do you use that measurement or the diameter from the spec sheet put out by the manufacturer?
I would contend the use of the ride hight measurement is, by far, the more accurate of the two.
BTW, ride hight is measured from the ground, through the axle, to even with the top of the tire (line of sight). Measuring it the way you suggest would introduce an error in that the distance from the ground to the center of the axle is less than the distance from the center of the axle to the top of the tire - remember, the weight of the vehicle somewhat "flattens" the tire where it rests against the ground.
MD
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JES
Dodge Dakota
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10/08/2003
10:57:10
| RE: Using RPM/Gear Ratio Calculators
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Twice the measured distance from the ground to the center of rotation (axle center) is the effective dynamic tire diameter. This should be used in all calcs for proper results. By the way, a calculator really isn't necessary. All other things remaining constant, if your gear ratio changes by 10%, so will your rpms; it's a linear 1:1 relationship. If more than one variable changes (tire diameter and gear ratio for instance), simply use both percentages as multipliers. Ok, back to my spreadsheets, my head hurts. :oP
Eric -- http://photos.yahoo.com/jes_96
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md
Dodge Dakota
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10/09/2003
09:04:12
| RE: Using RPM/Gear Ratio Calculators
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Jes - I contend that your method of measuring dynamic tire hight would mean that you assume that the two radiuses - ground to center axle and center axle to top of tire, are equal - they are not. The bottom tire radius resting against the ground and under load - and thus compressed - is less. In my case it is less by 1 inch.
That 1 inch must be accounted for, but not twice.
Am I missing something?
MD
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JZ
Dodge Dakota
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10/09/2003
11:49:51
| RE: Using RPM/Gear Ratio Calculators
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MD,
The tire is pushing the ground past the truck, always on the side that is flattened by one inch, in your case. The side of the tire that expands when it isn't loaded has no effect on the trucks movement. Think of it this way: you have two solid tires that don't flatten with the load, one the same diameter as your current unloaded tire, and the other is two inches less diameter (one inch for each side). You roll them both exactly one revoloution and measure the distance that they travel. The smaller diameter solid tire will move the same distance as the tire on your truck now.
Good luck
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md
Dodge Dakota
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10/09/2003
16:33:58
| RE: Using RPM/Gear Ratio Calculators
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Thanks, JZ and Jeff - I got it!!
MD
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