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03:14:48 - 12/29/2024
Dakota Performance
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Jim
Dodge Dakota
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8/06/2004
18:27:35
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 Subject: Gear Alternative
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Message:
For you drag strip / towing people
.
I’ve been thinking about your gear ratio questions for performance. Lots of people want to change the differential just to go to the track and turn a lower ET. Also most of you have a problem with the traction off the line. Well, I’ve came up with a quick solution.
On my example with stock gearing and a 04 Dakota with 265x70 R16 tires and a 3.55 LS differential, you would probably not get out of 3rd gear on a 5 speed or 2nd gear in and automatic in the quarter mile.
Using simple mathematics, you can calculate a new tire height to compensate for a gear ratio change just to run and see how your truck performs in the quarter mile. Of course your truck speedometer will be off, but not the speed/timers that clock you at the track. But WTF, you can run your truck with the equivalent of 3.92, 4.11 or even 4.56 (which I recommend) gears. Just bolt on 2 wheels with the proper tire height and bump you’re there. It’s the same for towing. If you already have 3.92 or lower gears, at your request, I will re-calculate the needed tire height for you or send you the ‘factor’ to do it your self.
For my example, if your truck has 3.55 gears and 265x70 R16 tires, your tire diameter is 31.6”, the following applies.
Diameter Now__Diameter New___New Ratio
____31.6”_________28.6”________3.92
Diameter Now__Diameter New___New Ratio
____31.6"_________24.6”________4.56
I’m not going into the contact patch area ‘length of tire with asphalt’ with smaller diameter tires at this time because I would hope that you would either borrow or buy either 2 slicks or wide street tires to put on your trial wheels, because with the changes in torque, you will gain has a monster wheel turning ‘torque multiplier’ potential. The same as changing gears. You would want to go through the traps in high gear ‘not OD’ at near the rev limiter RPM. With 24.6” diameter tires and 3.55 gears, at 5200 rpms, you would be doing 107 mph. Probably won’t make it that far, but will kick ass on the 90/92 mph. This is an unbolt / bolt on 2 tire ‘ rear wheel drive’ quick fix for gear substitute.
Try it at the strip this weekend and let me know how it goes…Bet you seasoned veterans will be a few tenths less..
Jim
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youre wrong
Dodge Dakota
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8/07/2004
20:30:50
| RE: Gear Alternative
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Message:
you cantr subsitute tires for gears.
what do you do to think that.
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mcarthey
Dodge Dakota
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8/07/2004
21:56:26
| RE: Gear Alternative
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All of these formulas take into account the Gear Ratio and Tire Radius. Obviously they are linked. Jim, and others, I hope these formulas provide some use to you. I thought they were interesting... but math always has been, to me. :)
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Formula for MPH
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MPH = Tire Radius / 168 * Engine RPM / Gear Ratio
Example: What MPH at 6500 RPM with a 4.9 rear axle and 14 radius tire in 4th (1:1) gear?
MPH = 14 / 168 * 6500 / 4.90 / 1 = 111 MPH
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Formula for RPM
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RPM = 168 * Gear Ratio * MPH / Tire Radius
Example: Using the first example, what will be the RPM after shift from 3rd to 4th gear at 83 MPH?
RPM = 168 * 4.90 * 83 / 14 = 4880 RPM
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Formula for Gear Ratio
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Gear Ratio = Tire Radius * RPM / 168 / MPH
Example: Using the first example, what Gear Ratio is required for 120 MPH at 6500 RPM?
GR = 14 * 6500 / 168 / 120 = 4.51
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Formula for Tire Radius
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Tire Radius = 168 * MPH * Gear Ratio / RPM
Example: Using the first example, what tire radius for 110 MPH but at 6000 RPM with a 4.11 gear?
TR = 168 * 110 * 4.11 / 6000 = 12.7 inches
Note: Approx a 25" diameter tire.
Mark
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Tnjones
Dodge Dakota
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8/09/2004
17:40:19
| RE: Gear Alternative
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You're right about the ratio, but it will kill your traction.
Why do you think top fuel dragsters use tires taller than the car body itself? It creates a contact patch that is enlongated in the direction of desired friction. You could concevably get the same amount of rubber on the ground with greater width (as you suggest), but it won't work the same because your contact patch will be smallest in the direction of desired friction. That's why Indy cars use tires that are low and wide, good lateral traction for going around corners.
Dropping some height and going to slicks is definatley a plus (an old racers trick for accomplishing just what you suggest), but if you try to go from a 3:55 to a 4:56...
Let's just say I don't think a 12.7" O.D. tire will fit on my 17" wheels.
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Jim
Dodge Dakota
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8/10/2004
00:14:54
| RE: Gear Alternative
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Message:
Tnjones
Did you read the last paragraph of my post about contact patch area? I think the offset in 4.56 gears will greatly off set any traction limitations with good slicks with a somewhat stock engine....not in a dragsters class at all.....
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