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Old 10-12-2012, 10:26 AM   #1
Mike
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Default Picking the Right Gear for Your Tire Size

The Quick and Dirty:

Your have increased the size of your tire, what gears should you buy?

Old Tire Size 31
New Tire Size 35
Current Gear Ratio 3.73

New Gear = (New Tire Size / Old Tire Size) * Current Gear Ratio

4.21 = (35/31) * 3.73

Since you cant buy 4.21 gears, it would be advised to purchase 4.56 gears. It is advised to round to the next lower ratio to account for increased rolling resistance, and reduced aerodynamics from being lifted.

Method 2:

This method is a bit more favorable as it takes into account a target RPM in your engine's power band for a given cruise speed.

New Tire Size 35
Target RPM 2500 RPM
Trans. Overdrive Ratio 0.70 (4L60E)
Highway Cruise Speed 70 MPH
Correction Factor - 0.8

New Gear = (Target RPM * Tire Size)/(Cruise Speed * 420.38 * OD)

4.24 = (2500 RPM * 0.8 * 35)/(70 MPH * 336.3 * 0.70)

In this instant you would use a 4.56 gear. Note that this calculation has a correction factor that provides a little insurance for passing, hills, towing, hauling, etc.. I used 0.8 which should be about right for most engines. If your engine makes A LOT of torque at your target RPM, you can use corrections factors of 0.7, 0.6, 0.5 etc... Also, this calculation assumes that a lockup stall converter is used in automatic transmission applications.

Here is a good link with some very good calculators. Bear in mind that no corrections are applied with these calculations.

http://www.bncoffroad.com/ratio/
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Old 10-12-2012, 10:44 AM   #2
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nice write up!
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Old 10-12-2012, 11:59 AM   #3
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1:1 ratio with overdrive off. Green is better fuel mileage orange is for little more torque while still gettin decent mileage
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Old 10-12-2012, 12:00 PM   #4
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And obviously the numbers are rpms that are highlighted
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99 gmc z71 5.3L-9" rcd lift- cam-tune-exaust-cai-4.88 gears- rpm 80e---Sold!!
2003 z71 4.8L- tune-exaust-cai-rpm 80e- interior mods
2002 cadillac Escalade 6.0- procharged awd- fully loaded- audio system---sold
01 tahoe- 22" welds. Front/back lights swapped. S3 tuned. Pretty stock for now.
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Old 10-12-2012, 12:03 PM   #5
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Is that using a 3spd tranny or 4spd tranny? Alot of the charts used a TH350, TH400, C6, or TF727 trannies back a few years ago.
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Old 10-12-2012, 12:51 PM   #6
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4 speed. Its basically the rpms in drive. 4th gear
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99 gmc z71 5.3L-9" rcd lift- cam-tune-exaust-cai-4.88 gears- rpm 80e---Sold!!
2003 z71 4.8L- tune-exaust-cai-rpm 80e- interior mods
2002 cadillac Escalade 6.0- procharged awd- fully loaded- audio system---sold
01 tahoe- 22" welds. Front/back lights swapped. S3 tuned. Pretty stock for now.
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Old 10-12-2012, 12:54 PM   #7
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I had 4.88s with my 37s and was turning 2100 rpm at 65mph in overdrive so thats prollu perty accurate.and actually i guess it could be for a 3denali spd tooif in third gear its 1:1
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99 gmc z71 5.3L-9" rcd lift- cam-tune-exaust-cai-4.88 gears- rpm 80e---Sold!!
2003 z71 4.8L- tune-exaust-cai-rpm 80e- interior mods
2002 cadillac Escalade 6.0- procharged awd- fully loaded- audio system---sold
01 tahoe- 22" welds. Front/back lights swapped. S3 tuned. Pretty stock for now.
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Old 10-12-2012, 12:57 PM   #8
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Posted twice. Dumb phone
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99 gmc z71 5.3L-9" rcd lift- cam-tune-exaust-cai-4.88 gears- rpm 80e---Sold!!
2003 z71 4.8L- tune-exaust-cai-rpm 80e- interior mods
2002 cadillac Escalade 6.0- procharged awd- fully loaded- audio system---sold
01 tahoe- 22" welds. Front/back lights swapped. S3 tuned. Pretty stock for now.
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Old 10-12-2012, 03:48 PM   #9
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So it doesnt take overdrive into affect. Those charts are nic, when everyone had 3 speed transmissions. Thats why the calculator is nice. Most OD transmissions are a .7 or so 4th gear so the RPMs will be lower than your chart.
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Old 10-12-2012, 04:48 PM   #10
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Yeah. Its without OD its 4th gear so in od u can account on it being lower. It was close on my truck but its 4 spd with OD
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99 gmc z71 5.3L-9" rcd lift- cam-tune-exaust-cai-4.88 gears- rpm 80e---Sold!!
2003 z71 4.8L- tune-exaust-cai-rpm 80e- interior mods
2002 cadillac Escalade 6.0- procharged awd- fully loaded- audio system---sold
01 tahoe- 22" welds. Front/back lights swapped. S3 tuned. Pretty stock for now.
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