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-   -   Picking the Right Gear for Your Tire Size (http://www.indianatruckscene.com/forum/showthread.php?t=102)

Mike 10-12-2012 09:26 AM

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/

Devs93 10-12-2012 09:44 AM

nice write up!

rj2 10-12-2012 10:59 AM

http://i1152.photobucket.com/albums/...-40-33_296.jpg
1:1 ratio with overdrive off. Green is better fuel mileage orange is for little more torque while still gettin decent mileage

rj2 10-12-2012 11:00 AM

And obviously the numbers are rpms that are highlighted

Mike 10-12-2012 11:03 AM

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.

rj2 10-12-2012 11:51 AM

4 speed. Its basically the rpms in drive. 4th gear

rj2 10-12-2012 11:54 AM

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

rj2 10-12-2012 11:57 AM

Posted twice. Dumb phone

Mike 10-12-2012 02:48 PM

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.

rj2 10-12-2012 03:48 PM

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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