Bigger tires...need to regear?

[Jdogg]

guys...i return from Iraq in Nov and plan to put my Kore leveling kit on and some larger tires. I have a 2500 6.7L 3.73 Mega. I will be using stock wheels (17x8).

I was planning on putting on some ProComp Xtreme ATs 35x12.5x17. I only tow about 4-5K occassionally on the weekends atv'ng/camping.

I live in Colorado and we play in the mtns. With bumping up from the stock 265s to the 35s....will I need to regear the rear end to 4.10 or 4.30 to have the same power/torque/umpppf...or will it not be noticed?

Also - where can I have my speedo calibrated correctly for the new tires? Thank you.

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08 2500 MegaCab 6.7 CTD
"Crush It!"

[JasonY]

You'll be fine. The 3.73's are a good all around gear and 4-5k isn't even breaking a sweat for a 6.7.

Jason

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2001.5 Ext. Cab Short bed 4X4-315 BFG AT's/AEM HD intake/MBRP 4"/Silencer ring M.I.A/Raptor 100/new VP44/Mag deep trans pan/Edge EZ/RV275's/Quad Commander/Kool-Vue power/heated tow mirrors

[dodgezilla04]

no worries with 35's and towing 5k.

a smarty will recalibrate your speedo.....

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04.5 CTD with more mods than my wife knows about.
618hp/1442 tq BD spring 09.
www.nwdiesels.com for all of your diesel insanity needs

[ban89]

I have an 07.5 6.7l with the 68rfe auto with 3.73 gears and a Carli kit on 37''s.

You do not need to regear.

I actually picked up an extra mpg going to yoko 37''s from the 35'' toyo MTs I was running before. These trucks make plenty of torque to turn the tires so spend the $ elsewhere. I tow a 7k lb baja truck/trailer and a 10k lb sand toy trailer no problem. Usually keep in in 5th though to keep the rpms around 2k.

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2007.5 w/ 6.7l, auto, XRT 175hp, Suspended by Carli Bilstein 2.65'' and some Carli extras
Class 7 Prerunner 520HP Ford, equal length i-beams/3 link w/ King Coilovers/Bypasses/bumps

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Last edited by ban89 : 10-16-2009 at 06:58 PM.

[Jdogg]

do i have to buy the smarty or could I have someone just recalibrate it with one?

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08 2500 MegaCab 6.7 CTD
"Crush It!"

[titan0454]

buy one they are vin specific. Hey with the extra hp you won't have to worry about that regear.

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2006 Mega SLT - Carli Starter - Carli trackbar - Fox SS - Magnaflow custom dual 4" - K&N Intake - Magtec tran pan rear diff. - PacBrake airbags - Moto Metal w/ 35x12.5x20 Toyo M/T - Smarty Jr.

[dodge3500]

When you install larger tires it has the same effect as changing your axle ratio. For instance:

(stock tire size/new tire size) X gear ratio = effective gear ratio

(31/35) X 3.73 = 3.30

By installing 35" tires, you have effectively changed your axle ratio from 3.73 to 3.30.

To correct your axle ratio for 35" tires you would use the following formula:

(new tire size/stock tire size) X gear ratio = new gear ratio

(35/31) X 3.73 = 4.21

Since there is not a 4.21 ratio, you would select a 4.56 ratio to put your engine back into its efficient operating powerband.

When I installed my 37" tires, I regeared my truck, and it brought my performance right back to stock. It also increased my fuel mileage because the engine was not lugging all of the time. Regearing was the best performance modification that I did after installing the 37" tires.

I did a short writeup on my 4.56 gear install: PICS of 4.56 Gear Install

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2004 Dodge 2500 4WD, 5.9L high output CTD, 6 speed manual, 4.56 axle ratio, 9.25" Truetrac, 11.5" TracRite, 4" Rancho lift, 37" BFG KM2, H2 wheels, Magnaflow exhaust, Smarty JR

[Bochco]

Originally Posted by Jdogg

do i have to buy the smarty or could I have someone just recalibrate it with one?

Hey Jdogg, I just installed the Kore HP kit and Dueller AT Revo II's in a 315/70R17E. The speedo was about 4 mph off, but I noticed NO difference in towing power when towing our 7500 lb boat/trailer. The truck also felt no different at all in terms of acceleration without a tow. I did splurge on the Smarty Jr though and recalibrated the speedo until it matched my GPS. I also programmed in the low setting (adding 40hp) and the truck was much more responsive offline as well as passing on the highway. Haven't checked mileage yet, because I'm still enjoying the heavy foot. I always wanted the Smarty and the speedo cal gave me the reason. You can take it to the dealer as well and they can recalibrate it for you, though I'm not sure of the cost.
Bob

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04.5 2500 4x4 Laramie CTD, Kore HP suspension, Firestone airbags, Smarty Jr, Dueller AT Revo II's 315/70R17E

[Jdogg]

Bocho - any pics of your truck you would share?

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08 2500 MegaCab 6.7 CTD
"Crush It!"

[elephantrider]

you'll have no issue going from a 35 " tire to a 37 " tire.
i had no issues, actually got lower rpm's at hwy speed
when i moved up.

the smarty will not hold the speedo recal. once you
move back to stock the update will go away.

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jason
04.5 6sp qclb whiskey tango foxtrot, over

[Bochco]

Originally Posted by elephantrider

you'll have no issue going from a 35 " tire to a 37 " tire.
i had no issues, actually got lower rpm's at hwy speed
when i moved up.

the smarty will not hold the speedo recal. once you
move back to stock the update will go away.

Hey elephantrider!
I believe you're correct since it will only change the speedo setting once it's locked to your VIN and the only way is to program in a performance level but honestly, why would you want to go back to stock? The lowest setting is 40 hp and is perfectly safe for a stock engine. Just my 2 cents though, no disrespect meant.

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04.5 2500 4x4 Laramie CTD, Kore HP suspension, Firestone airbags, Smarty Jr, Dueller AT Revo II's 315/70R17E

[Bochco]

Originally Posted by Jdogg

Bocho - any pics of your truck you would share?

Jdogg, I don't have pics yet due to poor rainy weather here for the past week, but will post some in the next couple of days. I checked into what a friend paid at the dealer for the speedo program flash and he said it was $97.

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04.5 2500 4x4 Laramie CTD, Kore HP suspension, Firestone airbags, Smarty Jr, Dueller AT Revo II's 315/70R17E