3.73 re-gear

[Sour_Squirrel]

A couple of months ago I installed a BDS 6” lift and started running 35” Toyo AT2 tires.  

The truck drives like a dog now.  I know it’s because of the bigger diameter tires but I didn’t think it would make that big of a difference. I was previously running 285/55/20 Toyo’s And the truck felt peppy.  

Right now the truck has the factory 3.42 gears. If I re-gear the front and rear differentials with 3.73 gears will the truck drive like it did before the swap to 35” tires?

I use my truck as a daily driver as well as towing a 17 foot fishing boat.  

My mileage went down from 18-19 mpg to about 12-15 mpg average.  I don’t drive with a heavy foot and I notice it doesn’t go into V4 mode that often anymore. 

Would 3.73 gears improve my mileage?

Also I’ve seen some mention about programmers that compensate for bigger diameter tires. Do these programmers just recalibrate the speedometer or do they also do some sort of reprogramming of the engine computer?

[tnchevy]

3.73 would help a little bit, mainly driving around town, pulling out from red lights, etc.  With your setup you may even need to go to a 4.10.  Have you had your trucked juiced up with a tuner?  You really should have done some research before you decided on a 6 inch lift.  They look cool but serve no real purpose and of course your mpg's were going to take a major hit, you lost a bunch of aerodynamic properties and your truck is having to turn a much heavier wheel/tire combination plus I am sure the tread is much more aggressive than the stock tires.  You have to pay to play.

[sdeeter19555]

Skip 3.73s, go 4.10s...

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[truckguy82]

1 hour ago, Sour_Squirrel said:

A couple of months ago I installed a BDS 6” lift and started running 35” Toyo AT2 tires.  

The truck drives like a dog now.  I know it’s because of the bigger diameter tires but I didn’t think it would make that big of a difference. I was previously running 285/55/20 Toyo’s And the truck felt peppy.  

Right now the truck has the factory 3.42 gears. If I re-gear the front and rear differentials with 3.73 gears will the truck drive like it did before the swap to 35” tires?

I use my truck as a daily driver as well as towing a 17 foot fishing boat.  

My mileage went down from 18-19 mpg to about 12-15 mpg average.  I don’t drive with a heavy foot and I notice it doesn’t go into V4 mode that often anymore. 

Would 3.73 gears improve my mileage?

Also I’ve seen some mention about programmers that compensate for bigger diameter tires. Do these programmers just recalibrate the speedometer or do they also do some sort of reprogramming of the engine computer?

You increased the size of the tire without calibrating the ecu. Which means your truck thinks it went 10 miles but you really went 12 miles. That’s why your mpg went so far down on the DIC. It’s not actually down that much, it’s just calculating it wrong.

[CDNSS]

Yes for sure 4.11 if it a 4wd  I went to 33” tires and 4.11 & 6 spd 3.90 wpuld have been perfect to maintain 3:42 and stock tire ht rpm. But not available for 4 wd transfer case With 35 4.11 gor sure go online and find a rpm  calculator and plug in different tire hts to see what gets you close to your old cruise rpm with 3:42 and stock tires  pbly 2000 at 70 mph  or what you want 

 

what yr truck ?

[Donstar]

According to online gear ratio calculators the 3.73 would bring you back to where you were before the tire change.  If you don't change gears you are still stronger than a factory 3.08.  You have changed the gearing of your truck but it will still perform fine.  You need to be more assertive with the gas pedal to get the same results as before.  Truckguy82 is correct and the money you save from not re-gearing will more than compensate for any further increased fuel costs!

[sdeeter19555]

Keep in mind that a gear calculator will show the RPMs with a given setup, but does NOT take into account the added weight or rolling resistance of tires or the increase wind resistance from the lift.

My rule of thumb has always been compare the RPM gear for gear, then go one step deeper to offset the parasitic losses. I have never been disappointed by doing so, and swapping gears is something you only want to do once because of cost.

Just my thoughts...

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[Grumpy Bear]

This is interesting. A mod I'm planning myself. Keep going guys! 

[Bikemobile]

Start with a programmer to tell the truck what size tire you have. That should adjust shift points and the speedo. See if that helps. 

[Sour_Squirrel]

My truck is a 2017.  I bought it in March of this year.  I haven’t done anything to the ecu and everything, other than lift and wheels/tires, is all stock.  

So if I go with 4.10’s would that actually bring my fuel mileage up?

I’m thinking because with the 4.10’s it be easier to get the truck going with less throttle, this getting better fuel economy. Is that a correct assumption?   As mentioned, I don’t drive with a heavy right foot. 

[Sour_Squirrel]

2 hours ago, Bikemobile said:

Start with a programmer to tell the truck what size tire you have. That should adjust shift points and the speedo. See if that helps. 

With my truck being less than a year old, would messing with the ecu (programmer) create any waranty issues with the dealer ?

[truckguy82]

36 minutes ago, Sour_Squirrel said:

My truck is a 2017.  I bought it in March of this year.  I haven’t done anything to the ecu and everything, other than lift and wheels/tires, is all stock.  

So if I go with 4.10’s would that actually bring my fuel mileage up?

I’m thinking because with the 4.10’s it be easier to get the truck going with less throttle, this getting better fuel economy. Is that a correct assumption?   As mentioned, I don’t drive with a heavy right foot. 

Did you even read the responses?

 

-You don't even know what your fuel mileage is. Your DIC is wrong

 

-spending $2-3k on gears for front and back for the purpose of saving 1-2mpg, (if it actually saves anything) is the worst idea of all time. It would likely take you 25 years of driving to recoup the cost of the gears with your decreased fuel costs.

 

-Yes reprogramming it could potentially have warranty issues. Unlikely but possible.

[Bikemobile]

The warranty issues with that lift would concern me more. You have to recalibrate for those tires first. 

 

 

[sdeeter19555]

No amount of recalibration is going to offset larger diameter tires without changing the gearing...

And didn't the OP state he was towing a boat a couple times a year?

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[Donstar]

How heavy is a 17' fishing boat?  I believe OP's truck will be fine without re-gearing.  He has made a significant modification which has altered the performance from when the truck was stock.  If increased speed off the line is a goal then bigger wheels and a higher lift are not the right mods.   - OP, what size are your new tires?