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3.73 re-gear

Sour_Squirrel

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Sour_Squirrel Enthusiast

Sour_Squirrel

Posted
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Posted
15 minutes ago, Donstar said:

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?

The tires are 35x12.5x20 Toyo AT2’s.  

I don’t care about muscle car like acceleration.  What I do care about is decent acceleration, and maintaining speed uphill, while trailering my boat. 

My 17 foot boat probably weighs just over 3000 lbs loaded.  Yes that’s well under the rated tow capacity but what people don’t understand is parasitic drag, not just weight, affects the towing performance. 

I am also planning on buying a bigger boat in the next two years.  Hopefully a 20-21 ft fishing boat.  The boat, motor and trailer I’m thinking about will bring the total weight to about 5500-6000 lbs. 

So that’s why I was asking if swapping in 3.73 (or 4.10) gears would be a good mod to do for towing and for decent fuel mileage (daily driver).  

 

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redwngr Senior Enthusiast

redwngr

Posted
Posted
9 hours 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?

Did you account for the difference in tire size when calculating the MPG?

(the truck doesn't 'know' it has a different size, so as others posted, the odometer will be wrong and the mpg on the dic is wrong.)

 

Since each rev of the larger wheel travels further than the OE tires, the truck has traveled further than the odometer reading would indicate -- so mileage not as bad as you are thinking if this hasn't been accounted for.

Z2Z Enthusiast

Z2Z

Posted
Posted
8 hours ago, Sour_Squirrel said:

The tires are 35x12.5x20 Toyo AT2’s.  

I don’t care about muscle car like acceleration.  What I do care about is decent acceleration, and maintaining speed uphill, while trailering my boat. 

My 17 foot boat probably weighs just over 3000 lbs loaded.  Yes that’s well under the rated tow capacity but what people don’t understand is parasitic drag, not just weight, affects the towing performance. 

I am also planning on buying a bigger boat in the next two years.  Hopefully a 20-21 ft fishing boat.  The boat, motor and trailer I’m thinking about will bring the total weight to about 5500-6000 lbs. 

So that’s why I was asking if swapping in 3.73 (or 4.10) gears would be a good mod to do for towing and for decent fuel mileage (daily driver).  

 

Simple answers for you:

 

Your MPG is not as bad as the truck is telling you because it is confused by change in tire size.

 

3.73 or 4.10 gearing will improve acceleration and towing performance 

 

You need to reprogram your ECU not matter what at this point. Start there.

 

Dont worry about your warranty. That big lift kit is going to void more of it than just a tune would.

 

None of these solutions will help get back the loss in braking performance.

~~~~~

Its not talked about often, but I believe that most of these “cool” lifted trucks that you see actually drive like dogs. I know that the ones that I test drove at the dealer (new) did. It’s all about just looking cool which is fine, you just have to be honest with yourself about it.

Donstar Senior Enthusiast

Donstar

Posted
Posted
17 hours ago, Sour_Squirrel said:

The tires are 35x12.5x20 Toyo AT2’s.  

I don’t care about muscle car like acceleration.  What I do care about is decent acceleration, and maintaining speed uphill, while trailering my boat. 

My 17 foot boat probably weighs just over 3000 lbs loaded.  Yes that’s well under the rated tow capacity but what people don’t understand is parasitic drag, not just weight, affects the towing performance. 

I am also planning on buying a bigger boat in the next two years.  Hopefully a 20-21 ft fishing boat.  The boat, motor and trailer I’m thinking about will bring the total weight to about 5500-6000 lbs. 

So that’s why I was asking if swapping in 3.73 (or 4.10) gears would be a good mod to do for towing and for decent fuel mileage (daily driver).  

 

I believe the 5.3 /6 spd/ 3.42  combination has ample power to handle a 6" lift and 35" tires.  These modifications will draw more power than stock but these trucks are designed to work.  We get use to running around in our unladen trucks and expect the same performance in all conditions.  I said earlier you may have to be more assertive with the throttle, and your truck may take the hills in a lower gear but you have ample power.  Re-gearing will return some of the stock-like responsiveness but I would be surprised if any lost mpg's will be regained!  The cost of conversion will not justify the results, imo.   Next tire change reduce the diameter by 1 - 2 inches and you will feel a surge in power!

KaptKyle Enthusiast

KaptKyle

Posted
Posted

Warranty was voided as soon as you put on the lift.  
Not even dealer installed lift kits keep the warranty intact. 

Grumpy Bear Senior Enthusiast

Grumpy Bear

Posted
Posted

3:90 gear would be 2% slower. 4:10 gear would be 3% faster on 35's. Since the 3:90 is a non-starter for 4X4's....4:10 it is. This will restore you 70 mph rpm to within 28 rpm. That still won't offset the weight of the larger tires flywheel effect and the aggressive tread pattern works against mileage as well. The AFM will not return to normal operation as it is a load sensitive device but it will be on more than it is now. 

 

A 4:30 gear adds 118 rpm at 70 mph and a 4.:56 235 rpm and a 13% increase in torque. Perhaps enough to offset tire weight. You can get your torque back but the MPG's are bye-bye. 

GRN69CHV Enthusiast

GRN69CHV

Posted
Posted

Picked up  a '13 2500HD with 6L90 trans (same ratios as 6L80), truck has 3.73's with stock 265/70-17 tires (31.60 diameter).  6th gear drops overall ratio to 2.48, which is slightly less gear than the 3.08 running in 5th.  I think with a 32-33" tire, 3.73 or 4.10 would be ideal. With 35" tires, 4.10 would be minimum.  More I drive the 2500HD with the 3.73's (keep in mind it weighs about 800# more than my 1500), the more I dislike that 3.08 gear in the 1500.  If I keep the 1500, it's getting either a 3.73 or 4.10 gear swap and that's with a 32" tires.  

Tauro_Z71 Enthusiast

Tauro_Z71

Posted
Posted

I have the exact Same issue, go with the 4.10,  there is also a 4.54 (or 4.56 i forgot what my mechanic told me)  available for the 5.3 6spd which is what im going to do in the spring time,  but gears wont change what the computer is telling you, which is why i am looking into tuners as well, still waiting for Bullydog to release their 2017 model

AgDoctor Senior Enthusiast

AgDoctor

Posted
Posted

Keep in mind that the higher numeric/lower ratio, that you go, the faster tour driveshaft will spin. If there is any imbalance in it that might not have been apparent at 75mph before, it will be present now. I love the low gears myself and I am if the opinion that a 3.08 has no business existing in a truck.


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sdeeter19555 Senior Enthusiast

sdeeter19555

Posted
Posted
Keep in mind that the higher numeric/lower ratio, that you go, the faster tour driveshaft will spin. If there is any imbalance in it that might not have been apparent at 75mph before, it will be present now. I love the low gears myself and I am if the opinion that a 3.08 has no business existing in a truck.


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Since tire size is increasing, that offsets the higher numeric gearing...truck will turn nearly the same RPM at the end of the day for the same given speed.

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Herc130 Enthusiast

Herc130

Posted
Posted

I’m running 5.3/3.42 with a 6in lift and 35’s. A loose rule of thumb is for every 1in of lift you loose 1mpg.

I now have 31,000 mi on my 2016 and bought it new with the lift and tires. My total average Mpg is 15.4 with 50/50 hwy/City. I have not done anything else to the truck but will consider a tune after 36,000 mi.

Grumpy Bear Senior Enthusiast

Grumpy Bear

Posted
Posted

My first car was a 65 Ford Custom with a 390, C6 and 3.0:1 gear on G60 15 Firestone. 70 mph was 2675 rpm. Piston speed 28 feet per second.

 

My trucks tires Bridgestone are  3.75 inches taller and the 6L80 gear box 6th gear is 0.67:1 instead of the C6's 1:1 ratio. To get like piston speed from my 4.3 V6 it would take a 5.38 gear. With it's current 3.23 gear it would be running 116 mph before I hit 28 feet per second piston speed. To slow the car to todays piston speeds I would have needed a 1.80 rear end! 

 

Unless your OLD you don't realize how tall these things are geared today. 

redwngr Senior Enthusiast

redwngr

Posted
Posted
14 hours ago, Grumpy Bear said:

My first car was a 65 Ford Custom with a 390, C6 and 3.0:1 gear on G60 15 Firestone. 70 mph was 2675 rpm. Piston speed 28 feet per second.

 

My trucks tires Bridgestone are  3.75 inches taller and the 6L80 gear box 6th gear is 0.67:1 instead of the C6's 1:1 ratio. To get like piston speed from my 4.3 V6 it would take a 5.38 gear. With it's current 3.23 gear it would be running 116 mph before I hit 28 feet per second piston speed. To slow the car to todays piston speeds I would have needed a 1.80 rear end! 

 

Unless your OLD you don't realize how tall these things are geared today. 

And it would have been running on crappy bias-ply tube type tires. 

 

 

KARNUT Senior Enthusiast

KARNUT

Posted
Posted

Tire technology was a little more advanced than that. I can remember going with my pops to get Michelin tires on an 65 Electra 225.


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