33s, Gears, and MPG

[WildBill]

So I've done some searching on this but I don't have a clear answer...

I recently added 305/55/20s to my 09 suburban and I find that it likes to drop to 5th when going up even small hills. This is ******* MPGs. I have 3.42 gears.

Has anyone on here upgraded to 33s and then changed over to 3.73s or 4.10s? Did the steeper gears improve mileage any?

 

[swathdiver]

Let's play with some numbers. Your tire size mathematically works out to 33.21 inches tall. Stock tire (265-70-17) is 31.61 inches.

Stock tires, 70 mph, 5th gear is about 2,175 rpms and in 6th 1,700 rpms.
33s, 70 mph, 5th gear is about 2,075 rpms and in 6th, 1625 rpms.

Now put a 3.73 ring and pinion set with those 33s and she'll turn about 2,250 rpms in 5th and 1,775 rpms in 6th at 70 mph.

With a 4.10 gear and the 33s she'll turn about 2,475 and 1,950 rpms respectively.

 

[WildBill]

Yep that looks about right.

Has anyone swapped gears after a tire change? was there any improvement in milage?

(I know it's higher rpm, but it's also less downshifting to 5th and less load on the engine)

 

[Matt_]

I did, it definitely helps but don't expect stock gas mileage. My city driving gas mileage improved more than highway, but my highway mileage can tank pretty quick if I want to cruise at 80. It drinks A LOT less in the 2000-2200 rpm range than 2400+

Mines 6.0 with 6L90E, stock gears were 3.73, new gears are 4.56, running 35s... YMMV (literally)

 

[WildBill]

Cool.

Anybody done just a modest change to say a 3.73?

 

[TxDrewski]

IMHO, the money invested to do the swap is not worth the slight MPG increase your going to net. The cost (in my exp), would be better used on a tune, or just to put back in the fuel tank lol. i should note my gear swaps were all on 4x4s, so I had to change gears in the front and rear, so my cost was double what yours would be.

 

[WildBill]

I'm 4x4 too. What was your cost to re-gear? What ratio? How did it affect your mileage?

 

[Flexx]

So I've done some searching on this but I don't have a clear answer...

I recently added 305/55/20s to my 09 suburban and I find that it likes to drop to 5th when going up even small hills. This is ******* MPGs. I have 3.42 gears.

Has anyone on here upgraded to 33s and then changed over to 3.73s or 4.10s? Did the steeper gears improve mileage any?

What is your actual mpg?

Just switched from NBS tahoe on 35"s to an 07 suburban, all stock now, ordered a 2.5"/1.5" kit. Trying to decide what tires to go with. Like the look of 305/55/20 but could do 275/65/20 or 285/55/20

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[jlee]

What is your actual mpg?

Just switched from NBS tahoe on 35"s to an 07 suburban, all stock now, ordered a 2.5"/1.5" kit. Trying to decide what tires to go with. Like the look of 305/55/20 but could do 275/65/20 or 285/55/20

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I just went a 2in rough country up front and kept my autoride on my 09 Tahoe. I'm just a little higher in the rear yet when the shocks are pumped. I'm running stock 20's with BFG 285/55r20 which I think are perfect, no trim required but a noticable increase in width and height. I dropped from about 16.9 mpg with my Duelers to 15.7 mpg which I think is still pretty good for how I drive. I'm at 43 psi with the BFG's right now and still get a factory like ride and not much road noise. I have 3.43 gears by the way.

 

[Flexx]

I just went a 2in rough country up front and kept my autoride on my 09 Tahoe. I'm just a little higher in the rear yet when the shocks are pumped. I'm running stock 20's with BFG 285/55r20 which I think are perfect, no trim required but a noticable increase in width and height. I dropped from about 16.9 mpg with my Duelers to 15.7 mpg which I think is still pretty good for how I drive. I'm at 43 psi with the BFG's right now and still get a factory like ride and not much road noise. I have 3.43 gears by the way.

Wow. If i can get 15 highway I'll be pumped. I'm going to stay stock tires for now until they wear out or i score a deal. Will also keep stock rims. The stock 20" wheels are my favorite factory rims ever made.

My tahoe on the 35" tires got 13 highway.

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[jlee]

Wow. If i can get 15 highway I'll be pumped. I'm going to stay stock tires for now until they wear out or i score a deal. Will also keep stock rims. The stock 20" wheels are my favorite factory rims ever made.

My tahoe on the 35" tires got 13 highway.

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I did the same, 56k on my Duelers from when I got it. Got a package deal on new BFG's and some newer LTZ takeoffs.
View media item 27238View media item 27237

 

[swathdiver]

I'm at 43 psi with the BFG's right now...

I lost a lot of life by running mine that low and lower. BFG recommends 50 psi, the ride is not much different and they wear better. Check the tread depths at each rotation at least to make sure they're wearing proper. Mine are a different size than yours.

 

[jlee]

I lost a lot of life by running mine that low and lower. BFG recommends 50 psi, the ride is not much different and they wear better. Check the tread depths at each rotation at least to make sure they're wearing proper. Mine are a different size than yours.

Thank you. I've read a lot of mixed information on this in my searches. I'll consider going higher.

 

[Flexx]

Hey guys. Bumping this. Would like some more input on sizes. I installed 2.5/1.5 lift Saturday. Really like 305/55/20 but worried about mpg taking a huge hit.

Anybody on 285/65/20, 275/65/20? Thinking tall and narrow may not ding the mpg so bad.

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[Tozan]

I am running 34.4 in tires with a 4 inch lift
The lift and tires killed the performance and MPG with stock gearing

With the stock 3.43 gearing I averaged 11 to 12 mpg.

I now have 4.11 gearing and average 16.8 to 18 mpg my next set of tires will be 35's to see if I can improve MPG a little more.

I think if you are running under 34 inch tires 3.73 might be a real good choice for good power and good MPG.

 

[Tozan]

I am running 34.4 in tires with a 4 inch lift
The lift and tires killed the performance and MPG with stock gearing

With the stock 3.43 gearing I averaged 11 to 12 mpg.

I now have 4.11 gearing and average 16.8 to 18 mpg my next set of tires will be 35's to see if I can improve MPG a little more.

I think if you are running under 34 inch tires 3.73 might be a real good choice for good power and good MPG.

I am now running 35's and MPG is about the same as before.

 

[swathdiver]

I am now running 35's and MPG is about the same as before.

RPMs wise, running 34s with 3.73s is near the same as the stock tire with 3.42s. Good choice IMO and my plans too; someday.