Rear Axle Ratio Change

[Ben Wilbanks]

I have a 2004 Yukon XL 2500 with the 8.1 4 WD and 51K miles. The truck is used mostly for highway driving with light loads and some trailer towing. The engine is turning faster than I would like. I keep checking to see if it is in top gear. Needless to say the mileage is poor. Is it possible to change the 4.10 ratio to a 3.73?

Is this practical? The 8.1 isn't available in the Yukon anymore and this truck is excellent condition I want to keep it.

Ben

[hockeypro1411]

Yes. It's completely possible. Just buy 2 sets of 3.73 gears since you have a 4wd.

[svensk]

Don't do it. It will have ZERO affect on your mileage. The 8.1 is what it is.

[pm26]

What kind of mileage are you getting now? Before changing gearing, try experimenting with simple things, like inflating your tires 5 psi higher than normal, try different brands and octanes of gas, change your driving habits some, like accelerating slowly, using the A/C less, etc. Do you have oversized tires on the vehicle now? If the vehicle lifted? If yes, consider going back to stock to decrease the rolling resistance and wind resistance.

 

I tend to agree that given the 4WD and the big engine, changing the gearing to 3.73 may not accomplish much. Maybe going down to 3.42 ratio would be beneficial for fuel economy, but then you would be giving up some towing potential.

 

I remember that several people who owned Avalanche 2500 with the 8.1 liter V8 always complained about poor fuel economy (in the 10-11 MPG range).

[Z'06]

I have a 2004 Yukon XL 2500 with the 8.1 4 WD and 51K miles. The truck is used mostly for highway driving with light loads and some trailer towing. The engine is turning faster than I would like. I keep checking to see if it is in top gear. Needless to say the mileage is poor. Is it possible to change the 4.10 ratio to a 3.73?

Is this practical? The 8.1 isn't available in the Yukon anymore and this truck is excellent condition I want to keep it.

Ben

 

Possible? Yes.

 

Practical? No. - Too much time/expense/effort for so little change in gearing (especially on both axles!). If you want a noticeable change, go to 3.42. Your engine size will be fine with that ratio for towing the light loads you mention. Your highway MPG will rise a bit, but your city MPG will suffer some.

[Gregorio]

I was just helping a friend with the same problem in the same truck. His 8.1 has been mildly worked over and probably puts out just over 400HP at the crank. Along with heavier tires and wheels, larger fuel capacity, a winch, dual batteries and a boatload of gear, he is tipping the scales over 7200lbs. His mileage is 8-11 depending on how much highway it sees. With 4.10 gears and 31" tires you guys are spinning that 500" big block pretty damn fast. At first we thought that changing the gears would be the answer but after doing some math, it looks like to get the highway RPM down enough, 3.42 is needed. However, the engine will be working much harder around town with much less torque multiplication available.

 

He is now thinking about swapping the 4 speed auto for a 6 speed. On paper, it looks like the right choice. First gear is much steeper and will get the truck moving quickly with lighter throttle input and with much taller top gear, the highway RPMs will drop about 10%. If that is not enough, 3.73 gears with the 6 speed would give him even better top gear RPM compared to the 3.42/4 speed combo and still have more torque multiplication than what he has now.

[brainpause]

I've got 3.42's. Wanna trade?

 

We could put 'em on racks side by side and tell them to switch. All we pay for is labor. Mine's an 06 Z71 1500 4wd with about 42K on the odo.

 

Only drawback is that I hear it takes someone that really knows what they're doing.

 

Larry

[pm26]

I've got 3.42's. Wanna trade?

 

We could put 'em on racks side by side and tell them to switch. All we pay for is labor. Mine's an 06 Z71 1500 4wd with about 42K on the odo.

 

Only drawback is that I hear it takes someone that really knows what they're doing.

 

Larry

 

The gears in his 2500 series Suburban are physically larger than yours, so you cannot swap them. His ring gear is larger in diameter..

[mitchtazz]

anyone ever consider the under/overdrive transmission from gear vendors? make your 4spd an 8spd and keep all of your torque for pulling but add gears to help with fuel economy.

[05silvycrew]

3.73 gears will net you about 1 MPG strictly on the highway

3.42 gears will net you about 2 MPG strictly on the highway

 

figure a gear swap will cost you about $1000 (mostly labor) total for front and rear.

 

if you drive 12,000 miles a year at 12 miles per gallon you buy 1000 gallons of gas

if you drive 12,000 miles a year at 13 miles per gallon you buy 923 gallons of gas

if you drive 12,000 miles a year at 14 miles per gallon you buy 857 gallons of gas

 

best case scenario, if you net a 2 MPG average increase over an entire year you save 143 gallons of gas. we'll estimate $3.00 a gallon for round figures. that mean you save $429 a year on fuel. it takes you nearly 2 1/2 years just to break even.

 

I really don't think you'll like how a 3/4 ton will drive with 3.42 gears. too heavy of a rig and not enough RPMs to support it. In my humble opinion, you're better off saving your money, or investing it somewhere else.

[DoMDrty]

just get one of these for a daily beater and be done with it

[herkdriver007]

..or get larger tires to reduce the effective ratio of the rear end

[05silvycrew]

..or get larger tires to reduce the effective ratio of the rear end

 

taller + narrower tires = yes

 

but if you just get bigger tires, you've introduced a lot more rotating mass which will require your engine to work harder to turn the new bigger tire. you're RPMs won't drop signifigantly, about 125 for every 1" taller you go. they pretty much offset each other. a 1" taller tire drops 125 RPMs at cruising speed, but the engine has to work a little harder due to the lower RPMs and small increase in engine load to keep the new heavier tires rolling.

[csheatz]

my buddy has a 8.1/allison in his 2500hd eclb 4x4, he gets about 8-9avg. What can you do? Just drive it, you bought it with the big motor. Thats what I tell him!

 

Someone said they are getting about 400hp from the motor with light mods, what was stock on a 8.1? that doesnt seem like the "light mods" were really worth anything. My friends 8.1 is running gibson cat back, tune, and k&n fipk, with 34" tires and has a hell of a lot more power than my 5.3. I mean he can smoke those big 34's first and second gear until he lets off. from the butt dyno it seems well over 400hp.

 

 

 

EDIT:

 

The Vortec 8100 (RPO L18) is a V8 truck engine. It is a redesigned Chevrolet Big-Block engine and was introduced with the 2001 full-size pickup trucks. It retains the same bore centers as the old 7.4 L big-blocks, but stroke was upped by 9.4 mm to reach 8.1L (496cuin) for a total of 107.95 mm bore and 111 mm stroke. It is an all-iron engine (block and heads) with two valves per cylinder. Power output ranges from 225 hp (168 kW) to 450 horsepower (340 kW) and torque from 490 lb·ft (664 N·m) to 455 lb·ft (617 N·m). Vortec 8100s are built in Tonawanda, New York. The Vortec 8100 is the engine used in the largest Uhaul, their 26-foot (7.9 m) truck. GM stopped installing big block V-8's in the Silverado HD trucks, when the GMT-800 series was discontinued in 2007. 8100 marine engines range from 300 to 500 hp and come with a stand-alone ecm. The last L18 was manufactured in December 2009.

[brainpause]

The gears in his 2500 series Suburban are physically larger than yours, so you cannot swap them. His ring gear is larger in diameter..

 

There ya go. I knew there was a reason this couldn't happen.

 

anyone ever consider the under/overdrive transmission from gear vendors? make your 4spd an 8spd and keep all of your torque for pulling but add gears to help with fuel economy.

 

I read about it in the RV mags, but never considered it for my truck. I can't get the DW to let me drop the cash on 4.10's. If I could afford Gear Vendors, I'd already have 4.10's. But an interesting thought, nevertheless.

 

Larry