Axel Ratio and MPG

[SKIDUDE]

Does anyone know how much of a difference the axel ratio makes? I bought my 2005 Trailblazer in Sept off the lot with the 3.73 ratio. I asked the salesperson if there was much difference from the stock ratio and he said "maybe a few miles per tank, but not many". Of course 2,000 miles later I notice how much my mileage sucks (like most everyone else on this forum) and I was wondering if the axel makes any difference.

Thanks!

[TBSS]

It's not axle ratio per se, it's RPM at cruising speed, of which axle ratio is only one number in the function. Other factors are trans gear ratios, tire size, etc. Whatever axle ratio gives you between 1800 and 2000 RPM will be optimum for gas mileage. As soon as you start getting into 2200, 2300 RPM territory and above, you mileage drops precipitously. One reason the TBSS will have such poor highway mileage (besides the 6.0L motor of course), is that it will be cruising down the freeway at 2500 RPM.

Why don't you have a gander at your highway RPM and then report back to us?

[ylab]

Mileage is a mixed bag, I've seen some claim fantastic numbers and others horrible. The EPA numbers vary by 1 mpg highway between the axle ratios in the SWB trucks and drops from 21 Hwy for the 3.42 to 20 for the 3.73 & 4.10 gears witht he city rating of 15 (yet right) staying the same.

Seems that city or stop and go driving may actually benefit from the shorter 3.73 and 4.10 gears, but highway mileages nudges down around 1 mpg per gear level. The good news is that the mileage does go up a little after you accumulate some mileage on the engine, these babies seem to take several thousand miles to loosen up a bit. (Good for durability, not so good for mileage). Consumer Reports achieved 15 mpg combined for a SWB truck on their economy loop and someting like 13 mpg for the EXT/XL versions. Stock, I averaged around 14.7 mpg with the 3.42 gears in my 2002 Envoy.....right in line with CR. (Mostly suburban driving).

What I have noticed in four years of ownership is that its the stop & go which really eats up the gas on these trucks.....they are nearly 2.5 ton trucks and not a car or car-based cross-over. This is especially true when the engine is cold, where the mileage plummets. On the open highway, I've seen trips of just over 22 mpg if I keep the speedo below 70 and hold a fairly constant speed.

Currently, with my Borla, CAI and Wester's tune the electronics are claiming that I'm getting around 17.7 mpg on the current tank of gas......up from the 15.7-15.9 average before the Wester's tune was installed.

[dach95]

ylab, your summary puts alot of questions to rest, thanks.

the EPA 15 city milage is a crazy number to benchmark with, since it's based on the most optimal driving condition. If I reset my avg mpg and drive in the NYC STOP and go, by the time I get home, my avg gets as low as 7.7. Mix of highway, parkways..etc. here in NYC gives me approx 15. My drives out of town gives me approx 21. So, it all depends on the driving conditions and my light vs heavy foot. I now just stop paying undue attention to all this and just enjoy my drives.

[SKIDUDE]

It's not axle ratio per se, it's RPM at cruising speed, of which axle ratio is only one number in the function. Other factors are trans gear ratios, tire size, etc. Whatever axle ratio gives you between 1800 and 2000 RPM will be optimum for gas mileage. As soon as you start getting into 2200, 2300 RPM territory and above, you mileage drops precipitously. One reason the TBSS will have such poor highway mileage (besides the 6.0L motor of course), is that it will be cruising down the freeway at 2500 RPM.

Why don't you have a gander at your highway RPM and then report back to us?

At 65-70 MPH the RPM's with the 3.73 are about 2000-2100. My last highway trip at that RPM was about 19 MPG. No mods (yet). I would think that it would do better than that on the highway. I complained to GM and their answer was basically "too bad so sad" Unless the check engine light is on, the car is fine and the MPG is normal.

[Toolman]

In my mustang I changed from stock gears to 373s and my milage got better.. I want the 410s in my SUV..

[tblazed]

I have 3.42, a friend here locally has a 2002 identical to mine expept his has 3.73 gears. I can consistently get 22, up to 23 mpg at 70, RPM is about 1900. His will not do better than 19-20 mpg. His tach runs right at 200 RPM higher (10%) at the same 70 mph. If I did most driving in the city (which I do) I would opt for 3.73 or even the 4.10. More low end torque, but you sacrifice highway mpg. Knowing what I know now... I would probably go with 4.10 for the performance. I drive around town in 3 with my 3.42 because 1100-1200 rpm at 35-40 mph in 4th OD seems to lug it and it isn't responsive until it downshifts to 3rd anyway. I never put it in OD until I hit the freeway or at least 50 mph. Darn, I coulda had 4.10!

[ylab]

I'm with you Tblazed. If I were to do it again I'd still buy my Envoy, but look for one with the 3.73 or 4.10 gears since I do mostly slower suburban driving.

[ScarabEpic22]

Ive started to do more city driving, and Im glad I got the 3.73 gears. They really help on the pickup and when towing. That is the big reason I didnt get the 3.42, towing.

[Gaspar]

Hi,
I don't have anything to tow and have no plans..
How easy/difficult & cheap/expensive it is to replace the 4.10 my TB has?

I do mostly Highway driving on the TB.

Thanks
Gaspar

[ScarabEpic22]

Hi,
I don't have anything to tow and have no plans..
How easy/difficult & cheap/expensive it is to replace the 4.10 my TB has?

I do mostly Highway driving on the TB.

Thanks
Gaspar
I think I remember hearing in another thread that it would cost about $1000 for the gear swap, but it can only be done to 2WD TBs. On 4X4s, the front differential cannot have the difference in gears, and will lock up almost immediately when put into 4WD. But, on any 2WD TrailVoy, the gear change could be done fairly easily.

[TBSS]

Actually, you can do it on an AWD or 4WD vehicle, but you would have to swap out both gearsets, which makes it quite an expensive proposition.

[Rescue16]

I have a 2002 4x4 Trailblazer EXT. How can I tell what axle ratio the truck has. Looking at pulling a travel trailer with it and want to make sure it has enough grunt to do so

[jimmyjam]

check the RPO codes on the sticker in the glove box.
G80 Differential, locking, heavy-duty, rear
GT4 Rear axle, 3.73 ratio
GT5 Rear axle, 4.10 ratio
GU6 Rear axle, 3.42 ratio

[Rescue16]

JimmyJam

Thanks I got the 3.73 with the Heavy Duty Rear. Appreciate the fast answer.

[Dave]

The Trailblazer SS will also get lower gas mileage if it is AWD because you are constantly powering all 4 wheels.

Gears do make a difference. It all depends on what you like to do with your vehicle. If you love to pull, go offroading a lot or carry heavy things than the 3.73 or 4.11 gears would be better. However, if you take long trips the standard gears would be better overall.

I would highly recommend getting the limited slip rear dif though. That is about the only thing I wish was different about my 03 TB EXT.

[Toolman]

It cost me 400 for parts and labor to switch out my rear gears in my Mustang..
I know other places can do it for cheaper..