Chevy TrailBlazer, TrailBlazer SS and GMC Envoy Forum banner

1 - 20 of 22 Posts

·
Registered
Joined
·
20 Posts
Discussion Starter #1
The 2008 TB, LT with the I-6, I just bought has a 3.42 gear. I think my 2002 had a 3.73. Ironically, the 2008 gets slightly poorer (although acceptable) gas mileage. It also has less grunt from a standing start (obviously).

I am curious as to others experience with these gears, and what views are as to the best rear end gears for this vehicle. Thanks.
 

·
Registered
Joined
·
20 Posts
Discussion Starter #4
i have the 3.73 gears and mine is ok (the very few times i have got on it) but i also have nothing to compare it too. how many miles do you have; most people report that as the engine breaks in the MPG go up after the frist 1000 miles or so
My 2008 has 10,000 miles on it. I bought it slightly used with 8500 on it. I am putting in a K&N and have switched to sythetic oil.

Mileage is not bad. I am getting about 20 hwy, and about 16-17 city. The 2002 I had got about 21-22 hwy and 18 in town, so not much difference. I was just curious if there was some type of consistent pattern or preference between these two rear end gears, which seem pretty close to me.
 

·
Registered
2002 gmc envoy_slt
Joined
·
3,203 Posts
i have the 3.73 gears and mine is ok (the very few times i have got on it) but i also have nothing to compare it too. how many miles do you have; most people report that as the engine breaks in the MPG go up after the frist 1000 miles or so
:iagree: The I6 takes a long period to break in, even after initially seating and gaining clearance. I recall the first tank or two giving horrible gas mileage. When the motor was replaced (bad liner) in '03 or '04, same thing, horrible mileage the first tank or two. That's just to get it up to the 'decent' department, these things remain tight for 10-20k miles, before they actually start to loosen up. My engine now has around 30k and it still seems to be loosening up as time goes on, the Atlas motor design is tight with great oil control, but require some time or mileage to break in.
 

·
Registered
2007 chevy trailblazer_ls
Joined
·
261 Posts
The 2008 model is also rated at less hp than 2005-2007 models. I have 3.42 gears and they arent too bad. But i would rather have the 4.10s, i dont care if the gas mileage goes down a lil bit. I have also heard the 3.73s are the best all around gearing for whatever your doing. Good highway mileage with pretty good power from a stand still. I dont think they changed the tire size from 02 to 08, just the rim size. 02- 245/65/16. 08-245/55/17. correct me if im wrong but if those are the sizes they are pretty much the same size.
 

·
Banned
2003 gmc envoy_slt
Joined
·
7,190 Posts
Adding that extra inch in rim size adds to the unsprung weight. It could have an impact on mileage and acceleration. :m2:
 

·
Registered
2002 chevy trailblazer_ltz
Joined
·
162 Posts
Tire size

There are a couple different sizes, but the stock 17 is 245/65. The only different size as far as I know in later models (other than SS of course) is 245/60-18. Even still, the difference in unsprung weight of a few pounds shouldn't be enough to hurt mileage. Give it some time to break in, and oh by the way, 20 mpg isn't exactly terrible. For us at least.
 

·
Registered
Joined
·
20 Posts
Discussion Starter #11
Thanks for all the replies.

The 02 was a loaded LTZ and the 08 is a loaded LT, so there is almost no difference as to equipment. The one difference is wheel size. The 02 had the 17" wheels, and the 08 has the 18" polished aluminum wheels (which look really sharp by the way). The larger wheels, combined with the 3.42 have this thing running about 1900 rpm at 70 mph.

And, I agree, 20 mpg hwy is fine for a vehicle this size, weight, and versatility.
 

·
Banned
2004 gmc
Joined
·
26,181 Posts
...The one difference is wheel size....The larger wheels, combined with the 3.42 have this thing running about 1900 rpm at 70 mph.
RPM isn't proportional to wheel rim diameter (16, 17, 18, 22) but the final outside diameter of the tire mounted on the rim, combined with the gear ratio. It's the tire that touches the ground after all, not the wheel. Since larger wheels almost always get fitted with tires with less sidewall, the final outside diameter of stock equipment is around 29", no matter what wheel option. Just want to clarify this common misconception.
 

·
Registered
2002 chevy trailblazer_ltz
Joined
·
162 Posts
:iagree:with you Roadie in that the outside diameter will be about the same in just about every factory offering, but one thing as well to consider is the section width. Most of the time people here upgrading wheel and tires go to a wider tire, which would have an effect on rolling resistance, thereby affecting mileage. There's a very distinct possibility that I'm wrong, but a change from 245 to 265 would mean an increase of approximately eight percent in width, meaning an eight percent increase in rolling resistance, leading to an eight percent decrease in mileage. If one were getting let's say twenty with stock and went to 265s with the same overall diameter, the result would be a little over eighteen mpg. Which seems about right to me.
 

·
Banned
2004 gmc
Joined
·
26,181 Posts
...There's a very distinct possibility that I'm wrong, but a change from 245 to 265 would mean an increase of approximately eight percent in width, meaning an eight percent increase in rolling resistance, leading to an eight percent decrease in mileage....
A couple of issues with that thought. A wider tire at the same internal PSI would have about the same contact patch area, so the rolling resistance wouldn't necessarily go up the same percentage. And mileage is a combination of rolling and wind resistance (assuming a steady speed), so the effect would be a bit less also.

I certainly lost a bunch of mileage from larger MT tires, from all sorts of factors including a loss of good aerodynamics from the OEM front bumper and underbody flow.

Here's some estimates from http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=29

During stop-and-go city driving, it's estimated that overcoming inertia is responsible for about 35% of the vehicle's resistance. Driveline friction is about 45%; air drag is about 5% and tire rolling resistance is about 15%.

Overcoming inertia no longer plays an appreciable role in the vehicle's resistance during steady speed highway driving. For those conditions it is estimated that driveline friction is about 15%; air drag is about 60% and tire rolling resistance represent about 25%.
 

·
Registered
2002 chevy trailblazer_ltz
Joined
·
162 Posts
I certainly lost a bunch of mileage from larger MT tires, from all sorts of factors including a loss of good aerodynamics from the OEM front bumper and underbody flow. [/I]
I would think the type of tire would have a great deal to do with rolling resistance no matter what size. I would think that say, a 245/75-17 M/T tire would have more rolling resistance than say, a 265/65-17 A/T or maybe even a 285/60-18 H/T due to tread pattern and construction. Assuming even air pressure and everything. The most important thing is one's right foot. A properly driven off road vehicle can get better mileage than a stocker. It's all about the driving style.
 

·
Registered
2007 gmc envoy_sle
Joined
·
112 Posts
The 2008 model is also rated at less hp than 2005-2007 models. I have 3.42 gears and they arent too bad. But i would rather have the 4.10s, i dont care if the gas mileage goes down a lil bit. I have also heard the 3.73s are the best all around gearing for whatever your doing. Good highway mileage with pretty good power from a stand still. I dont think they changed the tire size from 02 to 08, just the rim size. 02- 245/65/16. 08-245/55/17. correct me if im wrong but if those are the sizes they are pretty much the same size.
He used to have an '02 though which was rated at 270 compared to the '08 which is 285. Maybe his '02 was a freak...tq mgmt settings could be more conservative now? Who knows...
 

·
Registered
2004 chevy trailblazer_ls
Joined
·
182 Posts
I dont know about mileage differences but a friend of mine has an 02 and mine is an 04 both ls and only difference is the gear. mine is 3.42 and his is 3.73. Mine is a turd compared to his. I only get about 18mpg on the highway. Not sure what he gets but it deffinatly pulls better. We have the same tires too. bridgestone revos
 

·
Registered
2002 chevy trailblazer_ltz
Joined
·
162 Posts
I have considered "upgrading" to 3.73's from my current 3.42's, but I can't seem to justify it. Highway mileage is a pretty consistent 22, and if I get into it around town or getting onto the freeway, the power is just fine. I have never not been able to tow something I looked at and said, yeah, I can tow that. Of course, if I had the 3.73's now, I would probably be saying the same thing about them - don't want to give up my power, etc.
 

·
Registered
2007 chevy trailblazer_ls
Joined
·
261 Posts
He used to have an '02 though which was rated at 270 compared to the '08 which is 285. Maybe his '02 was a freak...tq mgmt settings could be more conservative now? Who knows...
Well if it was an LTZ model, chances are high that he had 4.10 gear ratio. I think the 4.10s were more common on LTZ models than any other models.:undecided

Oh and :iagree: 4.10s :drool
 
1 - 20 of 22 Posts
Top