Advice or Input needed. 3.42s w/ "32" tires.

[NastyBuzzard]

I have a 2004 GMC crew cab/short bed 2wd. I just got this truck. It has 275/55/20s and has had the speedo corrected for this tire size. The tires are about 32" tall and I have a 3.42 rear gear.

I currently have the 3.42s and 32 MTs on my 87 4wd and I know from driving it that the rpms I am running are too low. The 87 has to try way to hard after OD kicks in.

I have noticed that the rpms while I am doing 60MPH in the 04 are only about 1500 and sometimes when I slow down to 45-50 they stay around the 1100 RPM range.

I have a feeling this is too low. I know lower rpm=better MPG but at some point the lower rpm will make the engine struggle and not be in its power range and then its wasting gas.

I have a 2001 z71 donor that I am using for a ls swap in my 87. It has a 3.73 posi (g80) rearend.

What do yall suggest? Run with the 3.42 open rear or swap in the 3.73?

The 04 will only see light towing, it is mostly a 4 door vehicle that all the family can fit in.

Here is a pic. Anyone ever seen these rims before? I haven't, I checked and they are a GM rim. Maybe off a SS or Denali?

[Mr-Jack]

I had 32's on my 01 5.3L and the 3.73's were perfect...I wouldn't suggest running lower than the 3.73's what motor?

And those rims look similar to the denali rims but without the caps.

[NastyBuzzard]

Originally Posted by Mr-Jack

I had 32's on my 01 5.3L and the 3.73's were perfect...I wouldn't suggest running lower than the 3.73's what motor?

And those rims look similar to the denali rims but without the caps.

It has the 5.3. Truck itself has 128k miles but I have paperwork where dealer replaced engine at 79k miles. It has plenty of power/torque and pulls great in tow/haul mode, it also has corvette servo. Just when crusing around at those low of rpms is what worries me.

I think 3.73s would be great for it, however dont want to kill fuel economy either. Plus this truck mostly will see country highways 60mph and rarely see the interstate.

[Mr-Jack]

with my 5.3 and 32's I was getting around 20mpg driving around town. on the freeways, I could get around 24 IIRC

[hopped up]

I have 4.10 in mine with 35s , I get 16 mpg. But I know with3.42 it would be a dog

[NastyBuzzard]

Originally Posted by Mr-Jack

with my 5.3 and 32's I was getting around 20mpg driving around town. on the freeways, I could get around 24 IIRC

What is your truck? What size rim? I know it is getting pretty decent mileage now. Currently over 300 miles traveled on a tank and still have well over 1/4 tank.

I just worry that the low rpm is making engine/trans work harder. I don't want it to hurt anything. It currently has the shallow trans pan w/ no external cooler. I'm assuming its cooled through radiator. I am wanting to add the deep pan/filter and run an external cooler for trans for when I finally start towing loads of material to build my shop next year.

Hopped up, I know I could never get away with running 35s. Plus it is 2wd so no real need to. My 87 is 4x4 so it has that part covered. I didn't want to really put the 275/55s on, i was thinking about doing 255/50s all the way around but in the end I wanted more meat on those 20s b/c of all the potholes/bad roads around here.

Few years ago I had a 01 single cab step side sunset orange metallic 1500 2wd silverado w/ 4.8 and I am guessing 3.23 gears. I could squeeze out 23 or a little better with it on highway and being real easy on her.

[Mr-Jack]

That truck is long gone, but I was running some heavy 20" rims at the time so I know there was room for improvement. but I never drove trucks for gas mileage purposes. I drive trucks because guys are supposed to where I come from. I don't shop for a truck because of the MPG.

My 02' that I still own has the 8.1L/Allison combo on 33's and 4.10s and I get 10mpg even if it's parked out back.

My 05' denali has a 6.0 w/3.73's and 32's and I get about 18-20 mpg