rear end swap
 

now that I have been driving my LS1 swap with 460E. I am considering swapping out my 373 gear for something lower like a 308. I am hoping to improve gas mileage. Currently this truck will get up and run but the gas miles is bad. Interested in knowing how to go about getting one that bolts right in. Is there many different styles/configurations of how they bolt in or is a chev a chev? Will a Suburban bolt right in?

If anyone in the Houston area has a 308 and would want a 373 let me know.
Thanks

Re: rear end swap
 

Before I would swap gears I would check the tune on a chassis dyno. Maybe you already have. The good thing about the 4L60E is the low first gear.

Re: rear end swap
 

What is your rpm at 70 mph 2000-2200?? I would think those 3.73's would have you in the sweet spot.

Re: rear end swap
 

Your tune, your tire height, weight, rim size, stock height or lowered all play into MPG

Re: rear end swap
 

Not an LS swap, but my old Tahoe had 3.42s, with roughly stock size tires. Pretty much stock, but it had decent acceleration and top speed. Top speed was mainly limited to the stock motor and ECM. 70 was lil over 2k rpm.

Re: rear end swap
 

Like said.... Tire height plays a lot into gear ratio needs. Efficiency doesn't always mean the mildest gearing possible. What size tires?

Re: rear end swap
 

here's what I know as I am not up to speed with all the details as all I did was buy the motor and trans out of a 2001 Camaro and install it, Then sent the pcm to cal. for them to fix the harness. The only thing I know is I did tell them the raer end was a 308 when in fact it is a 370. I never sent it back for them to re set it. I didn't think it would matter that much. The truck is all stock except for a 2 inch drop in front(spindles) and 4 inch in back(shackles and mounts). Tires are factory steel wheels and 235/75/R15. don't know rpms I dont have a tach

Re: rear end swap
 

I would run it as is....if the speedo is off you can correct it later afaik.
I installed 3.42 in my 68 and wish I had the 3.73 instead.
The 700r4 4th gear is not really usable below 50 mph imho.

Re: rear end swap
 

Your tire size w/a .70 OD gear should yield 2100 rpm @ 70mph. Thats where you need to be. Anything lower will drop highway rpm's but they're in range of the 'small block' sweet spot.

Simply lowering rpm's beyond the sweet spot doesn't gaurantee better mpg's. The efficiency of mass going down the road will have an impact (these trucks are kind of a brick). Vehicle specific tuning is a must for your quest.

Re: rear end swap
 

thanks. So are you saying the fact I have the wrong info in my pcm for my rear end radio does or does not make a difference?

Re: rear end swap
 

Maybe not night & day difference but it can make a difference. Tuning can go a long way for steady rate highway cruise mpg efficiency.

Another thing.... I had an '02 TransAm. Took it on a trip & was trying to squeek out the best mpg possible. I was trying to keep the rpms around 2-2200/65mph thinking it would help. Turns out, the efficiency for the motor (amount of fuel consumed) was the same up to 25-2600. I got better mpgs @ 80mph (same fuel consumption but more mph). I ran the numbers on every tank of fuel for the trip there & back for better consistency on the numbers. Def didn't expect that.

Re: rear end swap
 

what was your highway and around town mpg?

Re: rear end swap
 

Kinda sounds like you need a vehicle specific tune. It seems you may have a simple generic "get it running" tune. My truck has a 1999 5.7 with a T56 and 3.73 posi. Guys that have a similar setup say it will be an absolute blast. I fit were me, I would put that moolah in a tune. It would be easier on the knuckles and possibly better results. If you change the rear gears, you will still need a tune.

Re: rear end swap
 

3800# f-body w/LS6, 4l60e, 3.23's, & 275/40-17's (25.6") yielded 16mpg in city; 20 on the local hiways.

For my trip mentioned above, it was 20mpg avg on the first 2 tanks. Once I increased speed & covered more ground/hour, we were seeing ~23/24mpg.

Re: rear end swap
 

If they set it up for a 3.08 gear your speedo is probably way off and not showing true mileage.

Re: rear end swap
 

I am running a cable manual speedo gear in the trans that is set for a 373. I am not using the ecm for the speedo. during the build I realized I had a 373 gear so I ordered the correct speedo gear the problem was I had already gotten back my ECM with the 308 setting completed.