Gas Mileage And New Transmission

cdreed06 · January 9, 2011

Hello, I am new here and have a question. We have a 07 Silverado crew cab 4x4 new style (GMT900) 5.3 with the automatic. At about 72000 miles the transmission went out, it would not shift past 2nd gear. We took it to the dealer and had absolutely no problem at all with them, even got a rental truck for free. They got the new transmission in the truck within a few days. Here is where my problem starts. With the new transmission it seemed to run at higher RPM than it normally did at driving speed. With the original transmission it would cruise at about 70mph 1500 RPM. Now it was running 2000RPM I didn't think the lock up converter was working right so I called the dealer, they were very nice and said bring it in. They looked at it and ran all kinds of tests, on road and in the shop and found nothing wrong. So now my 17+mpg pickup is getting 15+ if I'm lucky and there is nothing wrong? Is there any suggestions? Could the new transmission have a lower final gear? Thanks in advance for the opinions.

f0st3r · January 9, 2011

What gears do you have? My 08 with the 5.3, 4spd w/3.73's runs at about 2100 rpm @ 70mph so that sounds normal, unless you have really high gears like 3.08's or something.

My old truck had the 4sp and 3.42's and hovered around 1800rpm @ 70mph.

cdreed06 · January 9, 2011

How do I figure out what gears I have? Also I was wondering why it would be different now with the new trans? Thanks

Stevens03 · January 9, 2011

The rear end gears wouldn't have anything to do with this. The transmission is the only thing that changed, so somethings off.

If you want to know what gears you have, check the glove box for the RPO codes for GU6 = 3.42, GT-4 = 3.73, GT-5 = 4.10.

cdreed06 · January 9, 2011

Yes I think it is shifting into OD, I haven't checked the rear end code yet (its COLD outside!) Just wondering if there would be any different top gear in the trans?

music · January 9, 2011

You would have to have a rear axle ratio of less than 3.08 to hit 1500 rpm @ 70 mph w/ a 4L60, which GM didn't offer in 07 trucks. The standard 4wd ratio was 3.73, for which rpms should be ~2000 @ 70 mph in 4th w/ converter locked.

kdixer · January 9, 2011

The rear end gears wouldn't have anything to do with this. The transmission is the only thing that changed, so somethings off.

If you want to know what gears you have, check the glove box for the RPO codes for GU6 = 3.42, GT-4 = 3.73, GT-5 = 4.10.

The rear gear ratio is just to confirm if his rpm's are in the normal range. 1500 seems really low in a 4spd considering the 6spd with 3.42's is turning 1750 @ 70 mph.

8.1HD/ALLISON · January 9, 2011

If the shop that put the transmission in did the job right, the last thing they should have done would be to reset the transmission shift adapts and test drive it. It may take some time to completely learn your driving habits. Until it does this, it is possible that could be part of the change in mileage.

terzmo · February 3, 2011

What gears do you have? My 08 with the 5.3, 4spd w/3.73's runs at about 2100 rpm @ 70mph so that sounds normal, unless you have really high gears like 3.08's or something.
My old truck had the 4sp and 3.42's and hovered around 1800rpm @ 70mph.

what difference does the gears make .......it ran at 1500 before..now runs 2000...tranny changed...not rearend

tanked_darren · February 4, 2011

If the shop that put the transmission in did the job right, the last thing they should have done would be to reset the transmission shift adapts and test drive it. It may take some time to completely learn your driving habits. Until it does this, it is possible that could be part of the change in mileage.

+1 I'd give it some time to adapt to your driving habits.

HawkScot · February 4, 2011

If you think the torque converter is not locking up, test it. On my rigs,I hold a steady speed on flat stretch, keep your foot on accelerator and press the brake pedal with your other foot enough to turn on brake lights. The computer should unlock torque converter when brakes are applied. You should see the RPMs go up several hundred RPMs when you do that without increase in speed. If it does not, then torque converter either is not locking or is not unlocking.

terzmo · February 5, 2011

If the shop that put the transmission in did the job right, the last thing they should have done would be to reset the transmission shift adapts and test drive it. It may take some time to completely learn your driving habits. Until it does this, it is possible that could be part of the change in mileage.

+1 I'd give it some time to adapt to your driving habits.

Tranny..adapting...nothing personal here but wouldn't every car have to adapt to the driving habits of the new owner when purchased new ? I had a tranny repaired and it did the exact same thing driving out the dealer lot. Told Me it had to adapt. Well it never "adapted" and I asked the dealer if all the new cars had to "adapt"......I sold that car and will never go to that dealer again