GM trend continues on increasing ratio of Automatics and reducing Rear End ratio's.

[Larry E.]

A while back I started a thread of comparing a 3.42 rear end gear with a 3.08 for towing with a New Silverado. Looks like GM will
continue this way of thinking with the 2015 Silverado's. Here is the article:

The new GM-developed transmission is approximately the same size and weight as the outgoing 6L80 six-speed auto, the automaker says. The eight-speed has a 7.0 overall gear ratio spread, which is wider than the six-speed's to provide a higher first gear to help with heavy loads and towing. The new transmission also enables lower rear axle ratios to reduce engine rpms

Read more: http://wot.motortrend.com/1407_2015_...#ixzz386ugccjV

I suppose if the transmission can be "locked" and not searching for gears the trend should be OK. Comments always welcomed. Larry

[Duke W.]

Re: GM trend continues on increasing ratio of Automatics and reducing Rear End ratio'

It's all about meeting the onerous new fuel ecomonmy regulations that go into effect in 2016. There is a provision in the legislation for auto makers to ask for waivers if they can't make the cut, but so far I haven't heard of any requests.

A total ratio spread of 7:1 is three times the total spread of vintage four-speed manuals! They will likely pick an axle ratio that yields the greatest fuel economy on the EPA test, which is done with only a simulated driver and passenger load, and with the broad torque bandwidth of current engines and seven more speeds to choose from, there should be a good gear for towing on the flat or up a mountain grade.

Duke

[Paul D.]

Re: GM trend continues on increasing ratio of Automatics and reducing Rear End ratio'

I,m no engineer, but I would imagine the numerically lower gear ratios are also an attempt to slow down output shaft/ driveshaft/ pinion gear speeds which may help with EPA regs as well as parts longevity. Chip.

[Michael J.]