2015 8-speed Silverado/Sierra fuel economy set at 21mpg highway

[Zane]

By Zane Merva
Executive Editor, GM-Trucks.com
10/13/2014

 

Despite no official announcement or press release, the official fuel economy figures for GM's new 6.2L & 8-speed transmission seem to finally be out in the open.

 

Late last night, a GM-Trucks.com member noticed that the 2015 Silverado fuel economy numbers had been updated at fueleconomy.gov.

 

Only available with the 420-horsepower 6.2L V8 engine, GM's new 8-speed will net Silverado and Sierra owners 1-additional MPG in both city and highway driving. We will report on the "official" numbers if and when the company releases them publicly.

 

 

Compared side by side to the 2014 6-speed

(Thanks to gone_fishing for this graphic)

[Wintersun]

And we all know how accurately the EPA MPG figures translate into real world driving. There is also no mention of the gears used for the truck. I don't want to reduce the towing capacity by 25% or 1700 lbs. with 3.23 gears. If these figures were for the truck with 3.73 gears it would have merit. The devil is in the details.

[okfoz]

I really was hoping for better... 21 seems kind of low...

 

Real world highway will dictate that in order to achieve this you have to drive at 55mph on flat ground with a little wind behind you...

 

John

[Drautolock]

Hopefully this power train will soon show up in the 2500HD series.

[Nargg]

I too hope they put a version of this on the Duramax, and also on the 5.3L. Heck, why not make it available just about on every setup? They'd probably get a lot more sales on that setup. (I was going to say milage, but, you know....)

 

EDIT: I just checked out the gov web site on this. The 6 speed gets the same MPG....

 

 

6.2 L, 8 cyl, Automatic 6-spd, Regular Gasoline

15

City

17

Combined

21

Highway

[Krausmotorsports]

This talk about "gas mileage" is not allowed in my establishment! Hahaha

[Payton34]

I have a 2014 6.2 with 6speed and 3.73 gears and just averaged 21 on a trip chicago to nashville driving 75-80mph last weekend. Im sure the 8spd and 3.42s could get 23 at least.

[Brian1]

Looked up the gear ratio and 1 to 1 drive is sixth gear. 7th and 8th are overdrive. I thought the 6L series 6th gear ratio was .67 and the 8th in this one is .65. Please correct me on the 6L series gear ratio of 6th gear if that was incorrect. Just going off memory

[GPGuy]

Something doesn't appear to add up here...

[KARNUT]

Its not going to matter how many gears you have for gas mileage at hwy speed. You can only lower the RPMs so much, speed, weight and drag will affect the mileage. The added gears are great for performance and towing thou.

[Jsdirt]

Back in '07, my window sticker said 16/20. I have NEVER even come CLOSE to 20 ...

 

15/16 is closer to reality, and the rare 18 if I venture out where there's no ethanol in the fuel.

[DanMan_S]

Its not going to matter how many gears you have for gas mileage at hwy speed. You can only lower the RPMs so much, speed, weight and drag will affect the mileage. The added gears are great for performance and towing thou.

 

Definitely!

 

Really, the deciding factors for your highway mileage is RPM and aerodynamics. You want the engine to run at the lowest RPM possible, but at the same time, you need to balance the engine torque to make sure that you can actually resist the aerodynamic drag without using excessive throttle. But you only need one gear to accomplish this.

 

You really only need two gears...one to get rolling, and one for cruising. Everything else in between is to help keep the engine at peak hp/tq when driving in scenarios that aren't optimal for those two gears (accelerating, towing, passing, etc).

[Doug_Scott]

In real world driving, a trans with more gears should be getting better mileage. Real world driving is not 100% highway. When driving traffic or in the city, more gears keeps the engine at a more static rpm. In other words, once the truck is moving, the rpm will not be going up, on each gear shift, it will tend to stay within a much smaller window or range.

 

As for highway mileage, there is a "sweet spot" that the engine will return the best economy. This spot is dependant upon rpm, load, final drive ratio(high gear and axle ratio combined) and mph(aerodynamics). No two vehicles will be exactly the same, even given the same scenario and equipment.

 

Fuel economy ratings in the US should be pretty realistic. The US adopted the five-cycle fuel economy testing standards 7 years ago. This is a cut and paste:

---

US EPA altered the testing procedure effective MY2008 which adds three new Supplemental Federal Test Procedure (SFTP) tests to include the influence of higher driving speed, harder acceleration, colder temperature and air conditioning use.

 

SFTP US06 is a high speed/quick acceleration loop that lasts 10 minutes, covers 8 miles (13 km), averages 48 mph (77 km/h) and reaches a top speed of 80 mph (130 km/h). Four stops are included, and brisk acceleration maximizes at a rate of 8.46 mph (13.62 km/h) per second. The engine begins warm and air conditioning is not used. Ambient temperature varies between 68 °F (20 °C) to 86 °F (30 °C).

 

SFTO SC03 is the air conditioning test, which raises ambient temperatures to 95 °F (35 °C), and puts the vehicle's climate control system to use. Lasting 9.9 minutes, the 3.6-mile (5.8 km) loop averages 22 mph (35 km/h) and maximizes at a rate of 54.8 mph (88.2 km/h). Five stops are included, idling occurs 19 percent of the time and acceleration of 5.1 mph/sec is achieved. Engine temperatures begin warm.

 

Lastly, a cold temperature cycle uses the same parameters as the current city loop, except that ambient temperature is set to 20 °F (−7 °C).

EPA tests for fuel economy do not include electrical load tests beyond climate control, which may account for some of the discrepancy between EPA and real world fuel-efficiency. A 200 W electrical load can produce a 0.4 km/L (0.94 mpg) reduction in efficiency on the FTP 75 cycle test.

---------------

 

These are in addition to the traditional tests. Results are combined and divided by 5 I think. Mileage dropped when they switched to this formula. We (Canada) are just getting this system now. This helps explaining why Canadian mileage claims by manufacturers were always higher than the US numbers (after converting to US gallons).

[riverrat69]

I have a 2014 6.2 with 6speed and 3.73 gears and just averaged 21 on a trip chicago to nashville driving 75-80mph last weekend. Im sure the 8spd and 3.42s could get 23 at least.

 

 

Do you dream about gas mileage as you drive? Ain't no way that combo is getting 21 mpg while driving at any speed. I do know, I have the same combo. 43,000 miles. 13.3 mpg all day every day.

[Payton34]

 

 

Do you dream about gas mileage as you drive? Ain't no way that combo is getting 21 mpg while driving at any speed. I do know, I have the same combo. 43,000 miles. 13.3 mpg all day every day.

No dreaming I was surprised too. Trip to Nashville I averaged 18mpg and on the way back to chicago averaged 20 and then got to 21 the last 200 miles. No traffic there or back constantly moving 75-80mph. I was very happy and surprised since around town stop n go and my 2 mile commute to work I only average 14mpg. Truck only had 2800 miles when I left on my trip and put on a little over 1000 miles.