General Motors is phasing in the 8-speed automatic transmission into their higher-end trucks. It replaces the 6-speed that was there before. Naturally, TFLtruck wanted to try this 2015 GMC Sierra Denali 6.2L V8 with the new transmission. When we tested the 2014 Chevy Silverado 1500 and the 2015 Chevy Silverado HD gasser with the 6-speed, the gear spacing between second and third cogs was too wide to effectively put down the power on this extreme towing test, going up I-70 to the Continental Divide and the Eisenhower/Johnson tunnels. Can the new 8-speed (8L90) solve the issue and deliver a consistent run up the Ike Gauntlet?
This truck definitely has the power with 420 horses and 460 lb-ft of torque @ 4,100 rpm. It sends the power the AWD/4×4 system, although we are using 2WD mode for this and all Ike tests. However, this particular truck has a high 3.23 rear axle ratio that help fuel economy, but does not allow for maximum towing capability. This Denali is rated at an impressive 18 MPG in the city and 24 MPG on the highway. However, the maximum towing capacity is 9,100 lbs. Max Trailering package and lower 3.42 rear-end can allow you to tow as much as 11,800 pounds.
The lower rear-end gears also allow the truck and transmission to be more effective at “grade shifting” on the downhill. The transmission shifts down automatically to help slow down the truck when you tap on the brakes. This helps to a point, so don’t forget to always check your trailer brakes, the electrical connection to the truck, and the gain settings on your trailer-brake controller.
| 2015 GMC Sierra | |||
| Model | Crew Cab Denali 4×4 | ||
| Estimated $ | $57,800 | ||
| Engine | 6.2-liter V8 | ||
| Transmission | 8-speed | ||
| Power (hp / lb-ft) | 420 / 460 | ||
| Payload (lbs) | 1,740 | ||
| Tow Rating (lbs) | 9,100 (3.23 axle) | ||
| Test Load (trailer + cargo + ppl) | 7,700 lbs | ||
| Suspension Sag | 2 in | ||
| Ambient Temp F | 36F | ||
| Relative Humidity | Low (30%) | ||
| Barometric Pressure (at top) | 19.76 inHg | ||
| Calculated Power Loss | 32% | ||
| Up: Time | 7:57.0 | ||
| Up: MPG | 4.3 | ||
| Up: Interior dB | N / A | ||
| Up: Transmission Temp | normal | ||
Take a look at this detailed TFLtruck Ike Gauntlet video, featuring the 2015 GMC Sierra Denali 4×4 with the new 8-speed transmission.
Also, check out Ike Gauntlet results for all trucks.
Andre Smirnov is a life-long automotive enthusiast, writer, reporter, and software engineer. He has been writing and reporting at TFL since 2011.
