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2006 Chevy Silverado 2500 HD - Maximum MPG

The Search For Full-Size Fuel Economy

Photography by Courtesy of General Motors Corp.,

You don't have to drive a nerdy gasoline hybrid car to get good fuel mileage. You can maximize the miles per gallon achieved in a fullsize truck with a few flicks of a pen when you order a turbodiesel at the dealership. To test our theories, we ordered a two-wheel-drive Chevy Silverado 2500HD with the 6.6L Duramax engine and the Allison six-speed automatic transmission.

Transmission Truths
The Allison transmission has just as many speeds as the manual. Plus, it has higher overdrive gear ratios. The double-overdrive feature allows the Duramax to loaf along-even at highway speeds.

ZF S6-650 (ML6)
Allison 1000 (MW7)

6-speed manual
6-speed automatic

Gear Ratios
1st 5.79 3.10
2nd 3.31 1.81
3rd 2.10 1.41
4th 1.31 1.00
5th 1.00 0.71
6th 0.76 0.61
Reverse 5.23 4.49
Rear Axle 3.73 3.73
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