Probably the biggest advantage is not running the engine at full RPM just to maintain normal highway speed... This is good for a few MPG...
I like the idea of running the engine at a lower RPM, too. I can go 72+ MPH on flat ground (even at altitude here in Denver), but 65MPH just after the shift into 7th feels a lot easier on the truck.
Just for the record, I wouldn't say "a few MPG". It's basically 1MPG (10%) increase, and only on long drives where you burn most of the tank in one shot on the highway. On more normal "around town" driving, it's less (maybe 5%). Given the cost of the gears, the math does not work out in many people's favor for this being any kind of "cost savings measure". To oversimplify the math a little bit, if the gears give you a 10% increase, for every $1.00 of fuel you would have bought, you now pay $0.90... you saved one dime by paying nine. Whatever the cost of the gears, you'll need to buy roughly 9x that much fuel before you break even... so at $2500 for the gears you're looking at $22500 in fuel, 7500 gallons @ $3/gal, or about 45000 miles @ 6MPG average. For $4500 gears, $40500 in fuel, 13500 gallons @ $3/gal, 81000 miles @ 6MPG average. While there are exceptions, I assume most owners only put hundreds of miles on their truck a year, and it's going to take tens of thousands to break even. Plus, I spent way more than just the cost of the gears to do the install (e.g. tools, gear oil, seals, beer, etc.).
I keep good records of my mileage, and I have a post around here where I documented the first ~2500 miles I put on the truck after the gear change. I think the people who are claiming/predicting better mileage just aren't doing a good job keeping track. For example:
- they're eyeballing the numbers... "27.23 gallons is about 25, and 231 miles is about 250, so 250mi/25gal = 10MPG!" instead of the 8.48MPG it should be
- they fill the tank a little short and get a higher MPG calculation without realizing that it balances out the next time when they have to add that missing fuel back in and then get an extra low MPG number for that next tank, and they tout that "best tank" number
There will also be some MPG variation between trucks, drivers, geography, load, etc. But those should fluctuate more in the +/- 10% range, not the 10+ MPG claims I've seen around.
Note: Yes, for the mathletes out there, I understand it's not actually 9x, it's 9.09091x = 1/1.1, but I was trying to keep it simple.
