It's 10% shorter (higher) gearing
You lose about 500 RPM at highway speed.
It also means your bike accelerates 10% slower, at the "benefit" that it could theoretically go 10% faster top end. Except the stock engine doesn't have the power to spin a venture diff to redline in fifth gear, and even if it did, you'd never keep the bike stable enough to reach that theoretical 160ish top end. In reality a Venture diff bike isn't any faster top end than a stock, and takes longer to get there.
I'm not even sure it really helps with MPG, my bike came with one and usually got 38-40mpg, which seems pretty much right in line with the numbers from guys with stock bikes.
An interesting thing I noticed..I geared down my Z, added 2 teeth to the rear sprocket....picked up like 300RPM on the highway, and it gets better mileage. I'm getting consistently about 10-15 miles farther to the fuel warning than before, despite the engine spinning faster all the time.