Don't forget that 12.5 million SUVs are made each year - of the 50 million vehicles made (and presumably sold), 25% are SUVs.
so, the 16 million SUVs on the road is probably on the low side by 50% too low.

I think I saw figures mentioned earlier of 67 million "light trucks" of which SUVs are in that category, so lets assume given 15 years of SUV sales, that only half of the 67 million light trucks are SUVs, that makes your 16 million SUVs double to 32 million or more SUVs on the road.

Now that effectively doubles your "improvement" of 5 days of oil saved a year to 10 days a year if every SUV did 28MPG.

And when every SUV is able to do 40MPG like a lot of modern non-hybrid cars available now, then you're talking nearly a 3 fold improvement on those figures again (40-21 = 19 MPG better per gallon which is nearly 3 times your 7MPG improvementfrom your original figures of 7MPG better = 5 days worth of oil saved]

And when the Hybrid SUVs come out, with 50-70 or more MPG then you're really getting somewhere.

At that level you're talking about a situation where the US would be close to or past not having to import oil to meet its oil usage requirements.