Driving at a slower speed absolutely can save you fuel, at least on every vehicle with active gas mileage calculating that I've driven it did. Is there not more wind resistance against a car the faster it moves? If it uses more power to sustain a cars movement at a higher speed, it'll use more gas. The only factor that messes with this a bit is where your engines optimum cruising RPM is. For example, if you have a big cam with a lot of overlap and low gearing, you may be lugging the hell out of your motor at 55mph which isn't very efficient.

FYI, for those criticizing the Gear Vendors OD units .78 OD, I completely agree that I wish they made a unit with a numerically lower OD ratio, but remember with many muscle cars/hot rods running higher overlap cams and non lockup converters, it may not be a good thing to drop the RPM's so low.

As far as 400 hp viper smoking muscle cars despite the gearing, or any modern car for that matter, it has NOTHING to do with 4:10's being a poor choice for racing: modern fuel injected motors have very broad power curves, are making more power from a given HP rating than a muscle car with the same rating and the power to weight ratio as well as aerodynamics probably destroys most muscle cars you'd put it up against. I can guarantee you that if you took that viper and set it up with 4:10's, if it hooked, it would be faster through the 1/4 mile than the viper with 3:07's.

Simply put, unless you are talking about a turbo car which benefits from the load, the more gear you have the faster the car will accellerate up until the point where you run out of gear. That's just the way mechanical advantage works.

Motors that make more broad power curves may not benefit as much though, as a numerically lower gear will give them more average time in places where they are making good power versus a less technologically advanced small block that really only starts to build steam in the the high RPM- the idea is you need to get there quickly.

Look at the difference between some of these late model motors versus an old school small block. To make 500hp, my 340 needs a huge overlap cam that shifts the power band way up and won't make for the greatest street manners at low RPM's. 500hp late model motors are doing it with cams that barely make a chop in the idle and the engine still makes great power/torque off idle.

You really can't compare ideal rear gearing or OD gearing for a muscle car motor to a late model, fuel injected one.

As far as the O/P's discussion goes, I have alway seen about a 2 maybe 3 tops MPG increase on cars I have installed OD's in, but there are a ton of variables here. With our older cars, typically being MODDED cars, the biggest variable is the cam profile/converter. A car running a big cam with lots of overlap and a high stall speed to make efficient use of that cam off idle may not benefit from an OD at all, depending on what rear gearing you have. You don't want to be cruising the car so far below your cam's power band or at a range where you are burning up your converter.