[Big Dave]

Tires for MPG is something that gets endlessly discussed on pickup forums. I have seen literally thousands of guys telling me that going to bigger diameter tires reduce MPG every time. Tried it myself and got the same result.

Ask yourself: Why is it that the designers of the first-gen Insight and the Prius (two fangs-out MPG specials) always selected small tires and used gearing to reduce the engine speed at a given road speed?

The answer is angular moment of inertia. Anything that rotates has an analog of mass called angular moment of inertia. AMoI is what makes a flywheel work. Every time you increase road speed you have to not only accelerate the mass of the vehicle, but also accelerate the rotational speed of the tires.

The angular moment of inertia goes up with the square of the radius, so bigger diameter tires bring a very large AMoI that you have to accelerate. This also explains why lighter wheels improve MPG more than you’d think just from the weight advantage.

Gears, although dense, are of much smaller diameter and thus have very small AMoI. Gears, not tires are the way to efficiently control engine speed at a given road speed. This is why I like the T-56 transmission. It is a close-ratio four speed with two overdrives. Fifth gear is a 0.71:1 overdrive and sixth gear is either a 0.63:1 or (more commonly) a stratospheric 0.5:1 overdrive. You can have really small-diameter tires which have very low AMoI and still have a very efficient engine speed with the gearing a T-56 routinely makes available.

If you weren’t just shining us on about wanting 40 MPG, the 4.5 liter gas V-8 and any automatic simply has to go. There isn’t enough aeromodding and weight reduction available to offset that inefficient drivetrain. Probably the best drive train would be a M-B OW 617 and a Getrag 275 five-speed. I suspect you’ll have to go to Germany to source a Getrag 275, but it should mate right up to the OW 617. The OW 617/Getrag 275 combo is probably three hundred pounds lighter than a 4BT3.9/T-56 setup – my second choice. It still gives you a top gear in the 0.7:1 range. The OW 617 will give you about 125 HP max and is legendary for longevity. The OW 617/Getrag 275 is designed for a vehicle roughly the weight of your Crown Vic. (Benzes are heavy.) Downsides: The old-school fuel injection is complex and finicky and the engine is loud by today’s standards. But then, you gotta make sacrifices to get to 40 MPG.

To get to 40 you have to forget gas engines. A diesel is typically 25-50% more efficient than a comparable gas-pig. Look at fueleconomy.gov and contrast a gas VW Jetta vs a TDI diesel Jetta. Generally about 22 MPG for the gas pig and 33 for the TDI. To get to 40 you have to go to war and the diesel is the biggest gun you can use. Even a 2.0 liter EcoBoost (if you could find one) would not give you a 50% bump over your modular V-8.

A lock-up torque converter behind a diesel is going to get you real familiar with a transmission tech. They break down a lot. I had a Chevy pickup with a wimpy old 6.2 diesel and a Turbo-Hydro 700R4. In 300,000 miles I went through seven transmissions. With my Ford (having a ZF six speed) I’m at 290,000 miles on the factory clutch. The manual is also much more conducive to effective hypermiling.

40 MPG in a Crown Vic is do-able, but is ambitious. To get to 40, you are doubling the MPG. You cannot compromise at all.