A Thought About Relative MPG

[Big Dave]

Darcane:
Your point about wheels/tires is taken but it actually reinforces the superiority of the diesel/stick configuration. The truck has tires with a 31.7 inch outside diameter while the OD on the Impala tires is 27.0 inches. Since rotational moment of inertia goes up with the square of radius, I would expect the truck tires to have 38% higher rotational moment.

We have been over this before, but in the real world where stops lights and signs enforce stopping the higher rotational moment of inertia penalty of bigger OD tires rapidly overwhelms the lower engine RPM. With trucks the equation is always the same: Bigger tires = Lower MPG. Now that’s conventional drive trains I’m talking. It might be different with hybrids, but I notice that both Toyota and Honda put small-diameter tires on their MPG flagships.

One mitigator: The Impala has efficiency-robbing EGR where the truck does not. There is a EGR-delete kit for the LT-1, but I’ve been too busy/lazy to do it.

I hear you about the disappearing manual transmission. Let’s face it, ecomodders/hypermilers are a rare breed. The vast majority are just lazy. Talking on the cell phone is more important to most people than being aware of how you are driving. Also, I think an automatic is a way for the manufacturer tio hold the engine into a regime where NOx emissions are minimized. The very low RPM – high load regime that give optimum MPG tends to have high cylinder temps that generate a lot of NOx. Hence the rev-happy way most cars and trucks drive these days.

CFG83:
There are kits out there to convert the B-body to a Tremec T-56 (the manual available in Corvettes, Vipers, Camaros, and Mustangs). It is not a project for the faint of heart (although a cinch compared to Bennelson’s Chevedes project). Yeah, you have to cut sheetmetal. There may be such a project in this car’s future, but hopefully not this year.

As long as the gas engine has to tolerate a throttle, it will never catch the diesel for efficiency at part-load operations.

[darcane]

Originally Posted by Big Dave

Darcane:
Your point about wheels/tires is taken but it actually reinforces the superiority of the diesel/stick configuration. The truck has tires with a 31.7 inch outside diameter while the OD on the Impala tires is 27.0 inches. Since rotational moment of inertia goes up with the square of radius, I would expect the truck tires to have 38% higher rotational moment.

We have been over this before, but in the real world where stops lights and signs enforce stopping the higher rotational moment of inertia penalty of bigger OD tires rapidly overwhelms the lower engine RPM. With trucks the equation is always the same: Bigger tires = Lower MPG. Now that’s conventional drive trains I’m talking. It might be different with hybrids, but I notice that both Toyota and Honda put small-diameter tires on their MPG flagships.

I agree there is a penalty for the higher moment of taller tires... but I don't agree it outwieghs the change in gear ratios. This winter I changed to 5% taller tires and saw about a 5% increase in mpgs. It's the simplest way to change your overall gearing ratio and generally taller tires has meant higher mpg in the posts I remember reading.

Originally Posted by Big Dave

I hear you about the disappearing manual transmission. Let’s face it, ecomodders/hypermilers are a rare breed. The vast majority are just lazy. Talking on the cell phone is more important to most people than being aware of how you are driving. Also, I think an automatic is a way for the manufacturer tio hold the engine into a regime where NOx emissions are minimized. The very low RPM – high load regime that give optimum MPG tends to have high cylinder temps that generate a lot of NOx. Hence the rev-happy way most cars and trucks drive these days.

The other issue that increases emissions is letting off the throttle in between shifts. My wife's Vue "solves" this by having the drive-by-wire throttle stay open for a second or so and slowly close after you let off.

[Clev]

Originally Posted by ShadeTreeMech

A 350 in a car that could run great with a 3.8 boggles the imagination. The setup described screams out muscle car with no chance of economy. I'd be blaming the gears in the transmission for being geared for burnouts and low 0-60 times instead of high mpg and low emmisions.

Agreed. The 6-cylinder Impalas in the EM Garage get around 30 mpg. A turbo would help get some of that 350 performance back while allowing better mileage at cruising speeds.

Or, like was already mentioned, a 6 speed with a tall sixth gear.

[Big Dave]

The T-56 swap gives you a 0.50:1 top gear.