A Thought About Relative MPG

[Big Dave]

This might get moved but I think it speaks to the direction we should be moving.

I have two four-wheel vehicles: A 2000 F-350 with a 7.3L diesel and a ZF6-650 transmission; and a 1996 Chevy Impala SS with a 5.7L (LT-1) engine and an automatic transmission (4L60E). Both vehicles are 4x2 and have 3.08: gears.

The Impala weighs a little more than half what the F-350 weighs. (Curb weight forboth).

The Impala has a smaller frontal area and probably a lower coefficient of aero drag.

Both vehicles' tires are aired-up rock hard and both have fresh front end alignments.

I drive the F-350 with very moderate hypermiling (a bit of engine-on coasting and timing of lights) and it pretty much gets 26.5 to 27 MPG routinely.

I hypermile my brains out (coasting, timing lights, drafting, brakes-free operation (it corners much faster)) in the Impala and struggle to get 21 MPG on occasion.

The F-350 gets better MPG in the winter than the Impala gets in the summer.

Does this not speak volumes for the superiority of the combination of diesel engines and manual transmissions or what?

[Crono]

It might have something to do with the gearing of trucks in general as well?

[ShadeTreeMech]

I think it has everything to do with the superiority of a diesel engine with a manual transmission. Since a diesel has a limited rev range (the 7.3 redlines at 3K, correct?) it has to have higher gears. The impala, being a SS model, is likely designed for power and ignoring economy altogether. I'd be looking at a possible transmission swap for one that is geared more toward economy if the one you have is geared for performance. It may just need to be adjusted for economy.

A 350 in an impala screams old school muscle car, not econobox. Is it possible to force the torque convertor to lock up sooner than the speed it locks itself? Some TCs can be manually locked by grounding out the correct wire in the ECU harness.

Does the impala see mainly city or highway?

[ShadeTreeMech]

Does your impala have to have high octane fuel? I find it difficult to believe you couldn't use 87 octane on a properly tuned engine. Lower octane should give better economy assuming you don't have octane knock.

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.

Depending on your amount of creativity, you may be able to get a manual from a Corvette in that rig. The new Corvettes have a 6 speed and can fetch 30 mpg on the highway.....of course the variable valve timing and direct gas injection probably helps out a bit. That and the Corvette is a bit more streamlined....

I agree, diesel engines mated with a manual kick rear all over the place as far as mpg. Plus, if you have some leftover grease from your fry daddy, it's free fuel after a straining. Or used motor oil. Or old auto tranny oil......

Hold on a sec, your avatar shows your truck having a fuel saving bed cover. So your Cd is likely significantly lower than stock!!



PS sorry for the double post, thought my original post was lost in cyberspace!

[Big Dave]

Both vehicles arebeing compared over my normal test route - roughly one-third urban/suburban, one third superslab, one third state two-lane roads.

Both vehicles have 3.08 gears.

The 7.3 diesel can be coaxed up to 3,800 RPM, but it is a dead cat above 3,200 RPM.

The Impala 350 runs 87 octane. The LT-1 uses reverse-flow cooling (cold coolant flows first into heads. This supresses knock, so the engine has over 10:1 compression.

There may be a T-56 six speed manual in this car's future.

The listed Cd for the Imp is .34. I'm sure my efforts have improved my Cd, but I doubt I got it lower than the Impala. FWIW, the Impala coasts magnificently.

With a diesel and a stick, this is a 33-35 MPG car, easy.

Problem is that is not a clunker. It is pristine.

[tasdrouille]

Diesel engines have been superior to comparable gasoline engines in term of fuel economy, but gasoline engines have been closing the gap. That is especially true as we currently see gasoline direct injection engines go mainstream, while diesels are toned down by emissions requirements (which is fine with me).

[Big Dave]

One wonders when we will see DI gas engines in the US.

Will they be reliable?

[tasdrouille]

There are already. Hyundai's new Sonata has a 2.4 liter GDI engine for example.

[Big Dave]

What I'm seeing is maybe a 3-5% improvement over conventional MPI, not the huge advatage diesels have. They don't seem to have the big low-end torque that diesels have. That big torque is needed to push efficient gearing.

[RobertSmalls]

Plus there are a few Ford EcoBoost engines on the market in America. Of course, we'll miss out on the two and three cylinder versions.