Sorry about the vaguenesss of the reference to my high school grades in physics, but lately standing of some sort seems to be required.

I never worried too much about laws of nature, as they always seemed to be able to apply themselves, engineer or not. My concern generally was... would they show up before First Article Test? Or after start of Full Scale Production?

Hmmmmmmmm, I would have assumed a greater influence on fuel economy.

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Did you just pass my 740 at 200 kmh in a 300SD?????

1978 300SD 'Phil' - 1,315,853 Miles And Counting - 1, 317,885 as of 12/27/2012 - 1,333,000 as of 05/10/2013, 1,337,850 as of July 15, 2013, 1,339,000 as of August 13, 2013



100,000 miles since June 2005 Overhaul - Sold January 25th, 2014 After 1,344,246 Miles & 20 Years of Ownership

Brian has done one diff swap and now believes that nobody else could have different results unless they don't know how to calculate the mpg.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Very interesting thread.

The problem with anecdotal evidence is that it is highly subjective.

In all those diff swaps you did, did you also change the speedo and odometer?

Also, are you sure your kids drove the car in the same manner you were driving after the swap? Seems like there are too many variables.

In my experience, I swapped the 3.27 diff of my 230E, 4-speed manual, with a 3.46, and I actually observed BETTER mileage even after correcting for the error on the odometer.

It was this error in the speedo/odo-meter that made me go back to to 3.27. In city driving the error was not significant, but at highway speeds there was a 10-15km/h difference between actual and indicated speeds.

Last week I changed the speedometer with one which is appropriate for the 3.46 rear end - original speedo- had a yellow sticker on the back "12V, K=0.780, and this one has a sticker indicating "12V, K=0.825. So, I put back the 3.46.
The mpg from my last trip during the weekend of about 360km, of which half is highway, and the other half is mountain roads, is the same as with the 3.27, as I tried to drive with the same speeds as before. I will be collecting further evidence to confirm or refute my initial observation.

In general, I also tend to believe that gearing and fuel consumption are not directly related. A gas engine under load (no manifold vacuum) tends to consume 1.5-2 times more fuel compared to no-load (high vacuum) at the same RPM. Thus fuel consumption in city driving would depend more on number of accelerations/decelerations and how hard you accelerate, while highway consumption will be more highly dependent on speed rather than gearing. At about 100 mph (160km/h) almost all of the engine power is consumed for overcoming drag.

The standard 4-speed, 3.27 has a slight over-rev gearing, achieving 183kph at 5000RPM, where the peak 100KW occurs, while the drag-induced maximum lies beyond at somewhere around 200-205 kph.

With the 3.46, 5000 RPM equals to 173 kph, so I might as well go for a 5-speed, with 0.8 5th speed.

If it were designed for V-max with the current 4-speed, the rear-end would have to be 3.07, giving it 195kph at 5000 RPM in 4th, but I am not very keen on this combination, as it would be even more sluggish than the standard 3.27.

As a general rule, best efficiency for a spark ignition engine is at low revs and high load without fuel enrichment( which is typically 20 percent richer than stoichiometric except large supercharged aircraft engines of yore where takeoff power may have used as much as 50 percent enrichment with water-alcohol injection). This can be difficult to achieve with a carbureted engine since fuel enrichment generally starts at about 6" Hg manifold vacuum, but on KE and most EFI systems, fuel enrichment doesn't occur until the throttle is to the floor, which activates fuel enrichment via an electrical switch signal to the ECU.

Pumping loss from high manifold vacuum is a signficant parasitic loss, which is one major reason why diesels are more efficient, especially in city driving. At low manifold vacuum spark ignition engine pumping loss is no more than diesels, but most of the time SI automotive engines are operating with significant manifold vacuum.

The spark advance map is also an issue since most maps limit advance at low revs to keep the engine out of detonation, so there is a low speed limit in terms of peak efficientcy that usually can't be determined without laboratory testing.

In most cases, taller gears with an optimized spark advance map that keeps the engine on the ragged edge of detonation is best for fuel economy, and this can be seen in the very tall gearing of modern cars with six to eight speed transmissions that may be set up for at little as 1100-1200 revs at 60 MPH, especially on large displacement engines.

That's why modern Corvettes with 6.2L/450HP engines can knock down 30+ MPG on long freeway trips.

Duke