Tested: speed vs. MPG, 2008 Corvette Z06 (505 hp)

[MetroMPG]

The ScanGauge readings weren't verified against filling the gas tank of the Vette - that's what I mean by uncalibrated. So yes, there's the possibility the MPG figures aren't exact... but it's unlikely they're out by more than a few percent.

My experience with the SG (comparing with "zero offsets" to filling the tank in several vehicles) is that it's not usually far off for "normal" driving, like the kind done for this test.

I don't have or use a scanguage so maybe I just don't understand how it interacts with the car.

The ScanGauge calculates fuel use indirectly: it determines it based on air consumption, assuming a stoichiometric air/fuel ratio for a warmed up vehicle in closed loop operation (such as for this test).

[PaleMelanesian]

In my experience, the Scangauge tends to be slightly optimistic, but only by a small margin, like 3%. Whatever the offset is, it's a percentage and not a constant value. The curve on the chart will not change with calibration, just the scale on the Y axis would change. The most efficient speed will still be 30 kph / 37 mph, it just might be 36 instead of 36.5 mpg.

[MadisonMPG]

Someone asked did it have cylinder de-activation earlier, I don't think it does. Are the Vett's still fiberglass?

[PaleMelanesian]

I believe the standard ones are, but the Z06 is carbon fiber.

[gone-ot]

Originally Posted by Mustang Dave

In my experience, the best FE is at the lowest speed in your top gear.

...exactly! That's the condition where you get SLOWEST engine speed for the MOST throttle opening (without being WOT), ie: least amount of "pumping" loss for the hp generated.

...anybody remember the Oldsmoble "Turnpike" cruisers?

[gone-ot]

Originally Posted by MetroMPG

I've updated Post 1 at the top of this thread with this graph of the results, plus methodology & raw numbers.

...if my number-crunching is correct (6th=0.57:1), the missing rpms would be 1,093 rpm at 50 mph and 1,230 rpm at 56 mph...sound about right?

[PaleMelanesian]

I get 1080 and 1215, but it's close.

[MetroMPG]

Originally Posted by MadisonMPG

Someone asked did it have cylinder de-activation earlier, I don't think it does.

You're right - it does not.

Are the Vett's still fiberglass?

Yes. The Z06 has some carbon fiber (the owner mentioned the liners inside the wheel wells as an example, and a balsa wood/carbon fiber composite floor). Also the frame is aluminum, vs. steel in the regular model.

[Big Dave]

I think this data vindicates my approach of low RPM = max MPG, even for gasoline engines. The Z06 with its LS7 engine has such mammoth torque as to allow operation at vividly low RPMs. The same principle works for a high-revving Honda engine, just the cutoff for lugging occurs at a higher RPM.

As you can see from this, the Tremec 6060 transmission used in the Z06 has a 0.63:1 top overdrive ratio, but a 0.5:1 ratio could be had if MPG were to be emphasized.

Tremec TR-6060 transmission - Wikipedia, the free encyclopedia

BTW, I am seriously looking at getting the Challenger version to replace the ZF6-650 in my truck. That with a couple of aero tweaks might get me over 30 MPG in my mixed-driving circuit. It has an optimal ratio spread and its 600 ft-lb rating is A-OK for my Powerstroke which has been dyno-tested to deliver 560 ft-lb max torque. Having a 1.54:1 overall reduction should give spectacular results.

Too bad the GM 4.5 liter diesel is dead. Given the LS7 in this test is operating way below its peak volumetric efficiency, it is incurring huge pumping losses for not much power. The diesel suffers no such loss. It would be interesting to see what a 4.5 could have done if fed the 70 cetane Fischer-Tropsch fuel the Audi & Peugeot prototype racers use.

[PaleMelanesian]

Originally Posted by Big Dave

I think this data vindicates my approach of low RPM = max MPG, even for gasoline engines. The Z06 with its LS7 engine has such mammoth torque as to allow operation at vividly low RPMs. The same principle works for a high-revving Honda engine, just the cutoff for lugging occurs at a higher RPM.

I concur. I have no tachometer, so I finally paid more attention to my rpm on the Scangauge this morning. I can gain speed from about 800 rpm with my little Honda. I typically run P&G cycles in the 1000-1500 rpm range. That's lower than I thought I was doing.