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http://i268.photobucket.com/albums/jj29/starstuff_forums/familycarcomparison.jpg
The image shows a speed/mileage graph of my family's 3 vehicles, my wife's 2006 Hyundai Elantra GLS (2 L, 4AT), my 2002 Nissan Sentra SE-R Spec V (2.5 L, 6MT), and my 2006 Scion xB (1.5 L, 5MT).

The leftmost values for each car show low speed P&G results, and the two MT cars top the Hyundai by 20 mpg. Each graph then shows a dip, representing lower gear cruising mpg @ 30 mph. The remainder of each graph shows mpg in top gear for each car using cruise control.

The Nissan and Scion maintain very healthy mpg advantages until the gap starts to narrow at about 50 mph. By 62 mph, the Hyundai tops the Nissan. By 72 mph, it looks like the Hyundai will also beat the Scion.

Why? I can understand the Scion getting overtaken at high speeds. Its larger frontal area and large (.35) drag coefficient mean I'm pushing a barn door into the increasing air resistance. Plus its engine runs about 3000 rpm @ 60 mph, and about 2600 rpm at 55 mph.

The Hyundai and the Nissan, however, are evenly matched. They both turn about 2200 rpm @ 55 mph, have Cds of about .33, and have similar (2750-2800 lb) weights and frontal areas. Why does the Nissan's mileage to speed graph drop like a rock, while the Hyundai's graph is so much flatter? I can't visualize why the Nissan's thirsty larger engine isn't thirstier at both low and high speeds.

Your thirstier sentra guzzles more at an increasing rate with increasing speed compared to the elantra.

Also, your sentra has very short gearing, so each gear has a very short range. This also creates increased pumping losses as you're increasing speed.

You didn't specify what gear you were in at each speed however, so that answer might change a bit.

Wow, is that teh suck. If I lit out on I95 from southern NC towards Alexandra, VA with the cruise set at 60 MPH (57 MPH actual), the '99 Elantra will return at least a 42 MPG average. Of course, it wasn't a real comfortable ride at something like 95+ degrees F the last time I did it, but it was the best mileage I've had to date. It's the only tank I've broken 600 miles on. Hopefully the "blue" versions of the Elantra/Accent return much better highway mileage than that.

Could the reason for the difference be the engine size (2 L vs 2.5 L): 2006 Hyundai Elantra GLS (2 L, 4AT), my 2002 Nissan Sentra SE-R Spec V (2.5 L, 6MT)?

Maybe an advantage of the newer Hyundai engine's VVT?

It's not like MT's have much advantage at speeds where the torque converter is locked up, so you'd expect the engine with more sophisticated combustion management to win at high speed if other factors were equal.

I'm pretty sure the Nissan also has VVT, and I know the Scion does.

All the data points above 35 mph are in top gear (Hyundai 4th with torque converter locked, Scion 5th, Nissan 6th). The Nissan and Hyundai both run at about 2200 rpm at 55 mph, so it's not a gearing advantage for the Hyundai. In fact, the Nissan may turn about 100 rpm slower at that speed.

I imagine the Hyundai's smaller engine does drink less fuel at 65 mph, but shouldn't it also burn less fuel at 40, and 50? Why does the Nissan beat it at low speeds, and not at high speeds?

I'm pretty sure the Nissan also has VVT, and I know the Scion does.

All the data points above 35 mph are in top gear (Hyundai 4th with torque converter locked, Scion 5th, Nissan 6th). The Nissan and Hyundai both run at about 2200 rpm at 55 mph, so it's not a gearing advantage for the Hyundai. In fact, the Nissan may turn about 100 rpm slower at that speed.

Wikipedia said the Sentra didn't get it until '07 (http://en.wikipedia.org/wiki/Variable_valve_timing), but they've been known to be wrong.

The SE-R Spec V uses the QR25 engine, which has Nissan's version of VVT, called CVTC (continuous variable timing control), available since 2002.

If you had a bsfc map of both cars, you would notice that both are efficient at different rpms. Your data would seem to indicate that the hyundai is more efficient at 2200rpm than the sentra. The fact that the displacement of the sentra is 25% more than the displacement of the elantra (sentra is using more fuel to travel at the same velocity) would be enough to suffice the difference.

Maybe Nissan is lying about the CD.

What I see might show the Nissan having a more efficient drivetrain-MT maybe engine also-, but maybe less efficient body shape.The less efficient body would do just what your graph shows

It looks like the Nissan pays more to punch thru the air.

Charlie

I wouldn't count on Nissan being correct on the CD.

It's also my experience that AT cars tend to have taller final drive gearing than comparable MT cars?

I think you may be on to something, jcp. If the gearing keeps it at lower rpm, the ideal rpm range will span a wider range of speeds. The steeper dropoff of the other cars seems to match the faster-rising rpm typical of a sporty manual.

Not in this case. The Nissan and Hyundai both turn about 2200 rpm @ 55 mph. Within the same brand and model, my MT Nissan is only geared 4% shorter than its AT equivalent, my wife's AT Hyundai has a 14% taller gearing advantage over its MT counterpart, and my MT Scion suffers from 36% shorter gearing than its AT counterpart.

It-the MT car- has more aero drag than the AT car.

The graphs make it pretty clear.
Charlie
PS I know you said they have the same CD's and maybe the same frontal areas, but the graphs imply they don't.

You may be correct, Charlie. The Nissan has 5 spoke wheels and a spoiler, but the Cd figure of .33 appeared in reviews for this model of the Sentra.

Remember, the Cd is just a multiplier. If the the Elantra presents less frontal surface area it will have less drag to deal with.

Lots of the newer cars automatic transmissions are actually geared for better mileage than the same car with a manual transmission. They're using shorter gearing in lots of the manuals for more pick up. My brother has a Nisaan Versa with a 6 speed manual transmission and if I remember correctly from when I've rode with him it is turning about 3K RPM's at 60 MPH in 6th gear. My '88 Ford Escort GL with a 3 speed automatic turns about the same RPM with 3 less gears. While on that subject does anyone know how much work and modification it would take to put an overdrive automatic out of a newer Escort into the '88, or is it even possible? I've even entertained the thought of putting a 4 speed manual in it like is in my '88 Escort Pony.

Sean, the frontal areas of my Hyundai and Nissan are pretty close. The Nissan is a tad wider, but the two cars are about the same height. I doubt there's much more than a couple of square inches difference in frontal area. The Scion, OTOH, is much taller. One of these days I'll photograph the cars and plot their area in Photoshop.

Ford Man, my Nissan is a rarity, with a tall overdrive 6th gear that's geared only 4% shorter than its automatic counterpart. It only turns about 2500 rpm at 60 mph My Scion, OTOH is 35% shorter than its AT cousin, and it's churning at 3000 rpm at 60 mph. As mentioned earlier, my Nissan and Hyundai are geared similarly, with both turning about 2200 rpm at 55 mph.

SentraSE-R,

I would bet dollars to donuts that the sporty looking 2002 Sentra gave up something to get that sporty look.

In 2002-(your 2002 would have been designed years before)there was zero real interest in mpg.By 2006 however, we had the 2002-2003 runup in gas prices because of 9-11 and the Iraq war.I remember gasoline went from about $1.15 to $1.69 and we started to sweat that price(ha,ha little did we know). By 2006-cars designed in 2004 or so??-we knew that gas prices were on the way up,so Hyundai would have spent time cleaning up their car-(not sure when the Elantra was redone, so this is a guess).Hyundai used to have class worst MPG in many classes(like Dodge).They started changing that in 2004 or so.

Those published CDs are little more than advertising ploys.No GOV agency verifies those claims.

The Sentra mpg is dropping like a stone-27 mpg at 70 mph-the other 2 are still above 30 mpg. It is fairly steeply sloped downward also.

Motors/drivetrains especially in top gear with the trans locked up-are all fairly close in efficiency(ok, not really, but they are getting there). The steep drop sure looks like an aero effect.

Charlie
PS Like you imply-the spoilers-ESPECIALLY FROM 2002- are usually poison for FE. Tuner type stuff-

Good observations, Charlie. If I don't hear any more convincing theories, I'm inclined to agree with you. Maybe it's time to take that spoiler off, and see what that does to my mileage.

I removed the spoiler from my Nissan and ran some more tests at 55, 62, and 70 mph. Sure enough, removing the wing significantly improved the Nissan's fuel economy at higher speeds. The Hyundai still caught it at 70 mph, but at least it didn't happen at 62 mph, which occurs when the spoiler is attached to the Nissan.
http://i268.photobucket.com/albums/jj29/starstuff_forums/familycarcomparisonrevised.jpg

Wow, that wing cost 3 mpg from 60-70 mph-almost 10%?? I wouldn't have guessed it would cost that much!! The tuner look is costly.

Did the wing seem to help high speed stability??

Charlie

I think I only took the Nissan over 100 mph once, and it was quite stable with the wing. I didn't notice any difference today at 70 mph.

I think I only took the Nissan over 100 mph once, and it was quite stable with the wing. I didn't notice any difference today at 70 mph.

Where? The only place I have driven that kind of speed is on the German Autobahn. 227 km/h (142 mph) was my max speed, and I think I was sucking close to 10l/100km of gas, too.

Larry