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Comparing MPG at speed


Bruce Nunnally

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Consumer Reports did an interesting study which is documented in their blog here.

In the study they measured steady state fuel economy for a variety of cars at three speeds: 55, 65, and 75 mph.

For example, the Lexus RX350 3.5-liter V6 got 30.9mpg at 55, falling to 27.4 mpg at 65, and finally 23.0 mpg at 75mph.

I would love to see a longer table, with more models tested. My steady-state driving tends to be in the 73 mph range, and the CTS in my experience gets ~26 mpg in that range.

The Honda Fit got 36 mpg at 75mph, and the Toyota Yaris got 34 mpg at 75 mph. Still more economical than the CTS, but not by the huge margin you might think.

Also, the CTS 3.6L V6 actually does as well as some 4 cylinders.

Bruce

2016 Cadillac ATS-V gray/black

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My experience has been that 55 mph is certainly the optimum for most cars, but my northstar and other large engine cars seem to have a flatter curve at the upper end (ie. the drop in fuel economy from 55 to 75 is less with the big engines than it is with the small engines). This is a gross generalization based on limited vehicles, but I wouldn't be surprised if testing proved it out.

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I agree with Phillip...

On the last little trip I took to Colorado....about 3000 miles round trip, my mileage didn't vary by a huge amount, no matter how I was driving.

I had a few days where we were just piddling around, in no hurry to get ANYWHERE...50 to 70 mph.

we got 23 to 25 on those days.

Then we had a couple of days where I was just getting from point "A" to point "B"...70 to 100 mph.

Got 21 to 23 on those days.

I go to Waco TX about once a month.

200 miles round trip.

Very hilly terrain...lots of traffic...almost no flat stretches of road.

Usually run around 80 to 85 as much as possible...get about 22 to 22.5 on those trips.

I recently had to make a speedy trip to S. Carolina. Got 20.5 to 21.5 on the trip out there.

On the way back...we drove along with Darling Wife's brother and his wife.

They were driving a two year old Hyundai Sonata.

MUCH smaller car with a V-6.

From Charleston SC...to Texarkana TX...every time we stopped for fuel... fuel used never varied more than a gallon from my car to his car.

I honestly think that, if I was driving a Honda Fit...(heaven forbid)...

and drove IT like I drive my Cadillac...

it would not get much, if "ANY", better mileage than my Cadillac.

That would be with two pretty fair sized people...luggage, ice chest and all the junk you take with you for a 9 day trip... and then drive it like I do my Cadillac...

If I "HAD" to drive one...it sure would cut down on my driving for pleasure. :D

It would not be a pleasure driving one of "THEM THINGS" anywhere... at least for me.

Thanks but NO THANKS... I would rather drive my 2.5 ton Cadillac, with plenty of room, nice ride, all the creature comforts and probably / maybe ... 2 mpg less than the little cracker box.

Between this 2006 DTS and the one that burned in my garage fire...I have put almost 120,000 miles on them.

VERY, VERY seldom has my tank average "EVER" dropped below 20 mpg.

Very seldom is it much above 22/23, but a few times (very few times) I have seen it as high as 25 to 25.5.

On the road, tt is usually around 21 / 23 ...tank after tank after tank.

Just my nickles worth. :D

I wrote too much for it just to be worth 2 cents. :D

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My steady-state driving tends to be in the 73 mph range, and the CTS in my experience gets ~26 mpg in that range.

The Honda Fit got 36 mpg at 75mph, and the Toyota Yaris got 34 mpg at 75 mph. Still more economical than the CTS, but not by the huge margin you might think.

Also, the CTS 3.6L V6 actually does as well as some 4 cylinders.

Bruce,

A while back, I had a 2008 CTS with the 3.6L and used it to make a trip to Waco.

200 miles round trip.

My DTS was getting the drivers Air Conditioned seat fixed. :D:D

I filled it up here by the house...drove it down there and back at about 75 mph, while really trying to be pretty light on the "GO" pedal.

Filled it up when I got back at the same pump at the same station with the car facing the same direction.

I very carefully topped the tank off each time.. until I had fuel up in the filler neck. I was trying to make it as consistent as I possibly could.

It got just a hair better than 29 mpg. :D

I was pretty impressed with it.

It was a very nice, well appointed automobile...not a base model, stripped car.

Nice ride..plenty of power...very well driving automobile.

If the seats fit me a little better, I wouldn't mind having one of those at all.

(I have bad knees and a bad hip)

Last year, I had a base model CTS for a couple of days and had to make a 300 mile road trip in it.

I put a total of about 600 miles on it.

I was very disappointed in it.

20 MPG was the BEST it would do. One tank was only 17 mpg.

One "MAJOR ISSUE" I had with it was, it had NO POWER...NO GUTS to it at all.

You had to time your entrances into busy freeways so you didn't pull out in front of someone and make them slam on their brakes to keep from running over you.

I DIDN'T LIKE IT AT ALL.

Wouldn't have one with the base engine...no way...no how.

Just my opinions... :D:D and I have PLENTY of them. :D

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Was the base model CTS from "last year" one of the 2.8L models or a 2008 with the non-DI 3.6L? My 2005 non-DI engine moves the car right along, but a 2007 base model with the 2.8L might be noticeably different.

Bruce

2016 Cadillac ATS-V gray/black

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This is purely subjective, but I don't notice any loss of fuel mileage between 55-65 MPH. Around 75 MPH, however, I think I begin to lose about 1 MPG.

Just S.W.A.G.

Regards,

Warren

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There is no means of avoiding the final collapse of a boom brought about by credit expansion. The alternative is only whether the crisis should come sooner as the result of a voluntary abandonment of further credit expansion, or later as a final and total catastrophe of the currency system involved. - Ludwig von Mises

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I would love to see a longer table, with more models tested.

Should be easy to do for your car anyway. Just set it to "instant MPG" at the different speeds on a flat road with light winds.

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Was the base model CTS from "last year" one of the 2.8L models or a 2008 with the non-DI 3.6L? My 2005 non-DI engine moves the car right along, but a 2007 base model with the 2.8L might be noticeably different.

I had two CTS's last year.

One was a 2008 3.6L "NON DI"

It was a very nice car.

Ran good.

Good fuel mileage.

etc..etc..etc.

The other one was pretty much a base model.

I "REMEMBER" it being a 3.2...but maybe it was a 2.8.

Whatever it was, it had no power and no acceleration...when compared to the 3.6.

I wouldn't have it on a bet.

It was a very poor example of a Cadillac.

I didn't even take Darling Wife riding in it...it was that bad.

I have yet to drive a 3.6 DI.

I hope to do that soon.

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There are three (driver influenced) factors that dramatically affect MPG, and of course each can have varying extremes:

- tire pressure

- using cruise control

- speed

from what I recall it seems having the correct tire pressure, and traveling with cruise anywhere between ~55 and~70 for an extended time gave me approx the same MPG. once above about 75, all things start to change.

the fourth although not scientific at all is that when I leave the "AVG MPG" on the display, I seem to average out better MPG no matter what I do;)

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the fourth although not scientific at all is that when I leave the "AVG MPG" on the display, I seem to average out better MPG no matter what I do;)

:D Wrapping up the steering wheel with gas station receipts helps too. :)

The saddest thing in life is wasted talent

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There are three (driver influenced) factors that dramatically affect MPG, and of course each can have varying extremes:

- tire pressure

- using cruise control

- speed

from what I recall it seems having the correct tire pressure, and traveling with cruise anywhere between ~55 and~70 for an extended time gave me approx the same MPG. once above about 75, all things start to change.

the fourth although not scientific at all is that when I leave the "AVG MPG" on the display, I seem to average out better MPG no matter what I do;)

I agree with your first three...I have about the same results.

The fourth one...for me...I have better results if I leave it on "INSTANT MPG".

It immediately tells me when my foot is getting too heavy. :D:D

It drops from 22/24 to 16/17 and I know I need to ease up. :D:D

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At a steady 55 mph on a flat section of highway, my Cadillac Eldorado ETC delivers a big-oil-industry-maddening 34 mpg.

Yeah, had that same thing with my '99 STS. These are both pre-redesign of the Northstar, and the gearing is very similar if not identical.

Different issue with my 2008 DTS. I feel the gearing is such that 55 - 65 provides very little advantage in fuel economy. It begins to shine above 65 and extends to about the afore mentioned 75, where wind resisitance becomes a huge factor.

I feel it has to do a lot with getting the engine to a proper point of the operating RPM range. At 71 MPH, the DTS turns about 2050 RPM, and will return a very respectable 28.5 mpg. At 60 we're talking somewhere around 1750 rpm, too low to be in the peak of the power curve, and likely below the most economical operating rpms, but will still produce the advertised 26 mpg.

I feel that the different cams tend to narrow the "economy" band in the "Y" (SLS/DTS) engine, with the "9" engine (STS) being much wider.

Never underestimate the amount of a persons greed.

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Consumer Reports did an interesting study which is documented in their blog here.

In the study they measured steady state fuel economy for a variety of cars at three speeds: 55, 65, and 75 mph.

For example, the Lexus RX350 3.5-liter V6 got 30.9mpg at 55, falling to 27.4 mpg at 65, and finally 23.0 mpg at 75mph.

I would love to see a longer table, with more models tested. My steady-state driving tends to be in the 73 mph range, and the CTS in my experience gets ~26 mpg in that range.

The Honda Fit got 36 mpg at 75mph, and the Toyota Yaris got 34 mpg at 75 mph. Still more economical than the CTS, but not by the huge margin you might think.

Also, the CTS 3.6L V6 actually does as well as some 4 cylinders.

between 60 and 65 i get between 30 and 33mpg in my 00 deville dhs. thats with AC on full blast. turning off AC gives a gain of 2 MPG. im very happy with my Devilles MPG considering its a heavy car with a powerfull V8 ;) . My 05 montecarlo LS would get 37mpg at 60mph beleave it or not. great car for MPG

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