Speed and MPG

[breugel]

Some background
I have a 2004 Limited with a 3.3 V6. I just put new tires on and went to a very slightly smaller size because according my GPS my speed and odometer was about 3% off. Now it is with in about .5%. This weekend I have changed my oil with Mobil 1 with a Mobil 1 filter. Normally there is about 500Lbs of cargo and people in the car. All for tires were 32 PSI



In my job I drive allot and my mileage is from 24 to 21 mpg. This is going 70 to 80 MPH on the interstate and maybe about 50 miles thought the country and small towns.

Today I drove to Peachtree City Ga @ 280 miles. I drove 60 to 65 MPH and averaged 60 MPH per my GPS. It was tough but I stayed slow the whole way. Now this is the good part I got 28.2 MPG that is a 4 to 7 MPG increase!!!!

This reinforces what I knew already. To get the best economy do the following...

Accelerate gentle and keep the engine between 2000 and 2200 RPMs when cruising.

[DrRuckus13]

Is your 04 AWD? I have never got anything above 23. I have also noticed about 65 is the breaking point for the milage going down.

also how many miles do you have? and did you use 93?

[breugel]

Originally Posted by DrRuckus13

Is your 04 AWD? I have never got anything above 23. I have also noticed about 65 is the breaking point for the milage going down.

also how many miles do you have? and did you use 93?

Sorry forgot that... 2wd with 95,000 with 87. When I come back down in a couple weeks I will use 93 to compare.


Drive like there is a egg between your foot and the accelerator This helps allot. If I drive in the city for the whole tank it is approx 17 - 19 MPG.

[deadrx7conv]

What tire sizes did you switch to/from???

[LifeTech]

My Scanguage II shows that my ODO is approx. 3MPH slow running. I checked two other HL's with stock tires ...all the same.

I have yet to check another friends HL with upsized tires(235/70-16) Mich. CrossTerrain SUV's.

[imjin]

You're dead on with the speed/rpm threshold. The general consensus seems to be running at roughly 65 (max) or keeping the RPM's at 2K or lower seems to really help.

I'm wondering what the size differential is between your original tires and the new set. Your wheel speed sensors are calibrated at the factory to the tire size that came on the vehicle originally, and that calibration doesn't change. I'm not even sure the dealer can tweak that. In using my own tires as an example, with larger than stock tires having fewer rotations, the computer is interpreting the sensor data incorrectly which in-turn throws off the speedometer, odometer and trip meter readings into a negative direction, displaying a slower speed than you're actually going, which in-turn causes the odometer to add fewer miles than was traveled.

Imagine cutting into one of your original and new tires and stretching them out flat on the ground like a piece of spaghetti, side by side. The original larger tire would be longer than the new smaller tire. If you flipped them end-over-end, the larger tire would reach the destination in a shorter amount of time than the smaller tire traveling the same distance. When the computer counts the number of revolutions that it uses to calculate miles traveled, the actual versus the programmed revolution count would be off slightly. Over a period of miles traveled, that "slightly" starts to get multiplied.

With smaller tires, as in your case, they would turn more rotations and cause the opposite effect on the calculations, adding speed and distance to the speedo, odometer and trip meters. So, if you're speedo says you're going 55, you're probably going a bit slower than that and your odometer and trip meters will start to add up more miles than you actually traveled, which MIGHT account for the improved mileage readings you're getting. Your RPM's will probably drop because of the decreased mass the motor & transmission has to turn. If you're using these to manually calculate MPG, they might be a little off because of that. Have you tried using a GPS to see how close your speedometer is reading? Other than a dyno, it's probably the best way to gauge how fast you're going over what the speedo is reading.

Using my truck as an example, I went from the stock size of 265/70/16 (30.60") to a 285/75/16 (32.83") and it threw my speedometer off by about 4 MPH into the negative, so my speedo was showing a speed of 55, but in actuality I was going about 59. I just recently installed a Truespeed calibrator to fix the problem, but I had been driving it like that for about 4 years. I've also been keeping steady fuel data for the last 4 years, from each fill-up and I'll probably need to go back and adjust the mileage numbers to compensate now that I can get a good set of true mileage reading, which affects my MPG averages.

[breugel]

I did have 225/65/17 (I think) and I now have 215/65/17. Their tires were changed a month and a half ago. My trip computer is normally +-1 mpg of measured if I used my GPS mileage not odo. I re-calibrated my tire pressure sensors when I got the tires and it works fine.

[imjin]

Ok, so you went from a 225/65/17 (28.5") to a 215/65/17 (28"), so not much difference really. I doubt a 1/2" in diameter change would account for much. The sensors I mentioned are the speed sensors, not the tire pressure sensors. In that case, grats on the upped MPG!

[LifeTech]

Originally Posted by imjin

Ok, so you went from a 225/65/17 (28.5") to a 215/65/17 (28"), so not much difference really. I doubt a 1/2" in diameter change would account for much. The sensors I mentioned are the speed sensors, not the tire pressure sensors. In that case, grats on the upped MPG!

On the 1st gen HL's The TPMS is an indirect system. There are no actual pressure sensors on the wheels. This indirect system uses the ABS axle speed sensors.

The Scan Guage II tool operates on the fly while vehicle in motion. It's MPH is accurate and gets data from the ECM/TCM. It shows that the analog speedo is faster than actual speed. (actual speed running slower than displayed on the speedo cluster)

I feel that the analog speedo Guage itself is off calibration. Because it is getting the same speed impulse voltage from the ECM/TCM as the scan tool is getting. But the ScanGuage maintains the Data in digital form. The Dash Speedo doesn't.

LT

[kathyricks]

55 MPH delivers even better fuel economy.

[breugel]

Originally Posted by kathyricks

55 MPH delivers even better fuel economy.

Maybe but when the speed limit is 70 I do not want to get run over.

[LifeTech]

With my 3.3 4WD(5-Speed), I find that 60 MPH is the sweet spot for economy. I used my cruise control and my Scanguage II for reference.
These MPG readings also matched my "Trip Info" numbers on the Nav Touch Screen.

My best recorded average MPG was 26.8 on a 25 mile trip...always using Regular Grade Fuel.
Ambient temps was 70-72 degrees F. Little to none wind conditions. Flat grade roads.

A/C compressor was Off. Recirculate was "On". Windows closed. Driving alone with no Cargo.
Tire PSI @ 32 all around. (Mobil 1 oil).

FYI, it is very hard to duplicate this Economy time and time again. I feel Ambient Temps play a big part in the equation.

LT

[shanelabs]

Anything over 55 is going to drain your gas. There's an explanation and calculator at MpgForSpeed.com - Fuel Efficiency Vs. Speed that shows the effects.

[LifeTech]

On a side note, it is interesting to note that on the link posted, http://www.fueleconomy.gov/feg/findacar.htm

the I-4 cyl.HL's are not too efficient and it's Carbon Footprint is very poor when compared to the V-6 HL's.

Great info regarding Speed an MPG.