khuezee

So on my IS250 (auto), I can usually pull about 30 mpg on the freeway consistently w/ cruise control. A funny thing happend to me the other day, I filled up the tank completely and so I hop on the freeway immediately. I set it on cruise control at 70 mph, and I averaged about 37 mpg for the next 45 miles. Is this correct? I know that the computer resets itself at each fill up, I'm refering to the Tank Avg. read out. Is it accurate? I would think not since a more gas in the tank would make the car get less mpg cause of weight. also, i have the same driving habbits on half a tank and I only get about 32 mpg.

PhilipMSPT

iTrader: (5)

There are two problems:

1) the average MPG calculation is slightly inaccurate, as the computer reads that you are driving 99 MPG while coasting without stepping of the gas.

2) short distances of highway driving does not equal the long distances of city+highway driving.

In other words, your short 45 mile run may have "read" 37 MPG, but it does not mean you're getting 37 MPG altogether...

lobuxracer

iTrader: (2)

Weight has nothing to do with gas mileage at steady state cruise unless you are climbing hills/mountains. You accelerate one time, set your cruise, and all you are battling is friction and wind resistance, with wind being the larger component at 70 mph.

It is very possible this is correct for the run you made. You might have had a tail wind (reduces the major resistive component) which would definitely push up your mpg. I once got 47 mpg from my Supra (actual measured) over a 67 mile run because I was going basically downhill and had a very strong (30 mph) tail wind. I've never seen anything like it since, it surely was an anomaly.

PhilipMST - I don't know where you get the idea about the 99 mpg indication. I drive to the Bay Area regularly from where I live. It's 106 mile drive. I have filled up just before I got on the freeway, then accelerated essentially one time and driven the entire trip without stopping or even braking significantly on cruise control. I do not see 99 mpg in cruise, I see actual. It varies from 21 to 99 depending on the grade I am climbing or descending, but on the flat portions it never exceeds 31 mpg unless I'm 5' off the back of a tractor trailer in full drafting mode.

al503

Weight does have an effect. All other things being equal, the heavier the car, the more the tires will deform, absorbing energy.

ABC

I got 35.3mpg on my 250 once. Same situation, gased up and jumped right on the highway and set cruise to 75mph for about 25-30 miles.

lobuxracer

iTrader: (2)

With properly inflated tires less than 0.5% difference in fuel economy. Adding a passenger adds a minuscule amount of deflection, and adult passengers are all heavier than a tank of gas. There's only a ~100 lb difference between empty and full. I have LOTS of practical experience with this.

PhilipMSPT

iTrader: (5)

Sorry. I didn't mean Cruise Control. I meant coasting without pressing the gas...

al503

Care to share how you came up with .5%? As I understand it, the more weight, the more the tires deform. The more they deform, the more they absorb/convert the kinetic energy into heat.

Sounds like you've figured out a formula to be so definitive with your <.5% figure. I'm just confused at how arrive at this when there are so many variables such as the tires themselves, how you adjust for the weight of the car, etc. Do you measure the heat of the tires before and after and the road surface? Glad someone can put all of this together.

ihkskim

youre' not getting 37mpg. its taking the avg from when you first filled up w/ the distance. ever notice how quickly the numbers change when you first fill up? you can drive the same as you did when you first filled up and it will go down regardless because there are more numbers to be averaging out

al503

He didn't ask whether he's averaging 37 MPG alltogether. He's asking if he averaged 37 for that particular highway stint.

lobuxracer

iTrader: (2)

It's not a formula, it's empirical. Half a tank of fuel only makes a 1.2% difference in weight assuming driver only in the car. I monitor mileage most of the time while driving with the IS. I also used to keep a log with every fuel purchase and the mileage associated with the fuel purchase (but not with the IS.) I've never seen a significant difference in economy (~0.5%) between running the tank to reserve and filling up after only a quarter tank on any of my cars ('88 GTS Corolla, '93 GTS Celica, '94 MkIV Supra, even my '88 Corsica beater car.)

Also, I push my tire pressure up 2 psi for long distance drives to aid fuel economy. It reduces sidewall deflection, tread squirm, and the tires run cooler which means I am converting less kinetic energy to heat in my tires and follows all the conventions for fuel efficient operation and extending treadwear. At the same time, remember, your tires heat up as you drive which causes the pressure to rise, so your tire's contribution to economy actually improves over the life of a drive from less tire flex.

You are right about a ton of variables. I've never tried to quantify all of them. It's hard enough to determine engine efficiency calculations with weather variations, but when you add fluid drag from the driveline oils, aerodynamic drag based on weather and altitude, tire and road coefficient of friction, dynamic tire pressure, etc, etc, it gets absurd pretty quickly.

lobuxracer

iTrader: (2)

This is not what I have observed. I typically see the mileage per tank increase steadily over the first 35 or 40 miles of my trip. That initial acceleration kills your numbers for quite some time even if you are trying really hard to accelerate with best efficiency. After 40 miles it pretty much plateaus and variations in the road itself make a bigger difference than anything.

ihkskim

well yeah thats what i'm saying~ he's not getting 37mpg because he just filled up his tank.. but yeah i dunno maybe i explained it wrong

ToothDoc

also, a hot engine burns less gas. So, if you fill your tank, on a cold engine and start driving, you'll get less MPG. If you fill your tank after driving a while and then reset it, you'll get more MPG. In the winter, because it takes so long for the engine to warm up, my consumption goes form 20-21 mpg to 17 mpg. My commute each way to work is 10 miles and my average speed is 40 mph.

lobuxracer

iTrader: (2)

Yeah, that's where I "cheat." I have to drive 5 miles to the gas station before I fill up and hit the freeway. So I get most of the cold cycle inefficiency worked out before I get the tank mileage reset and get on the road.