Gas Mileage

[mav1178]

Originally Posted by AVodka14

Noooooo. I don't think so. It's hard to explain how it feels. I have downshifted into second at high speeds. Let's get back on topic.

Just saying, proper heel/toe while braking can always get me into any lower gear at redline.

2nd thru 4th possible, depending on my speed.

-alex

[strat61caster]

Originally Posted by AVodka14

I imagine one should follow the computer over the odometer if you change the wheels and tires. Right?

They would both be wrong.

My assumption is that the computer estimates fuel used based on the system that controls the injectors. No problem there (that's the gallons part of m.p.g.) 'I have this many injector pulses over this time step which equates to X amount of fuel per time step'

The 'miles' part of the mpg rating is based off of the speedometer/odometer which both function on the principle of 'I know how fast the output shaft of the transmission is turning (or possibly it uses the wheel speed sensors embedded in the ABS/TCS system), I know the circumference of the OEM wheel/tire setup, this is the resulting speed, distance traveled per time step' Changing the circumference of the tire means that this calculation will be different from reality.

Easiest way to adjust would be to plan a trip, reset the tripodometer and compare to google maps or something, say a 100 mile trip shows up as 110 miles on your odometer. You can carry that into your fuel calculations to make it more accurate, in this case you would subtract 10% off the mileage for each tank (268 miles on the odo is actually 242 miles). For instance it's well known that my truck is off by about 8%, lots of posts on the forums about it.

Another trick is to set your cruise control for 60 mph and watch for mile markers on the freeway, you can watch the odometer or pull out a stopwatch, you should click off 1 mile every minute consistent with the markers on the side of the road.

But the reality is you would have to make a pretty big change to the wheel setup to really affect your odometer/speedometer, but the variation exists nonetheless. Odds are unless you're building a showcar (20" rimz yo) it won't matter. It really only counts for armchair engineers and spreadsheet fanatics, just a fun idea to kick around.

[sero901]

Originally Posted by strat61caster

The computer in the car is always optimistic in my experience, and not just on this car. If you flip through the manual I believe most cars with the feature have a disclaimer that this reading is an approximation and may not be accurate.

Gas stations are calibrated at least once a year and your odometer is tied to your speedometer, I'd trust those over the onboard computer any day. Unless of course you've changed tire diameter which would throw off the speedo/odo.

Back in high school when i worked at a gas station our pumps were calibrated to 15deg C (59F). Due to this you actually get more fuel under 15 (the fuel has a higher density/gallon) and less fuel if its over 15 deg out (lower density/gallon). I don't know if this is enough to explain the discrepancy but its a theory.

[Wepeel]

In the summer, for DD, I average around 34-35. In this polar vortex nonsense I'm right around 30-31. Hand calculated.

I also noticed the mpg display is optimistic by a good 1-2 mpg. Consistently.

I actually calibrate the odometer - I'll run 50 miles on marked highway/interstate and compare that to the odo and see how far off it is. This helps account for different tire sizes. I've found the stock calibration isn't far off but there is a difference with different tire sizes (225/45-17) - not a lot, but if you're being anal about it it can shift the numbers a bit.

[nos145]

8.1l /100km lifetime average w 31k km on the car. aka good mix of HW and street