Quote:
Originally Posted by AVodka14
I imagine one should follow the computer over the odometer if you change the wheels and tires. Right?
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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.