Im running 18 inch wheel with 225/45/18 and getting 40.7 mpgs

One thing i don't understand about changing tires is doesn't increasing the tire size without recalibrating the computer to the new tire size throw off all the data regarding speed, distance (and thus mpg)? i mean all the car really knows is how many tire revolutions are being made, and uses this primarily to determine speed and distance.

Hypothetically if a 15 inch wheel is to travel a mile that would take 1344 revolutions but an 18 inch wheel rotating 1344 revolutions would travel 1.2 miles. That's a 20% increase and seems significant.

All the while the computer still thinks you only traveled a mile and has no idea you really went 1.2 miles. All that extra energy needed to travel that extra 0.2 mile increase is being applied to what it thinks was only a mile, so I would think you'd see a drastic calculated decrease in efficiency because the actual distance numbers are way off.

On the other hand it keeps your odometer reading lower than your actual mileage so that would help resale later on lol...

 

In short, tolerances. MDU MPG is known to be off.. this just contributes to it and a factor but still you get an idea what is happening.. better or worse. Of course there are limitations so if he put 22's on there like some of these "donks" now, then that's another issue. I just learned that word btw, DONK! OMG.

Urban Dictionary: donk

Video: Donk Shreds Really Expensive Tires, Still Fails. | AutoGuide.com News

 

My point is the wheels are the final gear ratio between the engine and the road, but one the computer is unaware of when changed. I doubt he is really losing the efficiency the computer is telling him he is. If he was able to take a more accurate measurement of his distance traveled versus fuel spent I bet it wouldn't be as bad as he thinks it is.

 

Actually now that I think of it his readings are probably more accurate than ours. One of the reasons we see such good mpg readings from the computer is because it is overestimating our actual speed and distance. I know most of you see speeds on your dash faster than your actual speed. You can use a garmin or other gps to verify this. That being said, his computer is probably getting more accurate speed and distance readings that we do.

 

This has inspired me to do 18s without dropping! Thanks for the heads-up!