• Your spedometer is wrong, too! Rather than give you the answer, I'll give you the link to the calculator so everyone can use it. https://www.miata.net/garage/tirecalc.html
• Modifying your odometer reading is a felony, regardless of the method. The federal fine is 3x the profit you make from the fraud or \$1500, whichever is higher. So, it's not worth saving \$400.
• mushroom
The question was "What mathematical adjustment should I make to my odometer reading when calculating gas mileage?" not "mechanical," so I don't think we're talking tampering which is difficult. Then again, on modern cars, there may be programming which can change tire size so the gauge will read accurately.
• bostjan64
I never said mechanical. I don't have the cases in front of me, but there are precedents that determined that replacing tires with larger tires and knowingly not recalibrating the odometer to reflect the correct mileage is, indeed a federal felony. Likewise, using smaller tires and altering the title to reflect a lower mileage has resulted in civil suits, predicated on the allegations of fraud during the practice. Proper odometer programming cannot be done by a private individual without risk of federal charges. [ for reference on the law itself, see https://www.justice.gov/civil/case/federal-odometer-tampering-statutes ]
• Bry-dog
I got the truck like that. Somebody made the mistake of putting wrong tires on it. There is no criminal intent here. Jesus: Is it a felony to fart?
• Bry-dog
I am not physically modifying the odometer if that is what you thought. It is an old 86 truck. It came the way it came to me. I am trying to figure real miles traversed based on math. The odometer never physically gets adjusted. I am trying to take note of real miles; not the lie the odometer is saying.
• bostjan64
If you fart on a odometer, and the government finds out about, probably yes. So, how do you know what the mileage when the tires were changed? If the bigger tires were 485 rev/km, and the smaller ones were 469 rev/km, then the ratio between the two is 0.967, regardless of miles or km. So, if your speedometer says you are going 65 mph with incorrectly assumed smaller tires, you are actually going 67 mph with bigger tires.