This is actually easier than you think currently. Most muscle cars (like mustangs/camaros) allow you to change your rear end gearing. Different size rear end ge

This is actually easier than you think currently. Most muscle cars (like mustangs/camaros) allow you to change your rear end gearing. Different size rear end gear ratios are different factory options. There are programmers that allow you to tell your computer what gear ratio you running so your speedometer is correct. There are also changes that can be made for bigger/smaller diameter tires. You could change the computer's gear ratio setting and tire size setting. Your speedometer would be way off but you'd also be accumulating fewer miles. It's not directly modifying the mileage of the car, but over time the numbers of miles on the car would be far lower than what they really were. Then once you sell or trade-in the car, just set the settings back to the original ones.