I have OZ Ultraleggera wheels and have noticed when I go by the radar trailers that post your speed that my speedometer reads about 2 or 3 mph faster than I am really going. If this is off, does this mean that my odometer is overstating or understating my actual mileage. I am still under warranty and don't want to overstate my mileage. Finally, if I am overstating my mileage, is it a dealer fix to recalibrate the speedometer. I've devoted brain power to this but alas I am not an engineer and hated story problems in math!

 

While speedometers are usually a few percent high, odometers are generally close to spot-on, at least with stock-size tires.

The fact that most speedometers are a little "optimistic" is a safety buffer for the automakers. If they were to produce cars whose speedometers read too low, they'd be asking for a class-action lawsuit from people getting speeding tickets because they were going faster than their speedometers reported.

 

Zuh? Isn't the speedometer (and also the odometer) reading off a gear in the transmission? It should read spot on, calibrated to whatever tire diameter Mini specified (I have no idea what that is). So, generally, if your speedo is off, your odo will be off, too.

 

Even among the stock MINI wheel/tire choices there are slight differences in overall height. So there will be a slight variation of speedo/odo readings even with stock cars. So manufacturers generally read a few percent high.

My Classic reads almost 10% slow (with a constant jumpiness of 3-5mph) So it is very hard to make sure that I am not speeding. Not to mention that everything feels faster in that little car.

 

Even though there's a single sensor for both systems, each system interprets and displays the information on its own. It's like the old purely-mechanical systems, where although there would only be one spinning cable from the transmission, the speedometer used a magnetic connection to couple the rotating cable with the speedometer needle. By adjusting the gap between the spinning magnet and the needle, you could calibrate the speedometer separately from the odometer, which used the same spinning cable to turn the odometer via a direct-connect configuration of gears.

Likewise, with the MINI, even though there's only one sensor and everything's electronic instead of mechanical, both the odometer and speedometer can have their own "correction factors". I believe that the speedometer is purposely given an "optimistic" correction factor so that it will read a few percent high (which virtually all of them do), while there's no reason to build any inaccuracy into the odometer.

Several posters have confirmed this experimentally, using a stopwatch and mile markers (or GPS) to show that although their speedometers are reading slightly too high, the odometers are correctly recording the actual distance traveled.

 

Good to know that the odo may be more reliable than the speedo.