Odometer Correction
 

Re: https://www.jaguarforums.com/forum/x...omputer-10878/

Anyone know if I can get the odometer corrected on the speedo for the X-Type here in the States (DC area if I can be even more precise). I see it's pretty common and popular for our friends in the UK/Europe, but I'm not finding much info for us here in the States.

Help.

spiel, you should be able to take the car to the dealership and if you can tell them that the speedo is off by X.XX% (where X.XX is some calculated percentage), they can hook up their computer and make the necessary adjustment to get it spot on.

Now, I am sure you are curious how to do a calibration of your speedo. It is really easy. Find a nice flat area of highway that you can set your cruise on and be able to go at that same speed for a few miles (around 5 is more than good enough). Now, looking at the milemarkers on the side of the road, as you pass a milemarker, start a stopwatch and note what milemarker you just passed. After a few miles that you have been able to travel at this constant speed and noting that speed, stop the stopwatch as you pass the milemarker. Find the distance that you traveled via the milemarkers. Take the time and convert it over to seconds (ie, if you traveled for 4 minutes, 45 seconds, you would have traveled 285 seconds). Now, using a calculator, perform the following calculation: 3600 x miles traveled / seconds it took you to travel that distance. Using what I mentioned above, assuming you traveled 5 miles in 4 minutes and 45 seconds, you would end up with 3600 x 5.0 / 285, or an average speed of 63.16 mph. Now, with this number, you will need a second equation to calculate the percent error. In this case, you take your calculated speed and subtract from it the indication from your speedo. Take that difference and divide it by the calculated speed. So, if your speed indicated say 60 mph when you did the run, you would end up with (63.16 - 60) / 63.16, or +5.00% error (ie, the car travels faster than the speedo says, which is normally the case due to most people putting bigger tires on their vehicles). If you end up with a negative number, that just means that your tires are smaller that what the factory thought you should be using. From there, the dealership can get into the ECM and there is a number inside the computer that the car uses to count the number of pulses coming from the ABS wheel speed sensors and after so many pulses, the computer increments the ODO a tenth of a mile. Adjusting this number will cause the speedo to be adjusted and the ODO to be more accurate.

For you metric guys, this same equation works, you just have to do KMs vice miles (I would recommend doing about 8KMs for a fairly accurate measurement) and your speed will come out in KPH. From there the rest is the same.

Hmm... this sounds like a reasonable gear ratio change might be accomplished without requiring a TCM reprogram. Just tell the systems that the car is on really small tires ;)

Plums, but telling the car that it is on really small tires is reprogramming the ECM, which is not any less/more dangerous than reprogramming the TCM. So, I am not quite sure what you would be gaining without reprogramming the TCM. if anything, I would think you would want to alter the shift points with the new gearing to help maximize the performance gains.

I played with my truck and at one point had a +16.7% error in my speedo (actually doing 70 when the truck said 60). The truck still drove like normal other than the shift points were off slightly because of the speedo error. Once I recalibrated my speedo (for the truck was nothing more than installing a new $8 plastic gear), I noticed that my mileage came back up 2 mpg (lost 4 mpg due to the bigger tires) and my truck seemed to shift at more appropriate points. Now, this is kind of an extreme case, but you can see through this how tire size can affect the whole balance that is established in modern day cars.

Most cars coming off the lot are designed to have up to a 3% error in the speedo without any negative effects. But, I'm sure that keeping the speedo as accurate as possible can help you squeeze the most out of the car since the engineers try to figure things out assuming no errors.

"really small tires" is just a different way of saying "speedo error".

The attraction of what you wrote about is that it is a standard setting performed by a Jaguar tech. It is the setting of a variable and not a reprogram. This is compared to the aftermarket TCM reprogram that is mentioned in similar settings. The aftermarket reprogram seems to be quite a bit more money than a couple of hours at the dealership.

As an aside, most vehicles that are manufactured for the global market will be skewed towards overreading rather than underreading. This is because there are penalties in the EU for under reading but a tolerance for over reading.

plums, if you are truely reprogramming the TCM, yes, that can be very involved because if you are familiar with the fuel curves inside of the ECM, imagine having one of those for each gear in your car and making a change to one of these affects the others. But then, reprogramming the TCM can allow the car to shift where you want, how you want, and potentially give you better mileage too. The big change that I would make in the shifting is simply forcing the computer to make the tranny shift faster. Most trannies shift way too slow to be good for them due to the fact that we are wanting that silky smooth ride. me, I don't mind the little bit of a jerk when the car shifts because I know the tranny is not slipping and generating destructive heat.