# Speedometer Problem

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**1****Speedometer Problem**

Guys,

So I possible found yet another problem with the speedometer on my XE, it consistently reads 4 mph above the actual speed. When it shows 64 I am actually driving 60mph. Installed 2 apps on my phone to confirm this, I also tried both apps in my two other cars and the reading was spot on with the car. Is this a problem or perhaps intentional? My biggest concern is can this actually affect the odometer reading? I can't say this for sure as there are so many variables but the few trips I made it seems like the car is putting on an extra 1 to 2 miles for each trip to and from work (1 or 2 miles each way). I have done the numbers with my previous cars and it was 22 miles each way, the same is shown on my phone GPS. In the Jag the couple times I checked I put on 22 or 23 miles and I tried to go shortest route possible so I am pretty confident something is wrong. The Jag's GPS actually shows 21 mile trip vs 22 of my phone so there are discrepancies but it still does not feel right. Can the speedometer reading affect the odometer ? I am not sure if they are tied in together.

So I possible found yet another problem with the speedometer on my XE, it consistently reads 4 mph above the actual speed. When it shows 64 I am actually driving 60mph. Installed 2 apps on my phone to confirm this, I also tried both apps in my two other cars and the reading was spot on with the car. Is this a problem or perhaps intentional? My biggest concern is can this actually affect the odometer reading? I can't say this for sure as there are so many variables but the few trips I made it seems like the car is putting on an extra 1 to 2 miles for each trip to and from work (1 or 2 miles each way). I have done the numbers with my previous cars and it was 22 miles each way, the same is shown on my phone GPS. In the Jag the couple times I checked I put on 22 or 23 miles and I tried to go shortest route possible so I am pretty confident something is wrong. The Jag's GPS actually shows 21 mile trip vs 22 of my phone so there are discrepancies but it still does not feel right. Can the speedometer reading affect the odometer ? I am not sure if they are tied in together.

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**3**I think odometers work off actual rotations of the wheel. Or at least that is how they used to work.

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**4**
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**5**Vehicle certification rules that that a speedo must never under indicate the speed of the vehicle and instead must be spot on, or over indicate the speed up to a certain %.

Since it would be uneconomical for an OEM to make a speedo that's 100% accurate at all speeds throughout the entire tire life of a vehicle, the units typically over indicate.

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**6**Vehicle certification rules that that a speedo must never under indicate the speed of the vehicle and instead must be spot on, or over indicate the speed up to a certain %.

Since it would be uneconomical for an OEM to make a speedo that's 100% accurate at all speeds throughout the entire tire life of a vehicle, the units typically over indicate.

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**7**Vehicle certification rules that that a speedo must never under indicate the speed of the vehicle and instead must be spot on, or over indicate the speed up to a certain %.

Since it would be uneconomical for an OEM to make a speedo that's 100% accurate at all speeds throughout the entire tire life of a vehicle, the units typically over indicate.

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**8**Maybe they're spot on today, but if you were to install brand new tires (larger diameter than worn out tires) your speedo would under indicate. This is clearly out of spec as per international agreement:

https://en.wikipedia.org/wiki/Speedo...nal_agreements

- The indicated speed must never be less than the actual speed, i.e. it should not be possible to inadvertently speed because of an incorrect speedometer reading.
- The indicated speed must not be more than 110 percent of the true speed plus 4 km/h at specified test speeds. For example, at 80 km/h, the indicated speed must be no more than 92 km/h.

Not sure why you've got your shorts in a twist about the distance error. What's the chances it would favour a dealer?

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**10**https://en.wikipedia.org/wiki/Speedo...nal_agreements

- The indicated speed must never be less than the actual speed, i.e. it should not be possible to inadvertently speed because of an incorrect speedometer reading.
- The indicated speed must not be more than 110 percent of the true speed plus 4 km/h at specified test speeds. For example, at 80 km/h, the indicated speed must be no more than 92 km/h.

Not sure why you've got your shorts in a twist about the distance error. What's the chances it would favour a dealer?

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**11**https://en.wikipedia.org/wiki/Speedo...nal_agreements

- The indicated speed must never be less than the actual speed, i.e. it should not be possible to inadvertently speed because of an incorrect speedometer reading.
- The indicated speed must not be more than 110 percent of the true speed plus 4 km/h at specified test speeds. For example, at 80 km/h, the indicated speed must be no more than 92 km/h.

Not sure why you've got your shorts in a twist about the distance error. What's the chances it would favour a dealer?

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**14**If a speedo is dead accurate with worn tires, it will under indicate with new tires. They will be slightly larger in diameter than worn tires so the car will physically go further/faster with each revolution. The speedo might indicate 60 but the car will be going faster than 60. Not permitted under the regs.

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**15**Veteran Member

I am swapping out my stock 20's for Jaguar 19 Venoms for winter months and they said they have to recalibrate the car ( Not just TPMS) for the wheels and tires each season I swap them.

So is it possible that all 3 wheel sizes offered by Jaguar on the XE need a different calibration which sounds very silly to me? They offer 17-18-19-20 inch wheels for the models of the XE and at a quick search I see there is a slight difference in overall rolling height which should equate to the slight/tiny MPH difference seen.

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**16**I am swapping out my stock 20's for Jaguar 19 Venoms for winter months and they said they have to recalibrate the car ( Not just TPMS) for the wheels and tires each season I swap them.

So is it possible that all 3 wheel sizes offered by Jaguar on the XE need a different calibration which sounds very silly to me? They offer 17-18-19-20 inch wheels for the models of the XE and at a quick search I see there is a slight difference in overall rolling height which should equate to the slight/tiny MPH difference seen.

Circumference of a tire (assuming it is a perfect circle) would be π*D

For 18 inch tires, the diameter would be 18inches + sidewall height. Let's take rear tires for example. Sidewall height would be 40 (which indicates 40mm) 18 inch is the diameter of the rim which equals 457mm. Thus the diameter of a tire would be 457+2*40=537mm. Using that circumference formula above, we get that at every rotation, the rear tire travels 537mm*3.14=1,686mm

Doing the same for 20in rims gives us: 20 inches to mm => 508mm. We add sidewall height of 35mm and get 508 + 2*35mm =578 total. Thus, circumference would now be 578mm*3.14=1,815mm

So at every rotation, 20in tire will travel for ~13cm more. I would say that is the reason why speedometer and odometer need to be adjusted.

I hope I did the calculations correctly. I am no expert on this subject by any means.

*Last edited by Demetre Gvaramia; 01-13-2017 at 01:08 PM.*

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**17**Veteran Member

Miata Tire Calculator | Online Miata Tire Calculator Tool

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**18**It is controlled via three separate settings (below). The values given here are the defaults for the UK:

**Jaguar speedometer calibration - Market**: Market 0 : 3.5% + 2kph

**Jaguar speedometer calibration - KPH off-set**: + 2.0kph

**Jaguar speedometer calibration - Percentage scaling factor**: 3.5%

The distance value recorded by the odometer is unadjusted and is a true indication of the actual distance traveled. Only the visual speedometer is affected by increasing the true figure by those settings.

(Note that the first is a combination of the latter two.)

The scaling factor can be set to 0, 2.5% or 3.5%.

The KPH off-set can be 0 or +2.0kph.

The market should be set to a matching combination.

Values are stored as bytes in the CCF data and can be changed with appropriate diagnostics software if desired.

**The Following 3 Users Say Thank You to xdave For This Useful Post:**

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**19**I did notice that the trip computer reading on my car was (impossibly) higher than the odometer reading when I picked up the car, by a few miles.

This lead me to believe that the trip computer may use the speedo reading for total distance while the odometer uses the correct total distance.

Either way the odometer reading is not inflated... no company in their right mind would want to deal with the legal headache I imagine that would create.

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**20**It actually sounds about right if you think about. The larger tire will cover more distance at every revolution and confuse speedometer/odometer that is not adjusted.

Circumference of a tire (assuming it is a perfect circle) would be π*D

For 18 inch tires, the diameter would be 18inches + sidewall height. Let's take rear tires for example. Sidewall height would be 40 (which indicates 40mm) 18 inch is the diameter of the rim which equals 457mm. Thus the diameter of a tire would be 457+2*40=537mm. Using that circumference formula above, we get that at every rotation, the rear tire travels 537mm*3.14=1,686mm

Doing the same for 20in rims gives us: 20 inches to mm => 508mm. We add sidewall height of 35mm and get 508 + 2*35mm =578 total. Thus, circumference would now be 578mm*3.14=1,815mm

So at every rotation, 20in tire will travel for ~13cm more. I would say that is the reason why speedometer and odometer need to be adjusted.

I hope I did the calculations correctly. I am no expert on this subject by any means.

Circumference of a tire (assuming it is a perfect circle) would be π*D

For 18 inch tires, the diameter would be 18inches + sidewall height. Let's take rear tires for example. Sidewall height would be 40 (which indicates 40mm) 18 inch is the diameter of the rim which equals 457mm. Thus the diameter of a tire would be 457+2*40=537mm. Using that circumference formula above, we get that at every rotation, the rear tire travels 537mm*3.14=1,686mm

Doing the same for 20in rims gives us: 20 inches to mm => 508mm. We add sidewall height of 35mm and get 508 + 2*35mm =578 total. Thus, circumference would now be 578mm*3.14=1,815mm

So at every rotation, 20in tire will travel for ~13cm more. I would say that is the reason why speedometer and odometer need to be adjusted.

I hope I did the calculations correctly. I am no expert on this subject by any means.

I'm sure you cited "40mm" of sidewall because you know the tire size for the rear is P275/40R18 (for example), but that's not what the 40 means. The 40 means that the sidewall is 40% of the width of the tire. In this case, the width of the tire is 275mm, which means the sidewall height is 275x.40=110mm.

Carry on.