Thanks for the correction. I was wondering 4cm sidewall seemed low but I was convinced that the sidewall marking on a tire meant the height in millimeters.

 

Just to raise the geek/dork factor another notch, the 40 does indeed refer to % of the tread width and can be used to calculate tire diameter/radius/circumference but it must be remembered that the section of the tire in contact with the road deforms temporarily reducing the side wall height to less than the non-loaded areas.

In a practical sense, it means that tire pressure and vehicle weight will affect the actual in-service circumference and therefore speedo/odo accuracy. Many TPMS equipped vehicles use this effect to detect a tire with low pressure.

 

Well, I just had my summer 2-'s taken off and put stock Jaguar 19's on and had to get the dealer to calibrate the car for the smaller wheels. The cars speedo was off by 3 MPH.

The dealer said this is important to do as there is a file in the ECU that know what the wheel and tire size is and they need to set it correctly.

So maybe some of you seeing a difference have the wrong setting and the dealer can set this for you?

 

I have this same issue. 35t AWD with the 20 Rims.

I took the car on a road trip and I had the cruise set at 80 mph in a 70 mph speed zone and there was still cars flying past me. I wonder if this is an issue with the 20 rims?

I am leasing and I am also worried about the milage racking up quicker than it should.

 

I believe I have the 18 inch wheels, not 20. I also do not see how setting a cruise and comparing to other cars on the highway proves anything about the milage and actual speed..... I could be just drawing a blank though. You need measure distance traveled in another car to confirm mileage.... and then install a speedometer app on your phone to confirm the speed.

 

Dealer said nothing they can do for speed or distance.... actually said there is no settings associated to this......

 

Try Paul Miller Jaguar in Madison NJ. They use the CCF file all the time to set Wheel/Tire size all the time to get the speedo correct. Don is the tech who does this.

 

I installed the speedometer app, set the cruise to 70 and the app showed 67. So about 3 mph off at the 70 mph range. So at 80 it would be closer to 4 mph off which to me seems a lot.

 

have you tried checking the mileage against another car? that is the real problem.... the speedometer is annoying but I can deal with it. What I Can't deal with is the extra mileage its putting on. The speed being set higher is actually done on purpose on these cars apparently.

 

Being that I run the 20" Propeller wheels in the summer and the 19" Venoms in the Winter I did check and the overall rolling height is much different. I will check my Speedo against another car and GPS to see how far it is off.

 

I noticed this weekend that the distance to a location I have driven to for several years is off by 32 miles. This location is 131 miles from my house, and has shown approximately that every time with my Jeep. First trip with the new XE and the odometer read 163. I will have to do this a few times to confirm, but this is really bad when you have a leased car.

 

thanks Cat I was talking to myjag though but if you can check it as well it would be helpful to all I am sure.... as to rgun, that is a huge difference, I would be very worried as well.

 

I just picked up my new 25T 4 days ago and am very happy with it, no problems, except the speedo reads 9-10% high which is the biggest difference I have ever seen. The Kia I traded in was almost spot on. When I next take it in to the dealers I will ask if they can reset it but it is not too big an issue now that I know that I can just fudge in 10%

 

I have a hunch the trip computer uses the inflated speedometer for distance calculation, while the actual odometer uses the correct value. This could easily be tested.

 

Set cruise to 60 mph reading on speedometer, reset trip computer and time for 1 minute. Should read 1 mile if my hunch is correct, and it's using inflated speed for distance calculation. Now repeat but set your your actual speed to 60 mph, either from OBDII port reading or some GPS app. Observe distance traveled as calculated by trip computer. Should be slightly over 1 mile.

 

Same problem with mine: Speedometer is showing from 5 to 10 kmh more than the gps speed.



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XES 3.0 V6, 340PS, 19" Alloy Wheel, Black pack, Lighting Pack, Active Safety Pack, Cold Climate Pack, Parking Pack, Memory Pack, HUD Pack, Sliding roof, Pro Navigation Pack Meridian 380W, Configurable mood lighting, illuminated treatplate

 

I wouldn't be surprised if some people who are seeing slight variances in their speed/odometer/trip numbers, have the wrong tire settings in the vehicle's computer. Hey, dealers have been known to swap out tires on vehicles, to please customers, then overlook something like that config.

It could definitely cause your vehicle to register more miles and hurt you on a leased vehicle.

 

My car has standard 18in rims and factory tires but still shows +4mph difference.

I will ask the dealer next time to fix it if possible. My first service is due very soon.

 

Is there really a way to tell the computer what tire size you are driving?
My dealer doesn't seem to know how to do this.

 

My dealer said there is no such settings so its kind of scary to think who is working on our cars in these places. It is something so obvious even many users around here seem to know about and a Jaguar mechanic doesn't have a clue.