Hi,
I'm showing 70mph @ 1900 rpm & 60mph@ 1600; is this normal?
or (as I suspect) is my speedo reading 4-5% fast? The wheel size matches the tire sticker on the door jam.

If I'm correct, can this be adjusted in one of the computers by the dealer or jobber? I only have a code reader..............


Adrian

 

70 MPH @ 1900 RPM is about what I have with my '08 and 19" wheels.

As far as speedometer accuracy, it would not at all be unusual for car's speedometer to be a few MPH off. Italian exotics are famous for having speedometers that are way "optimistic". If I remember correctly my Lamborghini speedometer indicated 9 percent high at 100 MPH. My much modified 3rd gen Mazda RX-7 pegged the 180 MPH speedometer at a calibrated speed of 172 MPH.

You can always check your speedometer to highway mile posts or to those occasional police radars that you sometimes see on public roads. You could also calculate the exact speeds from RPM, gearing and tire circumference. I do not know of any ways to make calibration adjustment to them.

Albert

 

Hi Adrian,

I think you're asking: "How accurate are my tach and speedometer?" That depends on the accuracy of the speeds and rpms you provided; your accuracy determines the conclusion. As explained below, based on your figures, your speedo appears to vary by less than 1.8% between 60mph and 70mph.

Here's the math: At 70mph and 1900rpm, you're going 36.842mph per 1,000 rpm (70/1900x1000). At 60mph and 1600rpm, you're going 37.500mph per 1,000 rpm (60/1600x1000). At 70mph, your speedo is reading 1.786% higher than at 60 mph (37.5/36.842). That 1.786% variance could be due to slightly misreading the tach and/or speedo, or errors in the tach and/or speedo. Is 1.786% really the margin of error? It could be higher or lower.

In my experience, electronic tachs are very accurate. So I think what you really want to know is: How accurate Is my speedometer?

I had the same question several years ago with my Lexus LS460. I usually drive 7-8 mph over the limit, but I was constantly being passed by other drivers going at least 10-15mph faster. I suspected my speedo was off, so I plugged in my old Garmin StreetPilot 2720 navigation unit to verify the speedo. Since that Garmin displays MPH using GPS, I suspected it was accurate, but to be sure I verified it in a measured mile against the sweep second hand of my watch. It was dead-on accurate at 60 mph, but my speedo was reading 63 mph. So I know that speedo reads 3-4mph high. I haven't used my old Garmin to verify my XKR's speedo, so I can't attest to its accuracy.

If you have a separate navigation unit, see if it displays MPH as measured by GPS. If so, that's a very easy way to determine your speedo's accuracy. Also, if you have a smartphone with navigation, you might also have this capability.

Hope this helps.

Stuart
.

 

Thanks; I have already done the same math; the Tach is too course to read accurately......, But I will crank up my old hand held GPS I had forgotten about it.
So Thanks again Guys, as you say, It may be close just every one going too fast.
I know rear end ratio is 3.31, tire size but not top gear (overdrive) ratio.

Adrian

 

Yes, the GPS is a good idea, something I forgot about too.

As far as the gear ratios, the 6th gear should be 0.69, same as the XKR. I picked the xkr specs because the XK specs are listed next to 18" wheels while the xkr is with 19".

2007 Jaguar XK 2dr Coupe XKR Specifications-Vehix

Albert

 

Albert,
Thanks..................

Adrian

 

Albert,
Based on 2000 rpm, GBX ratio O.69, Diff. ratio 3.31, and tire Dia 26.6 ( Dunlop
SP Sport 01 275/35 ZR19) speed should be 68.4 mph on a brand NEW tire. OR 65mph @1900 rpm.
Therefore my speedo is +5 mph fast on new tires or +4.2 mph on 50% worn ones.

Jaguar must have a way to reflect Wheel size effects ie. 18", 19" or 20". so wonder how this is done by Factory and or Dealer; will have to ask! Should be resetterable !

Adrian

 

Bear in mind that in the UK and many European countries, an UNDER reading speedo is almost a a hanging offense. With electronics what they are today, the degree of built in optimism is much less than it used to be, but if there is an error, it will almost always be that way in a European car. Back in the day of mechanical speedos, 5-10% optimism at a real 70 MPH was not unusual.

 

Anyone have an idea of where I can purchase a Brake Master Cylinder for my
2008 Jaguar XKR - I let the brake fluid get low from a leak in the right front braided line
resulting in over pressing the brake pedal for start mode since NO KEY.

QRAgoodmedicine@gmail.com is where I can be reached

 

Antonio,
Try Gaudin Luxury Cars Las Vegas, New & Used Porsche, Jaguar, Fisker Car Dealer Las Vegas, Nevada for jag parts in Las Vegas.............not sure what you did to the M/C! and unusual for a leak in braided line; are you a weight lifter or do you use both feet on brake pedal!
I think you need to start a new thread for this topic too...........

Cheers, Adrian

 

Press the start button BEFORE you press the brake pedal - and you won't have to "stand" on the brake. Pumping the brake pedal increases the distance needed to make contact with the starter switch, which means you really have to stand on the brake pedal to make contact. So press the start button first.

.Hope this helps!

 

Stuart,
Thanks for info; good to know; I'm 77 years old so can't press too hard!

Adrian

 

For what its worth, On my 2008 XK with 19" wheels, it seems that whatever number I set the cruise control for, my new Escort Passport 9500IX radar detector with GPS show my speed and says I am going one mph slower. I figure that's pretty close to accurate.

 

Adrian

That speedo calibration sounds about normal, showing +5.

Regarding the issue of variance in wheel and tires sizing, if I understand your question correctly, with all wheel sizing (18, 19, 20) the wheel/tire combo diameter should remain about the same, not effecting the speedo accuracy to any measurable degree. The small variance would be due to available tire sizing.

In your example with the 275/35 ZR19 size tire, let's calculate the tire diameter.

* 275 = the tire thread or section-width in milli meters.

* 35 = the aspect ratio. That means that the sidewall from the rim to the top of the tire thread is 35 percent of the section width of 275 = 275 x 0.35 = 96.25 mm.

* Convert the wheel diameter size to mm = 19 x 25.4 = 482.6 milli meter.

* Add the sidewall thickness twice (top and bottom) to the wheel size and you get the total tire/wheel diameter; 482.6 + (2 x 96.25) = 675.1 mm or, 26.57".

So, if your car would have the 18" wheels, you would simply use a tire with a higher aspect ratio to make the tire sidewall taller so the total wheel/tire diameter remain the same.

So, if you had an 18" wheel instead of the 19" you could use a 275/40 ZR18 tire and using the above calculation the diameter would work out to be 677.2 mm or 26.66" in total diameter.

The difference between the 18 and 19 wheel/tire combo is 1mm in ride height, which is negligible.

So, in effect, the 19" wheel/tire has 0.5" shorter sidewall than the 18" combo. Negligible difference in handling but, should be felt in increased ride harshness for the taller wheel/shorter sidewall combo. The tire sidewall is basically an important part of the suspension, supposed to soak up those road irregularities with its flex. Less flex, more harshness. I personally hope that this madness of going to larger wheel diameters and less and less tire sidewall stops really soon before we ride on rigid solid wheels with no rubber tire sidewalls :-). Not even talking about bent wheels due to insufficient sidewall thickness to soak up the shocks from those pavement joints.

Albert

 

Adrian

You don't have to be a weight lifter to produce a leak in a braided wire covered fluid line. It is enough if the flexible rubber (or whatever) line inside the braided cover had been exposed to a lot of heat and develops heat cracks.

For example, once I was pulling out of my garage in my V12 powered Lamborghini and I smelled gasoline. I stopped immediately, popped the bonnet and watched in horror a totally fine, atomized cloud of gasoline being sprayed from one of the fuel lines, through the stainless braids onto the top of the engine. I suddenly understood right there why there were so many Italian exotic-car engine fires that I knew of. If you have a plain rubber line, you can clearly see age or heat cracking and change it. With the braided cover you have no idea as to the condition of your lines. Needless to say, I replaced all my fuel lines with non-braided ones.

Albert

 

A lesson learned from my Audi may (or may not) pertain to a Jag.
Speedos today are electronic.
As the Audi ECU has been cracked and software available to consumers for $400, you learn an awful lot about modern ECUs settings. The Audi could calibrate the speedo in the ECU (you simply raised or lowered a numerical value). What was not clear is whether it was used to set accuracy at a given speed, or if it altered the ratio to allow different circumference tires.

Having a cracked ECU certainly answered another interesting question. A single unit was produced for world wide distribution. Features allowed in one country and not in another were simple "turned off" in the ECU. If you wanted European or Canadian functions like DRL, or auto closing windows via remote on a US car, you simply turned them back on!

I suspect most modern systems are built around this simple premiss.

Vince

 

Thanks Gents (Ladies Too?) for all your intelligent inputs about speedometers will check with a Dealer if an adjustment can be made.............I have an idea that previous owner may have had ECU changed because the XK was his wife's!! Not a sexist idea; it's just that the car is too easy to exceed the speed limit.............just an idea.
Albert, good point about the brake line materiel; that's why I never put stainless ones on my '68 E Type; I miss the E Type hood arrangement, amongst other things...........!!

Cheers, snow gone, power on.........Adrian