New Tyres for XR5

[7summit7]

Thanks for you response. The original tyres were 45 profile and the speedo was about 3-4 km/hr out at 100 km/hr. ie to achieve 100 on the GPS I had to set the cruise at 103 this matched the speedo. The new tyres are 40 profile,so smaller in diameter. Now to achieve 100 on the GPS I need to set the cruise to 108.
It must be possible to calibrate the speedo as both 40 and 45 profile tyres are included on the tyre chart.
I was wondering if anyone has had this done.

Cheers
M

[whitetdci]

Maybe there is although not confirmed.
http://www.talkford.com/topic/154780...bration-trick/

[mondo_broady]

Originally Posted by 7summit7

Thanks for you response. The original tyres were 45 profile and the speedo was about 3-4 km/hr out at 100 km/hr. ie to achieve 100 on the GPS I had to set the cruise at 103 this matched the speedo. The new tyres are 40 profile,so smaller in diameter. Now to achieve 100 on the GPS I need to set the cruise to 108.

hi, if that's been your experience, then i can't doubt you, but, it is opposite to the effect you should have seen? tyres with a smaller rolling circumference should make for less speedo error as they make the car "faster". ie: the sensor will read 100km/h sooner than it would with a larger circumference tyre.

as for re-calibration, not sure, but ADR standards dictate the speedo only needs to be +/-10% @ speeds in excess of 40km/h. there's no "roadworthy" issue for you as it currently stands, if that's your concern?

still puzzled as to why they would have two different aspect ratio tyres on the same diameter rims on the placard? don't actually have my car here at the moment to check mine...

[svo supporter]

Seeing as you have a speedo reading error (ie road speed VS speedo speed), I would put on the same profile tyre it originally came with....Yes, it is possible to have it corrected, however should you ever change back tyre profiles, you'll be digging back in your pocket to have it readjusted.

That slight difference you are experiencing could result in an unwanted piece of mail, or delay going to or from a particular place.

I'm not going to bang on about speeding and the rest of the bulldust associated with it...It's more a suggestion to save you a little heartache in the longrun...

As for the different profiles being on the placard....I have no idea why....Would it be because there are different spec Mondeos, therefore they are catering for both specs, with the one placard? After all, the speedos are calibrated according to the tyre/final drive ratios when the car is produced.

[mondo_broady]

Originally Posted by svo supporter

As for the different profiles being on the placard....I have no idea why....Would it be because there are different spec Mondeos, therefore they are catering for both specs, with the one placard? After all, the speedos are calibrated according to the tyre/final drive ratios when the car is produced.

hadn't considered that? maybe like you say different models, different tyres?


side note - not sure my logic is not flawed from earlier?

[AlanD]

Hi,

The tyre plackard on our car shows three "normal" tyre sizes. They are:

215/55 R16 (equals a wheel and tyre diameter of 642.9 mm unloaded)
235/45 R17 (equals a wheel and tyre diameter of 643.3 mm unloaded)
235/40 R18 (equals a wheel and tyre diameter of 645.2 mm unloaded)

It does not show a 235/45 R18 size (and I must say I am not surprised about that because it would give an error of almost 4% and it would be the wrong way ie at an indicated 100 kph you would be moving at 104 kph, rather too close to the 5% allowance currently used in Victoria)

You can see from the unloaded diameters there is a range of diameters which gives a total variation of 2.3 mm which is 0.36% variation. On this basis I can be confident that the speedometer is calibrated to the largest diameter and then the other two tyre sizes will provide "high" reading by comparison. But only by 0.36% at the worst.

No manufacturer will sell a car which has a speedometer that reads lower than the actual road speed, for to do so invites a claim by an driver that they were pinged for excessive speed when they were doing an indicated speed that was legal. For this reason the manufacturer will market a car that has an accurate speedometer or one which reads higher than actual. For the Mondeo calibrating to the 235/40 R18 tyre would cause the other two sizes to give a "high" reading, so if you are travelling at the legal limit indicated on the speedometer on such a tyre you will actually be very slightly below the limit. ( you might regard this a a good thing - or not)

Two other factors tend to provide speedometer errors, but both of them err on the "safe" side of the equation.

The first is that the loaded rolling radius of a tyre is smaller than the unloaded radius, because the tyre "squashes" as it accepts the vehicle load and this results in a smaller radius than the unloaded situation. How much smaller depends on the tyre pressure. If the pressure is low then the "squash" factor is larger and the loaded rolling radius is smaller leading to higher reading on the speedometer. High tyre pressures are the reverse.

The second factor is tyre wear. As the tread is worn away the loaded rolling radius decreases and this magnifies the speedometer error, but it will indicate a faster speed than the car is actually doing. Again a good thing?

The tyres on our car had a new tread depth of 7 mm and when worn to the limit would inject an error of 2.1 % so I would expect the speed error to be of this size. It means that at an indicated speed of 100 kph we would be traveling at 98 kph give or take a smidgen. Because the specified tyres are those in the middle of the range of diameters, I would expect an error of about 0.3% from the speedometer when tyres are new.

Our tyre plackard lists tyres specific to the various models by engine type and the diesel engined cars have the highest load rating requirement (98) because they are heavier owing to the engine mass being greater.

Best advice? Fit tyres that have the same size and load rating as those that came on the car when new.

Cheers

[JimStone]

MC Zetec Ecoboost tyre placard

[ope126]

Originally Posted by JimStone

MC Zetec Ecoboost tyre placard

image

Thats whats on my car!!!

Check this site; www.1010tires.com

Stock Tire - 215/50R17 Search Tires Tire 1 - 235/40R18 Search Tires Tire 2 - 235/45R18 Search Tires Section Width:8.46 in215 mmSection Width:9.25 in235 mmSection Width:9.25 in235 mmRim Diameter:17 in431.8 mmRim Diameter:18 in457.2 mmRim Diameter:18 in457.2 mmRim Width Range:6 - 7.5 inRim Width Range:8 - 9.5 inRim Width Range:7.5 - 9 inOverall Diameter:25.46 in646.68 mmOverall Diameter:25.40 in645.16 mmOverall Diameter:26.32 in668.52 mmSidewall Height:4.23 in107.44 mmSidewall Height:3.70 in93.98 mmSidewall Height:4.16 in105.66 mmRadius:12.73 in323.34 mmRadius:12.70 in322.58 mmRadius:13.16 in334.26 mmCircumference:79.98 in2031.4 mmCircumference:79.79 in2026.6 mmCircumference:82.68 in2100.0 mmRevs per Mile:816.9Revs per Mile:818.8Revs per Mile:790.2Actual Speed:60 mph100 km/hSpeedometer1:60.1 mph100. km/hSpeedometer1:58.0 mph96.7 km/hSpeedometer Difference: -Speedometer Difference: 0.232% too fastSpeedometer Difference: 3.378% too slowDiameter Difference: -Diameter Difference: 0.24%Diameter Difference: 3.27%

[Rastus]

I'm booked in to get a set of Goodyear Eagle F1 Asymmetric 2's tomorrow (235/40-18), replacing worn Conti's & Bridgestone RE001's.
Pays to shop around, that's for sure. Prices from 4 places in Ballarat ranged from $360 each! < Bob Jane's dreamin, to $305. Was all set to book in @$305, then thought to ring K-Mart. Glad I did......$259 each!
I see the Goodyears are the standard fit on the new Focus ST so they must be OK...well I hope so anyway.

[Rastus]

They're only new, and it was all dry weather grip, but the Goodyear F1 Asy.2's performed extremely well this weekend. Saturday, it was a spirited drive down the Great Ocean Road all the way to Warrnambool. When the kids were near crook due to the corners, I told them to pretend it was a ride at the show! It's a decent drive that's for sure.
Today on the way home we went a little more direct (Colac), vearing off at Lavers Hill to go past the "Otway Fly Treetop Adventures". Didn't end up doing the treetop thing but will in the future. Some nice twisties in & out of there as well!
Great drive. Car was excellent. Averaged 8.4L/100km for the round trip. Was 7.1 before Anglesea ;-)