Quote:
Originally Posted by AlanD
Hi msford,
The two tyre sizes that you have nominated are not equivalent in respect of rolling radius. The 235/40R18 tyre has a diameter of 645.2mm when new and the 235/45R18 has a diameter of 668.7mm which is a 23.5mm difference.
Now I'm relying on memory, but I seem to recall that diameter alterations for wheels are limited to 15mm (someone will correct me if I'm in error here) but it does mean a significant error on speed readout and distance travelled will occur (it's about 3.6%).
Are you sure that the two sizes are as you have quoted are correct. I ask because the tyre diameter difference if the tyres quoted on the placard are 235/40R18 and 235/45R17 is less than 2mm and I know many Mondeos were supplied with 17 inch rims - we have one.
The other factor is load rating, I think the higher rating was specified for the diesel engined cars as that engine is heavier than the petrol alternative which could use the 95 load rating rather than the higher 98 rating (excluding the XR5 models).
If the OE tyre was the 45 profile tyre and you have replaced it with the same profile then no worries. But if the OE was a 40 profile and you are now using a 45 profile then the car will be travelling faster than the speedometer reading by about the 3.6% I have mentioned ie 103.6 when reading 100 kph. However no manufacturer calibrates a speedometer with absolute accuracy and they always make sure the speedometer reads high ie you are travelling slower than indicated. This gets them out of being a co-defendant in speed related court cases.
Cheers
|
Hi Alan
I can only assume that the OE tyre was the 45 profile as I went to two places for quotes and they were within a few dollars of each other. I went with the local Bridgestone Tyre Centre. That was about 15mths ago, from memory.
Whereas the quotes I am getting now for a 40 profile tyre are much cheaper than the 45 profile tyres.
I recall from my GPS that the speed is the same on the original and current tyres, when I'm doing exactly 100 kph on the car (as measured using the ACC), the GPS says 97 kph. So no change there from the OE tyres and the current set.
If I'm reading the placard correctly, and I believe I am, then for 18 inch wheels it has 2.0D, 2.2D and 2.0GTDi with both the 235/45 R18 98W or 235/40 R18 95Y as tyre options.
And I rang the Bridgestone Tyre Centre today as they have a buy 3 get 4th tyre free sale on and he recommended the S001 and RE002 in 40 profile as a cheaper but equivalent option to RE050A, in fact mentioning that the S001 (or perhaps it was the RE002, I forget) is supposed to be a replacement for the RE050A.