My XJR has X308 Asteroid wheels with 245/45x18 tyres, which an online size calculator tells me has a circumference of 2129mm.
It came out with 17” wheels which I am fairly sure had 225/45x17 tyres with a circus of 1991mm
If this is true it means my current tyres are 7% bigger than factory original.

Now I have an OBDII reader (Kiwi3, works with Apple products, USD$85 from the manufacturer), I can get ECU speed readings, which are well below the speedo, ie: suggests the speedo overreads. But I presume the ECU thinks I have 17” tyres which are approx 7% smaller than what I have on the 18” Asteroids.
I used to use a speedo app on my phone called Speedo Hub, but with various Apple software upgrades, it is obsolete and I haven’t found another one yet. However the speedoHub app used to be very close to the actual speedo readings, eg: about 4 km/h over at 100km/h .

I think the answer is find another speedo app.
I want to graph some speed data on the OBDII software, but I suspect it is underreading by 7%.

 

Original size on the XJR/6 was 255/45x17


Cheers
DD

 

Thanks Doug
I thought that, but a US site said 225/45. Probably a typo - 225 instead of 255

255 would make the circumference 2078mm

2129/2078 = 97.6%. So only a 2-3% difference ...

Just downloaded a new speedo app, so will see what that gives. There always seems to be some time lag with the GPS speedo apps, whereas data from the ECU is much quicker.
I’ll play around with it all

 

The Speedometer reads 55km/h when my GPS app says 50 and the ECU reads 48. This is with X308 Asteroids on the car, as per the first post.
I will try at some higher speeds later

 

I have noticed a discrepancy between OBDII outputs and speedo display as well. I think it's a legal requirement that a speedo can't read low, but for ABS or traction control purposes the systems can use the real speed. In a Ford shop manual I saw years ago it said that for a true ground speed of 60 mph, an acceptable range the speedo could display was 59-69 mph.

I suspect the speedo cluster simply has a multiplier in it that makes the needle display more than the true speed.

 

This is the reason that I went up a ratio to bring the speedo reading closer to correct. Had to do the sme with the wifes Mazda 6

 

The stock size of tires on non-SC X300 is 225-60-16, and is supposed what the factory speedometer was calibrated to. There was a very thorough discussion (note in particular comments by al_roethlisberger in post #41) about different tire size and speedometer readings under the thread "New tire size help with tramlining" in 2014.

 

Hi AL NZ,

My car has Penta wheels on. I have been through the same thing as you.

The original tyres on the 17s were:
1995-97 XJR 255/45ZR17 Pirelli P Zero Sport Cast Alloy 8x17

The Pentas are the same size as the Asteriods I believe (nice wheels by the way) They should have:
1998-99 XJR 255/40ZR18 95W Pirelli P Zero A Penta Forged Alloy 8Jx18

With the 255/40 18s the rolling circumference of the wheels plus tyres should be the same as the original 17s plus original tyres.

Cheers,
Nick

 

Thanks everyone
When I first got the Asteroids my tyre guy tried a 235/50x18. It would fit in the wheel arch, but the front tyres would touch the inner wheel arch when the steering was turned, so the next size we tried was the 245/45x18. I think it looks better than the 40 profiles - just personal preference
I wanted to keep as high a sidewall as possible - our NZ roads are not autobahns and there are enough potholes that tyres/rims get trashed. I had a Renault in the UK with 225/45x17s and the Surrey roads opened up like craters when the winter snow melted. I bent two rims and destroyed one tyre, when I couldn’t dodge an 8 foot wide pothole due to oncoming bus.
I like high profile tyres - I love the way my Daimler rides on 225/60x16s. And it still handles well.