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The car will still travel the 70 miles in the hour regardless of the wheel diameter. The wheel diameter being less than standard will entail the wheel performing more revolutions per minute (hence an increase in displayed speed) and in total (thus giving an increased odometer reading). I was trying , however, to make the point that these changes are small enough to be ignored. There is a more general point here which is that by identifying the magnitude of effects you can see which ones can be ignored thus simplifying life. I once attended a series of local meetings on the issue of Sustainability at which many of the prominent contributors got themselves really worked up about trivial matters. I remember one man agonising over the fact that the Imperial ton was different from the Metric tonne .....I pointed out that the difference was only 1.6 percent which really didn't matter unless you were buying or selling coal.

Do the sums. Typical wheel plus tyre is just under 600 mm. OD. Tyre wear range is 6 mm. , two sides so 12 mm variation in OD. That's 2 percent variation in diameter and therefore in recorded speed between the extremes of wear. That's 0.6 mph at 30, 1.4 mph at the National Speed Limit on motorways. My roadside meter readings are generally In 30 zones as that's where they are most frequently located.

By no detectable error I mean no more than about 1 mph which is the closest that I could /can read those speedometers whilst the roadside indicators and satnav are digital readouts and so also to 1 mph. I've owned the Jazz for all its 13 years and had the Jag for 16 years so I've had plenty of time to observe these effects. Your quotation simply shows that the spec for speedo accuracy is remarkably loose ...it doesn't prove that my claims are wrong.

We were certainly able to correct the wind speed and speed through the water on our boat instruments about 10 years ago via software and user instructions. That is why I raised the matter with the Lexus dealers and they saw no problem .... we shall see.

My IS300h speedo reads 10 percent too high as judged against those roadside speed indicators. As the car is still under warranty the dealers say that they will correct this when it goes in for service and first MOT next month. My 13 year old workhorse Jazz and my late 20 year old Jag XJ8 had speedometers with no detectable error when compared with satnav and roadside meters so I'm used to accuracy in these matters.

I seem to recall that bi-xenon headlights feature a solenoid operated shield that creates the cut off for the dipped beam. Sounds very old fashioned like those 1930s headlamps where a solenoid tilted the whole reflector. Anyway, maybe that shield or solenoid had become defective, necessitating replacement of the whole "bulb". That would explain the £95 charge.

I have now used my IS300 h Sport for some rural night driving and can compare the lights with those of my old Jag. which had a four 5.75 inch 60 watt main/ 55 watt dipped quartz halogen setup. The bixenons give a much more uniform and whiter distribution of light in dipped mode but switching them to main beam appeared to add less additional light than in the case of the quartz halogen system. Overall I prefer the bixenon system as dipped beam performance is more important to me from a safety viewpoint.

The essential point of the Atkinson cycle, as invented by Mr Atkinson over 100 years ago was to have a longer working stroke in order to extract more work from the combusting fuel mixture. He achieved this with a very weird linkage between con rod and crankshaft (Google to find an animation). Toyota achieved the same end by crafty valve timing so that the effective compression stroke was shorter than the actual working stroke. The geometrical c.r. is 13:1 but the actual c.r is nearer 10:1. What I don't understand is where the surplus unburned mixture goes to when it's pushed back out of the inlet valves.

Don't go quoting "rules of the road" too much when you have 100 watt bulbs fitted yourself.

Thanks, I'll have a look there. Imaginative thinking to put them down there. I have to say that the diagrams in the handbook aren't very clear or is that just me?

My IS300h Sport has TPMS according to the handbook but I can't find any button on the lower dash to use to normalise the system. Any clues, please?

I don't know the legal position in Eire but in the UK max wattage for vehicle headlamps on public highways is 60 main/ 55 dipped beam. Xenon are 35 watts but give about twice the light output of quartz halogen and so ought to be brighter. Regarding focussing.... the old "sealed beam" lamps used metal parabolic reflectors to do the main focussing with beam shaping being done by the patterned glass. The Austin Maestro introduced "homofocal" lights .....pressed polycarbonate reflectors which did focussing plus shaping so the glass was plain, which gave a slight gain in light output. Such lamps are sometimes called "free form".When I rebuilt my MGA roadster in 1981-83 I fitted some 1970s Mini sealed beams but they were just tungsten, not even halogen and were hopelessly outclassed so a couple of years ago I went for Wipac Freeform halogens ....big improvement. My Lexus IS is my first car with bi Xenons but I haven't driven it enough in the dark yet to be able to comment fully except that I feel that the beams are more sharply defined than on other cars that I've owned.

I seem to have started something here. My comments on overtaking grunt were based on subjective feel so I looked for test data. The Autocar XJ8 3.2 test shows 0-60 in 7.6 seconds but by 2015 Autocar were quoting 0-62 times ....8.4 seconds for the IS 300h. A bit of maths converts the XJ8 figure to 7.9 seconds for 0-62 so only 0.5 seconds difference in 8 seconds. More relevant for overtaking would be 50-70 times....3.8seconds reported for the XJ8 and (by my calculation, no data found) 4.0 seconds for the IS. Surprisingly little difference in times. With reference to noise, I must say that the factory fitted Jag exhaust ( which needed no attention in 20 years) emitted a lovely V8 burble which became just audible at around 2500 rpm and pretty muscle car style under kick down conditions. I frequently played games keeping the rev counter on a certain setting and letting the torque converter provide the acceleration ..2000 rpm for mild, 3000 rpm for faster progress. I have yet to try that with the Lexus ecvt.

Whilst I've always been aware of the bigger Lexus models I've always thought the styling to be heavy and clumsy compared with the XJ series Jaguars and particularly the x308 version of 1998 to 2003 which is what my second one was. I feel the same about all the 4x4 s, pseudo 4x4 s, softroaders and SUVs that seem to be so prevalent now. I also see plenty of people struggling to fit those vehicles into parking spaces. The IS suits us fine and we have an oldish Honda Jazz for utilitarian purposes (plus a nearly 60 year old MGA for fun).

Have owned two different Jaguar XJ s over a period of 26 years. The second one, a 1998 XJ8 3.2 which I bought in 2002, I had intended to keep forever and pass on to my son with the hope that it could become a show car. Big shock in January when, after advisories since 2009, the front sub frame corrosion was declared terminal. What to do? I didn't want a diesel nor a black car (the Jag was a superb Madiera Pearl) and eventually went to the nearest Lexus place and rather quickly (it was a very wet day) bought a Nov. 2015 IS300 Sport in pearlescent Arctic White , 6600 miles on the clock. My early impressions? Very quiet, smooth and comfortable along with 45 versus the Jaguar's 26 mpg. I do rather wonder about the lack of any spare wheel .... both the Jags had full size spares and hefty 60 profile tyres at that and they also had better all round visibility (need those proximity sensors in the IS) . Inevitably the Jags had more grunt for overtaking but overall it's good to have a change. My daughter, though, thinks that it's too boy racer for an 82 year old.