www.jec.org.uk

I'd be grateful if someone can tell me how the trip computer fuel consumption reading works .. eg: where it is take off point? Is it independent of the gearbox (such that eg converting to a manual box will not affect it? Will it automatically compensate for different profile wheels and tyres, or just misread if different wheels and tyres are fitted?

Hi there - to take you questions in order
In the later cars the trip computer and speedo both take an electronic feed from the same sensor mounted in the rear diff. Earlier cars had a mechanically driven speedo taken from the gearbox. I would expect yours to be the later type. Easy way to see is to get underneath and see if there's a feed coming out from the rear casing of the differential.
Good news if there is and you want to fit a manual gearbox as it makes that aspect much easier - in fact it doesn't affect it at all.
Bad news is that it won't compensate for a change in wheel size. What the sensor does is count the number of revolutions of the crown wheel in the diff and is calibrated for standard wheels and tyres to translate that into a speed.
The important thing with wheel and tyre sizes is the rolling radius. In other words, you could have a larger wheel with a lower aspect ratio (ie skinnier) tyre and still have the same rolling radius as a smaller wheel with a thicker tyre.
If you make the rolling radius smaller then the speedo will over-read and acceleration and fuel consumption will be greater as you've effectively geared the car down slightly.
If you make the rolling radius bigger then the speedo will under read and the acceleration will be slower but consumption lower as you've geared the car up. These are however marginal effects as you can't change it that much.
For example, my 1989 V12 HE has 235/60 tyres on 15" rims and my 1996 Celebration has 225/55 tyres on 16" rims but both have a static rolling radius of 300mm although the circumference of the Celebration wheels is slightly less than the HE. The overall effect of swapping them round would be that each speedo would misread by 1.4% or 1mph error at 70mph or in other words I wouldn't notice!
Hope that helps

Thanks for the handy explanation David. I was wondering why my speedo had become less accurate. I'm running 245/45 R16 compared to the previous 225/60 R15 (still 2% inaccurate) versus the original 235/60 R15.

Ta, Rich

Rich - after a quick bit of maths I think your rolling radius on your new setup is actually smaller than the standard and your speedo should read just over 5% optimistic. Is that what you find?

so........

if a new tyre starts with 8mm of tread ( top and bottom ) and is legally allowed to run down to 1.6mm ( top and bottom )

thats a difference of 8+8 minus 1.6+1.6 = 12.8mm in the reduction of the diameter of the wheel over a ' normal ' life of use


so that MUST effect the speedo as described above

so, at the end of the day and all things being equal and the earth being flat and space aliens coming to earth

it doesn't really matter that much ??

BB

As you say BB - tyre wear can make a bigger difference than the potential wheel swap in my example

And remember that Jaguar had the option of 2 different tyres on late XJSs - 225/60 x 16 & 225/55 x 16. So, one or both of the cars were wrong from Day 1. I've tried to find out over the years whether the speedos were set for the larger or smaller tyre but to no avail.

My best guess is that because it is apparently ok for a speedo to be set to over-read from new but not to under-read, then I think they set the speedo to read correct for the 225/60 tyre, which meant that every late Coupe + 4 litre Convertibles with Sports Suspension (with 225/55) had over-reading speedos straight from the factory!

Paul

that and it helped the top speed figures , lol


BB

Morning All,

Yes, about 5% optimistic, so erm 70mph shows around 73 / 74 mph on the speedo.

This introduces yet another variable into my qualitative assessment of my recent AJ6 intake manifold upgrade. So now I have to consider;
- different circumference of the tyre / wheel combo
- new distributor cap
- timing adjustment to correct over-rich b-bank
- ecu fueling adjustment for new twin tails and intake manifold tuning pipes

Oh well, might as well enjoy the weather and the performance!

Cheers, Rich

Thanks very much for the helpful info. just what I needed.