speedo reading 15% high on hymer exsis2.8

[hymer]

The speedo on my 2005 Hymer Exsis reads 35 at a true 30mph and 70 at a true 60mph. It has been back to Brownhills who have checked the ecu output and it is correct Where do I go from here ?

[johnsandywhite]

:-D Hi Hymer. Has the tyre size been changed? Our old 1988 Coachmen Classic was the same. The dealer warned us about it. He say's it was due to the tyres being a different profile to the one's they use in the USA. Who am I to disagree? (lol)

[Dave Newell]

Hi Hymer, JSW is probably right on the button here. Hymers especially seem to be being fitted with 15"alloys when they originally had 16" steel wheels. The difference in rolling diameter accounts for the excessive error. 10% over read is the legal maximum and 6% is about typical. Check with your supplying dealer as in it's present state the speedo is illegal and therefore the vehicle is not fit for purpose. D.

[Mel E]

The best way to check is on a level road with a good GPS. I have found my Citroen Xantia also overreads more than I thought. 30mph true is displayed as 34-35mph, while I'm still legal when the speedo reads 78mph. It is also worth noting that the difference between under-inflated and over-inflated tyres can be around 2 or 3 mph at about 60. The new van and my wife's car are both well within the legal limit. Having checked, I now know why I wasn't sent a summons when I inadvertently passed a mobile speed camera at 66mph displayed - I was actually doing just 60mph. One note: a GPS only gives a true mph reading on level ground. On a down or up slope, the GPS underestimates actual speed, because the equivalent level ground distance is shorter and is what the GPS actually measures.

[Derek Uzzell]

Intriguingly, a 70mph speedometer-reading at 60mph-true complaint that appeared in an "Interchange" letter in May 2006 MMM (page 244) also related to an Exsis, though, in that instance, to a 2.3 litre motor version. As Dave Newell suggests, wheel size will be a factor if 15" alloys have been specified for a chassis starting out with 16" diameter wheels, however, I note that the Exsis Special Equipment list indicates that either 15" OR 16" alloy wheels are available as an option. (Frankly, I'm startled to learn that Hymer are deliberately fitting 15" alloys to replace original 16" steel wheels as the latter are there to take the heavier duty tyres needed to cope with a high load-capacity chassis and to provide clearance for the larger diameter front disk brakes that go with it.)

[Dave Newell]

Hi Derek, I'm not sure it's Hymer that are fitting the smaller wheels but suspect a few under-informed dealers may be fitting "pretty" alloy wheels of the smaller diameter not realising what the effects are. I've definitely heard of several Hymers that are on the Maxi Ducato chassis, which as far as I'm aware, should come with 16" wheels, that have been fitted with 15" alloys. I've not heard of any other converters offerings suffering the same problem but of course that doesn't mean there aren't any. D.

[Derek Uzzell]

How weird! I think fitting alloy wheels on motorhomes is plain daft. The standard steel wheel is cheap, rarely leaks air, can take a tremendous amount of bashing about and, if irreparable damage does occur, a new wheel can easily be obtained. Suitably heavy-duty alloy wheels are costly, corrosion and leak prone, a good smack against a kerb or road debris will instantly finish them off and, if this happens, sourcing a matching new alloy wheel may prove impossible as production numbers are low and the designs are changed so often. I can't say that 'prettiness' in the motorhome wheel department is a particular fetish of mine but, if it were, I'd much prefer to put dinky wheel-trims on steel wheels rather than waste good wine-drinking money on alloys.

[Mel E]

Derek, You're right (as usual). Alloy wheels on vans should be limited to those with dark windows (actually illegal!) used by pimps, etc.

[Dave Newell]

Derek and Mel, how true, how true! Very dark tinted windows are only illegal on the cab windows though. There is a limit on how much light cab windows must allow through and at least one police force had a campaign against the heavily tinted ones using a special light meter to establish if they were too dark or not. D.

[Brian Kirby]

I think, with all due respect to my learned friends above, that nice Mr Collings answered this - and at some length - in the MMM Interchange for May, p244, in response to an e-mailed query on much the same subject from one Peter Taylor.

The essence of George's response is that a speedometer that read 70 mph when the vehicle was travelling at a true 60 mph would just about be within accepted tolerances.  However, if I understand George's answer correctly, one that read 59 mph when you were travelling a 60 mph wouldn't.  In other words, all of the tolerance is upwards and no under-recording of speed is acceptable. 

Bearing in mind the affects of tyre wear and possible differences in rolling diameters between makes (or possibly even alternative tyre sizes being fitted); I guess manufacturers err as far on the side of caution as they can.

I don't really think it matters that much in reality.  I tend to adopt a speed that feels reasonable most of the time and really only use the speedo to check whether I'm still legal.  When I find I'm not, it's a bit of a comfort to know the speedo is just trying to frighten me and in reality, I'm probably still just about OK.  I always wondered why I hadn't got a ticket yet, so thank you oh great and wondrous speedometer!

Oh yes, and the only reason I remembered reading the Interchange bit is that I've only just read it (been away you see), so you don't all need to rush out and buy those memory pills.

Regards