There are thousands of older vehicles on the roads whose speedometers could be (quite legally) up to 10% inaccurate. Difficult to see how a prosecution would be appropriate for anything less than 10% over the limit, I have never heard of one, 10.1% maybe. I note that the Sentencing Council table makes no mention of an Awareness Course in lieu of a fine and points? Have they been abandoned?

My understanding is that speedos are allowed to over-read by up to 10% but it's illegal for them to under-read. I doubt this is part of the MoT test though.
Speed awareness courses are still going - my daughter attended one last month for doing 48 in a 40 limit, she didn't notice the limit change from 50.
Good summary by s130 above.

TrevC said:

My understanding is that speedos are allowed to over-read by up to 10% but it's illegal for them to under-read. I doubt this is part of the MoT test though.

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UK Legal Vehicle Regulations are:

1. The vehicle shall be fitted with a speedometer capable of indicating speed in mph at uniform intervals not exceeding 20 mph at all speeds up to the maximum speed of the vehicle and capable of being read by the driver at all times of the day or night.
2. For all true speeds up to the design speed of the vehicle, the true speed shall not exceed the indicated speed.
3. For all true speeds of between 25 mph and 70 mph (or the maximum speed if lower), the difference between the indicated speed and the true speed shall not exceed—
V/10 + 6.25 mph
where V = the true speed of the vehicle in mph.

Well found, why didn't I think of looking it up! I'd be interested to know the thinking behind the 6.25mph. Clearly to somebody it's much better than 6mph or 6.5mph!

TrevC said:

Clearly to somebody it's much better than 6mph or 6.5mph!

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Comes from EU Requirement. 6.25mph = 10kph

Wheels and tyres fitted.

Went with alloys from Italy. And 215x65 Pirelli AS tyres.

Not managed a full test yet but.........on the side road leading to my house, the surface has had years of various tv and internet companies laying cables and botched resurfacing. The result is an undulating random surface.

This road seemed to bring out the worst in the low profile tyres with every bump being a sharp jarring sort of feeling.

Just drove it with new tyres and its a transformation. Yes - still bumpy but the impacts are "muffled" or soft rather than jarring or harsh / sharp.

No apparent issues with gearing. Acceleration is unaffected. Speedo was 8% out on standard tyres. New tyres reduces the error so speedo is now more accurate. Almost spot on !

So what is the increase in overall diameter? Presumably it's small enough not to risk anything fouling under full suspension and steering deflections.

Ok - managed to take it out for a proper run.

Absolutely fine at all speeds. Handling still as sporty but just that little more soft / comfortable.

I have a gps speedo to compare to car speedo. Its within 1 mile per hour at 70 ( real speed 70 shows 71 on speedo.

Noise - about the same really.

Adds .5 inch all around compared to 60 profile 16" or 45 profile 18". No fouling at all.

Now here is an unexpected finding...........and I know its easy to get a placebo effect here and "imagine" improvements..........but I used to find the brakes harsh but they seem less harsh now. Maybe the higher profile is soaking up some of the initial braking force ?

theshootist said:

Ok - managed to take it out for a proper run.

Absolutely fine at all speeds. Handling still as sporty but just that little more soft / comfortable.

I have a gps speedo to compare to car speedo. Its within 1 mile per hour at 70 ( real speed 70 shows 71 on speedo.

Noise - about the same really.

Adds .5 inch all around compared to 60 profile 16" or 45 profile 18". No fouling at all.

Now here is an unexpected finding...........and I know its easy to get a placebo effect here and "imagine" improvements..........but I used to find the brakes harsh but they seem less harsh now. Maybe the higher profile is soaking up some of the initial braking force ?

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Interesting. And looking at the photo, the smaller wheel/bigger tyre seems to suit the car.
So the cost all in is around £500?

Yes- though you could spend more or less depending on make of alloy and make of tyre obviously.

Had another longish run this morning. 22 miles each way - all undulating and bumpy b roads in the Fens.

Really noticed the increased comfort there !

Just put the original alloys and tyres on Ebay

I think it looks good. Your initial £750 for steel wheels and tyres did seem a bit toppy to say the least. Glad your noticing the improvement, would be a shame to go through all that trouble and it to be similar.

I presume the £500 was money well spent then?

Regarding your braking comment, I'd never thought about it before but with extra tyrewall I suppose theres more of a soaking up/ spongey effect.

When I test drove a 18 inch model I noticed the brakes were very grabby while on my 17 inch they are better.

In short - yes, totally happy with the increase in comfort and better "feel" to the braking.

Money well spent which will be recouped by the sale of the originals.

It may be worth keeping the originals to maintain the value at re-sale time. Modifications tend to bring it down. Of course, if you intend to keep the car until it is worth very little this matters less.
I think you are required to inform your insurance company if you change the original wheels.
Seems a very cost-effective way of improving the ride.

s130 said:

On the 500X the TPMS is a passive system. There are no pressure sensors within the tyre chamber. Passive TPMS uses the ABS system to monitor wheel rotational differences where an under inflated tyre will deliver different results to the other normally inflated tyres.

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My 500X does have pressure sensors inside the tire chambers. It may depend on a configuration or a region (the car is from the US).

Where I live it's a common practice to have two sets of wheels, because it's mandatory to change the tires twice a year, and replacing tires frequently is bothersome and problematic. With my previous car I kept the factory R16 wheels for winter, but in summer I used wide (225) R17 wheels, with low profile tires.

Now I'm planning to do the same with my 500X. My present factory dimensions are 215/60 R17, and the summer set will be 225/50 R18. The change has been approved by the local FIAT service, so it should be alright. I asked for a possibility to increase the profile to 55 for a softer ride, but they said it could be problematic, as it increases the overall diameter noticeably. So I guess 50 is the optimal choice for the R18.

Based on my driving experience with low profile tires, I imagine the car would feel considerably more rigid and that's the obvious downside, but wider wheels should provide a better attachment to the road, and the aesthetics of bigger wheels with lower profile tires should contribute to a certain "summer feel", if you know what I mean.

Bouncing along down a country lane reminded me of the good old days with my Citroen BX. Standard fix for the suspension on those was a bag of cement in the boot. Going home from the garden centre with 4 large bags of compost in the back, the ride was almost perfect. No room for luggage, but a vast improvement on the ride.

Goodcoin said:

My present factory dimensions are 215/60 R17, and the summer set will be 225/50 R18. The change has been approved by the local FIAT service, so it should be alright. I asked for a possibility to increase the profile to 55 for a softer ride, but they said it could be problematic, as it increases the overall diameter noticeably. So I guess 50 is the optimal choice for the R18.

Based on my driving experience with low profile tires, I imagine the car would feel considerably more rigid and that's the obvious downside, but wider wheels should provide a better attachment to the road, and the aesthetics of bigger wheels with lower profile tires should contribute to a certain "summer feel", if you know what I mean.

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Update on that: The 225/50 R18 turned out to be quite alright. Yeah, the ride feels definitely more rigid, but hey, like my service manager said, you can't have the best of both worlds when it comes to tire profiles.

I Know this post is a bit old but it was helpful.

On your advice I changed upto Michelin Primacy 4 225/50 R18 W (99) and they fit the arch better make little difference to the speedo. Much smoother quieter ride. I have used the Michelin Primacy 4 on another car and they are very good all round.

1 point to mention if you do go up to 50s you will have to do your spare as well.

Re: Higher profile on 18" wheel ?

TrevC said:

I think the only way to change the tyre profile properly is to change the wheels as well, to 17" as suggested above. This would allow significantly higher profile tyres to be used, which should reduce road noise and improve ride comfort. Furthermore, it would allow the fitment of all-season tyres which, as well as being better in the winter, also give improvements in noise and ride. We fitted Michelin Cross-Climates to our other car and the difference is noticeable. Unfortunately they are not presently available in the 225/45 18 size for the 500X.

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Hi,
just joined the forum as i have just bought a 500x cross plus with 18" rims and the ride isn't great i would call it choppy.
Reading everything here i feel it is the 18" wheel/tyre and moving to the 17" or 16" should be better (smoothes out the room more) would you guys concur?
cheers
James

theshootist said:

In short - yes, totally happy with the increase in comfort and better "feel" to the braking.

Money well spent which will be recouped by the sale of the originals.

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Hi Bud,
been having a similar issue on my 18" rims, i have a cross plus, so was looking at going to 16".
I haven't been able to see your pictures of the alloys, do you have one or at least a link to where you got them please?
thanks
James