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BigJimnyMeet (North) 2024 (12 Jan 2024)


BigJimnyMeet 2024

14th July 2024
Parkwood Nr. Leeds

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235/75R15 on JB74....

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26 Jun 2021 00:03 #236153 by Navigator
235/75R15 on JB74.... was created by Navigator
Propose putting Cooper Discoverer AT3 4S 235/75R15's on a new JB74. Interested to hear any observations on this - especially from real life experience of the size if not the brand. Considerable chatter on various blogs suggest this is a bridge too far. Others say it's fine, and suggest impact on performance and fuel economy, while clearly evident, are manageable/acceptable. These tyres do not have a particularly aggressive tread but they are obviously considerably larger, and heavier, than the standard issue ( in Oz) of 195/80R15. The vehicle is about to have a 45mm lift, so about 2 inches, more for GVM impact than the lift per se, so the tyres will certainly fit - on 7" aftermarket rims - but the principal concern is whether the increased weight/rolling resistance is too much to ask of the motor and other components. Appreciate any input.

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27 Jun 2021 00:44 #236169 by Navigator
Replied by Navigator on topic 235/75R15 on JB74....
Navigator again on this. Subsequent to post came across a considerable discussion on this precise size for JB 74 on BigJimny - and it only heightens my interest in informed input: Various views expressed in the discussion that mirror what I have said in post above: Some say regearing needed. Others say not. Prepared to put up with need to be more mindful of tyre influence when in low range - and when tackling, especially, steep gradients. Also relatively content with some reduction in fuel efficiency - but remain much interested in feedback on whether this proposal likely to have significant negatives for vehicle overall.

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28 Jun 2021 08:22 #236198 by 300bhpton
Replied by 300bhpton on topic 235/75R15 on JB74....
I'm running a 215/75R15 AT's on mine. I'd say it has had a significant knock on performance and mpg vs the factory fit tread and size.

I don't think I'd want to go bigger without potentially re-gearing and maybe a bit more power. Although it is hard to know if the next size up would have much more of an impact or not. But I suspect it would.

I think with a 235 your low 1st crawl speed would suffer too.

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28 Jun 2021 08:58 #236202 by saxj
Replied by saxj on topic 235/75R15 on JB74....

Navigator again on this. Subsequent to post came across a considerable discussion on this precise size for JB 74 on BigJimny - and it only heightens my interest in informed input: Various views expressed in the discussion that mirror what I have said in post above: Some say regearing needed. Others say not. Prepared to put up with need to be more mindful of tyre influence when in low range - and when tackling, especially, steep gradients. Also relatively content with some reduction in fuel efficiency - but remain much interested in feedback on whether this proposal likely to have significant negatives for vehicle overall.
I have 235/75R15 Muddies on mine. Off-road they are great, and wouldn't change them for anything, they handle even the steepest gradients without any issues. I drove home from a trail yesterday at 75mph,  with no issues at all, except for the fuel consumption which is around 20mpg . 

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28 Jun 2021 09:05 #236204 by Roger Fairclough
Everything you will read will be negative except possibly the aesthetics and/or the ground clearance.
Is this why you want to do it?
Aesthetics are personal and not open for discussion. Ground clearance increase will be nominal and only you can decide if the cost outweighs the benefit.
The negatives are legion, reduced braking efficiency, acceleration, top speed. Probability of bodywork touching, need to re-gear, extra wear on mechanical items, desire for more power to compensate for reduced performance, reduced stability.
There is a lot to consider.

Roger

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28 Jun 2021 10:41 #236207 by saxj
Replied by saxj on topic 235/75R15 on JB74....

Everything you will read will be negative except possibly the aesthetics and/or the ground clearance.
Is this why you want to do it?
Aesthetics are personal and not open for discussion. Ground clearance increase will be nominal and only you can decide if the cost outweighs the benefit.
The negatives are legion, reduced braking efficiency, acceleration, top speed. Probability of bodywork touching, need to re-gear, extra wear on mechanical items, desire for more power to compensate for reduced performance, reduced stability.
There is a lot to consider.

Roger
20mm may not sound like much, but in real terms it makes a massive difference. 

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28 Jun 2021 13:34 #236212 by Roger Fairclough
That's why I asked the question, is ground clearance important?

From my own experience, 20mm extra clearance under the diffs is not a major issue, especially when compared to the possibility of a roll over.

If you ground out, you can back out and try another line - no problem, but if your extra clearance leads to instability, then your chance of recovery are very slim.

Trust me, been there and done it.

Roger

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01 Jul 2021 05:27 #236273 by Navigator
Replied by Navigator on topic 235/75R15 on JB74....
Thank you all for the input. In the meantime - committed. The 235's are on the vehicle. The main prompt was a perceived lack of quality in the alternative sizes (they all seemed to be trying to look tougher than each other) and, ultimately, an indication that in the "outback," if there is tyre trouble, the 235 size much more available - because much more common. May be an expensive mistake. Will report after a couple of major on/off-road journeys, and also plenty of urban zipping about. Am hopeful the relatively conventional tread will reduce the inevitable impact on fuel consumption and drive train etc.

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24 Jul 2021 23:52 #236941 by Navigator
Replied by Navigator on topic 235/75R15 on JB74....
Very early days (1000 clicks) but vehicle delivering, almost precisely, the claimed fuel consumption figures of 8.2 litres per 100 kilometres for urban driving and 6.1 on the highway - at 100Km/h and 3000 revs - despite the increased tyre size/circumference/weight over OEM. The greater circumference (5.8% greater than OEM's) is also delivering, serendipitously, a speedo reading that is virtually spot on. (Assume the UK is the same as here in that manufacturers have built in an overestimate of speed.) As some reviewers have suggested, the feel on the open road is pretty "agricultural", and suspect I will settle for a pretty modest 90Km/h on long trips, which will need to be undertaken with good rest breaks: 100% concentration, while always at play in any vehicle, takes on a new meaning in the Jimny. You are "driving" it every inch of the way.

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12 Dec 2021 03:26 #240599 by Fullbloom
Replied by Fullbloom on topic 235/75R15 on JB74....
I went from stock to 235/75/15 A/T a week after getting mine. While I love the look I have a slight sense of buyers remorse. Although I wonder if any size up from stock would have had the exact same effect? It feels great on the road, but I don't know if I'm over thinking of the drastic reduced power and economy, or if it was simply always bad.

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12 Dec 2021 10:22 #240601 by Busta
Replied by Busta on topic 235/75R15 on JB74....
Consider that the tyres are 6% bigger than standard and the ways that this will affect the car.
In a given gear the car will move 6% further for every rotation of the engine. This means that the engine has 6% less mechanical advantage (torque) at the wheel. At the same time it will be travelling 6% faster. which means it will be fighting much more drag and resistance (drag increases exponentially with speed).
Now also consider that your speedometer and odometer are reading 6% slower than before, so from the drivers perspective you are subconciously drivng 6% faster than you did before, and basing your fuel economy calculations on 6% fewer miles... and the engine is having to work more than 10% harder.
All of these things combine to make the impact of a small change in tyre size seem massive. However if you re-callibrate your driving speeds and adjust your fuel economy calculations accordingly and the impact will be much less. Re-gear the transmission and the only difference becomes the increase in tyre weight.

As a side note, wide tyres aren't advantageous in any situation other than those requiring flotation (e.g. driving on top of snow or on sensitive ground such as peat bogs). For road/track/recreational use 235 is wider than necessary on a Jimny, and results in unnecessary weight and drag. 205/80r16 tyres would achieve the same increase in ground clearance and will give much better performance in all situations. The weight may be similar, but they will generate considerably less drag, especially noticeable when driving in mud and sand.

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12 Dec 2021 14:22 #240602 by Roger Fairclough
8.2 liters per 100 km = 34.4 mpg. The lowest I get is 42mpg.

6.1 " " " " = 46.3. " I have reached 49.5 mpg

As Busta said, the only benefits, that we can see are the aesthetics.

Roger

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