4th Gens on bigger than stock tyres

zukebob wrote: Fairly early on I changed to 215/75r15 tires. Based on all that I read on the forum, and without other modifications, I had expected no notable difference on highway performance.

I don't know what transmission you have but my Jimny with an auto behaved much like you described. The final drive ratio is a bit different between the AT and MT. What I noticed wasn't enough to make me grind my teeth but it was recognizable. I could really tell the difference when driving an incline on one of the mountain highways. The good thing is that I didn't really notice a difference off-road.

If you do have an auto transmission, the only thing I can add is that you will get used to it.

Edit: As far as the transfer case goes, if they put the wrong case in (AT vs MT) the gearing difference would be far more noticeable than what you are describing.

Thanks. It is a manual 5 speed.

The gearing doesn't seem changed since the transfer box swap. Do you know what the ratios are for each? Not saying they couldn't have mixed it up, but it would be quite a major blooper if they had.

As a side conversation, I'm quite used to transfer box ratio changes. Land Rover Defenders normally run a 1.4:1 transfer box. However my Land Rover runs a 1.2:1 and my brothers 90 has a 1.6:1

35kg of bumper and 18kg of rock sliders, that's nearly 10 1/2 stone. My 16 year old son weighs about that but I can ask him to get out and push. There's bound to be an effect but my guess is you'll get used to it. Don't let it spoil your weekend. Chris

300bhpton wrote:
And it was directly after the tyre swap, like hitting a light switch that the performance suffered hugely.

Perhaps that is your answer then? It's the tyres not transfer box?

Also, could it be more to do with the actual tyre not the size? Those you fitted are quite an aggressive tread compared to some AT tyres, and what is the compound like, are they a soft grippy tyre? That would add to the effect of the size increase.

Just thinking out loud really.

mlines wrote: We are on 215/75 R15 KM3 and think the Jimny drives just fine.

Jimnys have always needed the revs keeping up to keep it bowling down the road and the gen4 is no exception. Diesel 4x4 do all their power at low revs and most do not even rev to the point you have to get to before the Jimny engine starts to produce it's power.

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Thanks Martin.

I do use all of the revs. Not all the time, depends on the conditions and circumstances. However it has had a pretty big negative impact on all aspects of performance. Mid range and top end, etc. It will still pull from low down as smoothly as it did before, and is fine off road. It is more general road running. I'm now having to quite literally ring its neck to keep up with other traffic coming off of round abouts and the like. No point short shifting, 4000rpm+ required for every gearchange.

Actually, it is super hard to express in text on a forum. So I hope it isn't being misinterpreted. All I can say is, I have noticed, what I'd consider, a significant impact on performance of the vehicle, by fitting some 21575/R15 All Terrain tyres. Which was quite unexpected. The best example I can give is, on a section of tarmac previously, it would easily clock 82mph on the speedo (which wasn't all that far behind my 4.6 V8 Range Rover, which I've also said about in other threads). Had done it many times, including with the winch bumper and other bits. They made no impact to this that I could ascertain. Now however, it is more like 77-78mph. Which is quite a big difference.

Logically, either there is something wrong with my Jimny and it is amazing coincidence to have happened the day of the tyre swap (Obviously I don't have enough evidence to rule out the transfer box change, but I'd have thought it unlikely).

Or other people aren't noticing the performance change despite it existing (or are unwilling to say there is one). Rationally I cannot see any other answers.

I'd have thought a set of MT's would make an even bigger impact. I know the BFG KM03 MT's weight quite a bit more than their BFG KO2 AT's. And with bigger lugs would have more rolling resistance too. I'd assume and guesstimate my Maxxis Wormdrive AT's, are somewhat akin to the KO2 AT's in weight and rolling resistance.

mlines wrote:

Its not being mis-interpreted, its being taken in the context of an interesting discussion.

As our KM3s were fitted by the dealer we have only the 2 hour "test drive" of the demonstrator to compare with (plus our Gen3) and we are comfortable with the way ours performs.

Maybe that's it. I've spent quite a bit of time driving mine before the wheel swap. It's not that I thrash it, but as said, I like to use every part of the rev range. I love the fact it'll pull 30mph in 5th. But most times I go out in it, it will hit the red line at least once, but usually multiple times.

Wish I'd taken some performance stats such as 30-50mph or 40-70mph. I have an app on my phone. So I might try a few benchmark tests and post the results. Others might want to download the app (it's free on the iOS App Store) and also perform some measurements.

Yes it does seem a lot of people have swapped tyres straight off with getting to know there Jimny first,like you have done. So maybe a lot of people are not realising the drop in performance, so ate unable to comment.

How about actually getting a tape measure and comparing the results

Seriously though, different manufacturers state a given size, but the actual size can vary a fair bit. For comparison maybe some of the others here could post a measurement too? E.G. a 215/75r15 tyre is supposed to be 704mm tall according to tiresize.com/tyre-size-calculator/

I mention this because I had a horrible driveline issue with an old subaru which was caused by different brand tyres front to back. They measured nearly an inch taller on the back ones even though they were the same theoretic size...

There were no KO2 tyres at a fair price when my Gen 4 landed. So i waited months, in the end I got them from Germany delivered cheaper than you could get them from out of stock UK. Yes I did expect a drop in power as this had happened on other 4x4 vehicles. But was very surprised it was minimal. I think this was because the others were diesel & the Jimny is petrol. It might be your choice of tyre.

Rolling resistance is a phrase they use now and again, the run silent tyres on electric cars have virtually none apparently. Perhaps your chosen tyres have a high level of this black magic. Chris

KO2 Tyres 29/30 PSI still have a large foot print front & rear (empty car )

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Try putting more air in ( this might upset some diehards but give it a go )

PS. must weed drive. MMM