4th Gens on bigger than stock tyres

Question to anyone with a 4th Gen JB74 that is running an AT/MT in 215/75R15 or 235/75R15

Truthfully, what impact have they had on performance?

I'm quite shocked at fitting some 215/75R15 AT's to mine. It feels like I've lost 10% plus power or I'm driving with the handbrake on.

The tyres are barely any bigger than standard in terms of diameter. The maths say there should be about 11mm in it. But according to GPS it has made no impact on the accuracy of the speedo at all.

However it has had more impact on performance than fitting a set of 34" extreme mud tyres to a Discovery Tdi.

On a test stretch, the Jimny now clocks 4-5mph lower speed than it did previously. Which is a fairly significant drop. And you can feel you have far less overtaking capability now.

I'm a bit shocked at the impact on performance. The reason I'm questioning it is. The week before fitting the tyres, I had a new transfer box fitted by Suzuki (to solve it jumping out of low range). I'm sure this is unrelated, but it is the only other recent change to the vehicle. With the new transfer box and standard tyres, it went as well as it ever did. It was pretty instant on swapping the tyres over that caused the performance loss (or an amazing coincidence). But I want to rule out all scenarios, as going back to the standard tyres would require getting some extra wheels.

The Jimny seems to freewheel absolutely fine. Doesn't pull to one side or the other at all. Can't smell or feel anything getting unduly hot.


When people say the Jimny is slow, I've always contested it. As I thought it went rather well. And importantly felt very peppy, zingy and willing. Now though, I would indeed say it feels sluggish, underpowered and slow

I didn't have the original tyres on for long, but don't recall much of a difference fitting them. The larger ATs are likely to be significantly heavier than the stock tyres (about 4kg difference per wheel between stock alloys with 215/75 Yokohama ATs and stock steels with 195/80 Duelers) which will damp performance a bit. I didn't measure performance when I tested steels, just economy (about 2-3mpg worse with the larger tyres).

This is to be expected. You are now running with wider tyres so your footprint is bigger and that will create more drag. Add to that you have changed the gearing and cumulatively you have dented performance and increased fuel consumption. On my Landcruiser in standard trim 50-70 mph in 5th gear was 10 seconds. Change to A/T tyres and it went to 13 seconds. change again to M/T in 33" dia as apposed to 31" and it went to 16 seconds. Also adding lots of extras will make things worse. The Jimny is a lightweight motor and cannot cope with all the gear that my landcruiser could lug around.

Roger

300bhpton wrote: Question to anyone with a 4th Gen JB74 that is running an AT/MT in 215/75R15 or 235/75R15

Truthfully, what impact have they had on performance?

I'm quite shocked at fitting some 215/75R15 AT's to mine. It feels like I've lost 10% plus power or I'm driving with the handbrake on.

The tyres are barely any bigger than standard in terms of diameter. The maths say there should be about 11mm in it. But according to GPS it has made no impact on the accuracy of the speedo at all.

However it has had more impact on performance than fitting a set of 34" extreme mud tyres to a Discovery Tdi.

On a test stretch, the Jimny now clocks 4-5mph lower speed than it did previously. Which is a fairly significant drop. And you can feel you have far less overtaking capability now.

I'm a bit shocked at the impact on performance. The reason I'm questioning it is. The week before fitting the tyres, I had a new transfer box fitted by Suzuki (to solve it jumping out of low range). I'm sure this is unrelated, but it is the only other recent change to the vehicle. With the new transfer box and standard tyres, it went as well as it ever did. It was pretty instant on swapping the tyres over that caused the performance loss (or an amazing coincidence). But I want to rule out all scenarios, as going back to the standard tyres would require getting some extra wheels.

The Jimny seems to freewheel absolutely fine. Doesn't pull to one side or the other at all. Can't smell or feel anything getting unduly hot.


When people say the Jimny is slow, I've always contested it. As I thought it went rather well. And importantly felt very peppy, zingy and willing. Now though, I would indeed say it feels sluggish, underpowered and slow

I may be wrong, but havnt you also fitted some under plate protection recently too??

Ive lost a bit of zip too with 215/70/16's, but im still happy with performance.

Ive never understood folks who thinknthe jimny is slow, nor with cornering lean. I musr be a bit of a grandpa behind the wheel or something.

Why not try to borrow some standard jim wheels for the weekend (Gen3 will do). You can always lend them yours, you would then get a better idea if it's the tyres or all of the mods you've done. I suspect Lambert is right, there's a limit to how far you can go without penalty. Chris

Fitted 215 75 15 KO2 tyres thought this would make a big impact "but no". Fuel filter & TC case guards fitted still OK.
I like your new front bumper but 35/40Kg isn't going to help. As i drive on road most of the time tyre pressure is 29/30psi any lower and they look flat (MY CHOICE) fuel consumption is 39.6 mpg. Keep it light.

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.

I weighed my 215/75r15 BFGs and they are 5kg heavier than the standard Bridgestone Duellers on my gen 3. Adding 20kg of rotating mass is not insignificant!
The increase in circumference might only be 1.5% but the affect of any change is compounded if the speedometer is not adjusted accordingly. Effectively you have 1.5% longer gearing, and are trying to achieve 1.5% higher speeds.
I recently changed my diffs from 3.9 to 4.1 which putts the gearing close to standard and It makes the Jimny seem much more sprightly. Still not the same as it was on standard size lightweight road tyres, but an improvement nonetheless.

Roger Fairclough wrote: This is to be expected. You are now running with wider tyres so your footprint is bigger and that will create more drag. Add to that you have changed the gearing and cumulatively you have dented performance and increased fuel consumption. On my Landcruiser in standard trim 50-70 mph in 5th gear was 10 seconds. Change to A/T tyres and it went to 13 seconds. change again to M/T in 33" dia as apposed to 31" and it went to 16 seconds. Also adding lots of extras will make things worse. The Jimny is a lightweight motor and cannot cope with all the gear that my landcruiser could lug around.

Roger

Don't get me wrong. I understand where you are coming from. I'm very used to swapping wheels and tyres. We have an entire shed full of Land Rover tyres on the farm.

What I am surprised at, is the impact on the performance. Compared to other 4x4's I've owned/driven.

Especially considering the fact I haven't really changed the wheel size all that much on the Jimny.

I'm used to running things like these:

Andy2640 wrote:
I may be wrong, but havnt you also fitted some under plate protection recently too??

Ive lost a bit of zip too with 215/70/16's, but im still happy with performance.

Ive never understood folks who thinknthe jimny is slow, nor with cornering lean. I musr be a bit of a grandpa behind the wheel or something.

No under body protection.

It does have tree sliders, diff guards and a winch bumper. BUT.... it ran fine with all of these fitted. You'd not really know they were there. And to be fair, added up, they weigh less than an average person. And by quite a bit I should imagine. So are less noticeable than having a passenger. And it has always gone fine 2 up in that guise too.

The only changes were:

- new transfer box
- some days later (maybe a week) the tyre swap

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