POWER!

Contrary to most people in here I think there is almost no such thing as a "too wide tyre". It depends on how you drive it offroad and how prepared you are to be changing wheel bearings and king pin bearings.. bigge tyres do put more load on the suspension.
I had 325-60-15 on a SJ with 63 hp.. Floating over most obstacles. I had to change the gears of course.

On my Jimny i currently have 11% too big tyres (235-75-15) and still the original gears.. When i had the 1.3 engine i had to use one gear lower everywhere compared to original size tyres and often that did not quite cut it either. Changing to a 1.6 engine helps but still it is far from a racer.

I agree Yakuza, but, for DanRigg94's specific situation, the most likely reason for such low power output is that the tires are too wide for his current set up.

I've put some proper Carlos fandango's on tarmac cars and tyre width does have a considerable impact on drag.

My mini was 7mph faster (measured on a circuit) on standard road wheels (145) than it was on it's alloys (175). The rolling diameter of the two wheel and tyre combinations was near identical and the steel wheels (fastest) were heavier. The only explanation was increased rolling resistance and additional aerodynamic drag.

You don't notice so much in a jimny, they are just not quick enough to really see it.

Put mountain bike tyres on a road bike, you'll soon see it, in an extreme.

sniper

Fit vvt diffs 10% lower.

Tyre height formula I use.

Width / 50. X. Profile / 25.4 then add wheel size or 15" give height in inches

One thing to remember when changing tyre size is the effect on the speedo is the same as the effect on the gearing. If your tyres have a 10% bigger circumference, you gears are effectively 10% longer and your speedo reads 10% slower. So a reading of 45mph is equivalent to 50mph on the standard tyres. This error is compounded when it comes to calculating fuel economy- You've unwittingly being trying to drive 10% faster than usual, with the engine under greater load, and your odometer has clocked up 10% fewer miles. I think this is the main reason people complain of poor mpg when changing to bigger tyres.

Another thought that I often have when people talk about the speeds their Jimny will comfortably do- what gear are you in when you are 'fighting to keep it above 45mph'? Like most petrol engines, the Jimny 1.3 makes peak power around 6,000rpm. If you are lugging up hills at 45mph in 5th the engine is making barely 50hp. Drop down a gear or 2 and you can use the engine to its full potential. I do lots of towing and often use 3rd gear up to 60mph on slip roads so I can match my speed to traffic. 5th gear is very much a cruising gear.

Busta wrote: One thing to remember when changing tyre size is the effect on the speedo is the same as the effect on the gearing. If your tyres have a 10% bigger circumference, you gears are effectively 10% longer and your speedo reads 10% slower.

Except for one point, nearly all speedometers I have encountered on standard tyres read around 10% heavy (by that I mean it says 30 when the road speed is 27ish) and going up a size or two will nearly always put the speedo closer to bang on. I always do a check before going big to see what the discrepancy is. Manufacturers have to ensure the speedo does not under read so to speak and so IMHO play safe. It is part of the reason so many travel at around 25 in a 30 limit because not only are they trying to keep below the limit (as they perceive it on their dial) they are running about 3mph shy to start with.

That is of course true (although rarely by a linear 10% at all speeds), but my point still stands. If your reference, however inaccurate, has moved by 10% then your expectations are also off. The system for measuring speed and distance has changed by a significant amount.

Running BFG AT's at 215/75x15 is only 40mm larger in circumference than standard and whilst it is noticeable, it does seem to have lined the speedo up with road side speed warnings.

sniper

sniper wrote: Running BFG AT's at 215/75x15 is only 40mm larger in circumference than standard and whilst it is noticeable, it does seem to have lined the speedo up with road side speed warnings.

sniper

Yes same here - same tyre size and same result.

I'm running 215/75s and my speedo now matches GPS (true) speed and speed warning signs. From memory they are 5-6% bigger than standard, and my average miles per tank went from 210 to 200 (5% drop), so it all correlates quite nicely. My fuel economy has not suffered from running bigger tyres.

I have the same results with 215/75/15. Speedo now spot on according to my GPS. My mpg is down slightly around town but a little better on long runs.

When I change from alloy rims and road tires to steel rims and MT there's a difference, most noticabely in braking performance (same dimensions, only heavier). I can only guess how it handles with bigger tires and stock engine/gearbox