bushwhacker Posted March 22, 2020 Share Posted March 22, 2020 I am not actually doing this. I was told a long time ago to not do it as it would wreck the diffs. But does anyone know if there is any leeway? I have just got 285 75 16 tyres and wanted to check if the 30mm difference in 265 75 16 would wind up the diffs if I run it as a spare? Quote Link to comment Share on other sites More sharing options...
Bowie69 Posted March 22, 2020 Share Posted March 22, 2020 You run the middle diff in a constant different speed between front and rear outputs. This will create huge heat and knacker something in quite short order, especially if doing long motorway cruises. Just not worth it. Quote Link to comment Share on other sites More sharing options...
miketomcat Posted March 22, 2020 Share Posted March 22, 2020 I've always had same size spare though not always the same make for the reasons above. You might get away with it but for the cost of a second hand tyre or a new one for that matter it's not worth it. Mike Quote Link to comment Share on other sites More sharing options...
ballcock Posted March 22, 2020 Share Posted March 22, 2020 If you have an open diff it should make no difference in theory, as the whole idea behind a differential is to allow the outputs to travel at different speeds. it would be like traveling in a circle. Quote Link to comment Share on other sites More sharing options...
Bowie69 Posted March 22, 2020 Share Posted March 22, 2020 Yes, but it over speeds the tiny gears in the diff, and overheats them. The centre diff in the LT230 is particularly susceptible as it tiny... Quote Link to comment Share on other sites More sharing options...
bushwhacker Posted March 23, 2020 Author Share Posted March 23, 2020 I can't say I understand the whole workings of diffs but from what I was told I will stick to same size spare. Thanks guys, regards Mark Quote Link to comment Share on other sites More sharing options...
miketomcat Posted March 23, 2020 Share Posted March 23, 2020 7 hours ago, ballcock said: If you have an open diff it should make no difference in theory, as the whole idea behind a differential is to allow the outputs to travel at different speeds. it would be like traveling in a circle. A continuous circle that's getting tighter. If you put it on the rear your Speedo will also be out. Heaven forbid you put it on the front steering and handling will likely be seriously affected. A final note different widths will be ok it's the overall diameter that needs to be the same. Mike Quote Link to comment Share on other sites More sharing options...
FridgeFreezer Posted March 23, 2020 Share Posted March 23, 2020 Yep - the planet gears in diffs are made to take up the small/slow difference in speed when turning a corner or a brief wheel slip so they are (relatively) under-sized - the LT230 centre is a prime example and it's not very well lubricated I believe. What this means is if you spin a wheel really fast or drive for miles with wrong-size tyres etc. you're making the little weak gears spin all the time and get a lot hotter / deplete their oil supply at which point something's going to go bang. This is why LR manuals and the sticker on the dash says to lock the centre diff "if traction is likely to be lost". Quote Link to comment Share on other sites More sharing options...
Escape Posted March 23, 2020 Share Posted March 23, 2020 I have to agree with all the above, ideally you want same size tyres, and even same brand/type/wear, to limit the difference in rolling radius. However, if you consider your spare for emergency use only, much like the space savers found in a some cars, it will be OK for a short distance. If you're on a longer trip, the correct size will give you piece of mind and allow you to continue your trip after a puncture. Me and Ben had to abort a greenlane about a year ago, after he got a flat and had the use the smaller spare. With the P38 having a viscous coupling in the center diff, we decided not to take any risks and drove to the neares tyre shop to get a replacement, and then home. Which meant we missed out on some fun in the mud further up the trail... Since then, we carry a full size spare. 😉 Filip Quote Link to comment Share on other sites More sharing options...
Snagger Posted March 23, 2020 Share Posted March 23, 2020 Apart from destroying the centre diff quickly (or axle diff if you have different diameter tyres on each side of the same axle), it can cause dangerous handling characteristics. It was one of the issues that lead to a badly modified 110 ending up in a ditch and drowning the owners kids. Just don’t even think about it. You shouldn’t even have different tread patterns of the same tyre dimensions - all the tyres should match, including the spare. 2 Quote Link to comment Share on other sites More sharing options...
Mutley Posted March 23, 2020 Share Posted March 23, 2020 Not wanting to High Jack this thread so feel free to remove..... As a bus driver on a regular route it never ceases to amaze me the amount of people that run on a SPACE SAVER tyre for weeks at a time, quite often this will be taxis!?! I have even seen one car with both near side wheels on space savers! Oh and as for driving at the lower recommended speed when using these tyres, not a prayer, they still drive at 60 plus mph!!! Quote Link to comment Share on other sites More sharing options...
Bowie69 Posted March 23, 2020 Share Posted March 23, 2020 Probably the same people that ram themselves into an underground carriage during a pandemic. 1 Quote Link to comment Share on other sites More sharing options...
Tanuki Posted March 23, 2020 Share Posted March 23, 2020 What vehicle? And how are you running the different tyres? Different sizes on the same axle? or are you running one size on the front axle and another size on the rear? On the same axle (assuming it's stock LR and not some odd unofficial locker-thing), a few percent of different-diameter's not going to be a problem - it's only the difference between having a new tyre on one side and a worn one on the other. In this situation though, it *may* flag ABS errors. Equally, if you're running matching tyres on the ront axle and on the rear axle but the front's different to the rear - with an 'open' centre-diff [like a LT230 in non-difflock-mode] if won't make any difference at all provided the difference is a few percent. The problem comes when you have 'locking' diffs - either clockwork ones or viscous-coupled jelly-pots, which don't like long term significant rotational differences. In times-past, those of us who ran Sierra XR4x4s as service-vehicles always made a point of replacing all four tyres at the same time, because that was a lot cheaper than cooking the viscous-coupling at the side of the M6 after a 100-mile 'fast' callout while running with even-slightly-different-diameter tyres. The Vauxhall Calibra was also famous for diff-damage with different-diameter tyres. Series-1 Freelander-owners can no doubt tell their own tales about VCD failures and fractured rear-diff-mounts. The probability-of-damage increases with vehicle speed, since this increases any difference-in-rotational-speed between different-diameter tyres. How fast will you be regularly driving?? I've run 750-16 on one axle and 235/85 on the other axle of my Defender without any issues, but it's probably only ever done about 10 miles with the difflock engaged. Quote Link to comment Share on other sites More sharing options...
bushwhacker Posted March 23, 2020 Author Share Posted March 23, 2020 Thanks everyone for the info. I have 5 x 285 75 16 I was just interested to know if there was any leeway as I have seem others with a different size stuck on their back door. And they dont realise the potential failures. Quote Link to comment Share on other sites More sharing options...
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