Rolling Circumference

[ab56452001]

Hi just picked up a second spare for outback travel , a slight problem , the 4 on vehicle plus spare are 275/70 r16 , the new second spare is 265/75 r16 , 35mm extra circumferance , about 1.5% I think , wouldn't be an issue would it ? , you could have this difference from wear alone I think ! , cheers in advance.

 

[Beau]

I think in the short term, like using the tyre as a spare for short distances is fine, but in the long run you want the same size all round, otherwise I believe it can damage the transfer box. 1.5% is more than just the usual wear, and I believe Toyota normally recommend having the most worn tyres on the back in situations.

 

[ab56452001]

Thanks for reply , yes I understand about keeping same size , but , if after a month ( about 1.5% of tyre life ) of putting on new tyres , 1 blows or gets wrecked , nobody is going to put 4 new ones on are they ? , cheers.

 

[ab56452001]

Im a bit sus on this , 25 years ago I blew a tyre in the scrub , since then ive travelled many , many km on dirt roads with 2 spares and never , thankfully had any tyre go on me , the day I only have 1 spare Murphy's law will surely come into effect !

 

[Beau]

Maybe I confused you a little. I advised it's best to run 4 tyres the same size. No matter what the tread on those tyres, if one blows, replace just the blown one. Try to match the tread per axle as evenly as possible, and I believe Toyota recommends having the most tread on tyres at the front. Wear on tyres does not equate to 1.5%.

Like I said, you should be fine with running the odd size tyre for short distances, but I would advise against it as it could potentially damage the transfer box.

 

[AndycruiserguyLomas]

Beau, I get your point about what Toyota say, but I can't get my head round how running 2 slightly different size tyres on an axle (with an open differential) can affect let alone damage a transfer box (also with an open differential) ?

 

[mettisse]

Hello, good point of discussion, when I recently changed 2 tyres ( BFG 285 all terrains) the fitters were firm in saying best tyres to the rear, they said Michelin recommend on it for best handling. Well, with the "claim" mindset prevalent today I don't think they would put out false info.
One of the wonderful things about differentials is that the unit will " deal" with uneven wheel diameters, it's just as if it's going around a constant bend in the road. However, it's beyond my pay level to determine what effect it will have on steering and handling. Of course 4 perfect tyres of identical circumference would be ideal, but we have to get on with the circumstances we have. I hope someone will post their experience to enlighten us all.
Another point to think on is that many cars have these emergency tyres, I believe 50 mile max is recommended but if it was gonna trash the diff, 50 miles is enough, what do you think?
Best regards to all, Bill Westley

 

[frank rabbets]

With a 2 wheel drive car whether FWD or RWD you put the good tread on the rear. It is more important to keep the rear axle from skidding than the front. The same logic applies to the brakes, the rear axle brakes are designed not to lock the rear wheels up.

If you lose control of the rear it's very difficult to control compared with losing control of the front. That's LOSING control not just power sliding the rear on purpose under control.

I don't know the tread rule for the 80 but I would put my good tyres on the rear.

 

[Chris]

Looking at the differences, I would say that for short distances this would be better than being stranded with no spare. It's the rear differential that is going to get worked, more than the transfer box. I would think that this might be equivalent to having one brand new AT one one wheel and a totally worn tyre on the other side. There's no chance that you will always have precisely the same tyre on each end of the axle and with turning corners this effect would be unnoticeable. The real tester would be miles and miles of straight tarmac.

Let's not lose sight of what the question actually was.

 

[ab56452001]

Thanks all for very informative replies , I reckon as a second spare the tyre I just acquired will be o'k'.

 

[Towpack]

A 35mm difference in RR will mean the bigger tyre will turn approx 86 more complete revolutions per mile than 'tother one on a straight road. I would think a standard open diff would handle this OK personally. A clutch or viscous type LSD would be more at risk though. JMO

 

[ab56452001]

A 35mm difference in RR will mean the bigger tyre will turn approx 86 more complete revolutions per mile than 'tother one on a straight road. I would think a standard open diff would handle this OK personally. A clutch or viscous type LSD would be more at risk though. JMO

Are you sure ? , if the tyre goes around say 1400 times a mile , 1.5% is about 20 times . Anyhow it would only prob be used on dirt so some slippage available there id say , cheers.

 

[Towpack]

Are you sure ? , if the tyre goes around say 1400 times a mile , 1.5% is about 20 times . Anyhow it would only prob be used on dirt so some slippage available there id say , cheers.

I thought I was, so much for wine assisted fag packet maths! A quick check on a calculator says a wheel with 252.55cm RR will rotate just over 637 times/mile. A wheel with a RR 3.5cm bigger will be just over 628.5 rotations so the difference is even less than either of us thought! I got an A in biology though

 

[Chris]

Errm 'slippage' has nothing to do with this. If the wheel is on the ground it will go around. This isn't about wind up. But answer established I think, no it wouldn't matter if this was a spare for a while. and it would ONLY really matter in a very straight line.

 

[clivehorridge]

A 35mm difference in RR will mean the bigger tyre will turn approx 86 more complete revolutions per mile than 'tother one on a straight road. I would think a standard open diff would handle this OK personally. A clutch or viscous type LSD would be more at risk though. JMO

I think the smaller one would do the most revolutions, but that aside, I think you're all on the right track.

 

[IRLGW]

I have driven 2000miles on 3 x 265/80/16 and a 235/70/16 with no problems. For all the diff knows you are on a very twisty road. With difflock on its a different matter and probably more of a problem with electronic traction control but dont think mine is working anyway. You shouldnt even notice it really.

 

[ab56452001]

Thanks again everybody , I really don't want to by a new tyre as this has 95% tread left , and I only paid 30$ for it including original mag wheel! , cheers

 

[AndycruiserguyLomas]

I have driven 2000miles on 3 x 265/80/16 and a 235/70/16 with no problems. For all the diff knows you are on a very twisty road. With difflock on its a different matter and probably more of a problem with electronic traction control but dont think mine is working anyway. You shouldnt even notice it really.

Probably the most concise answer, the diff works constantly anyway so it will make f**k all difference if they were all different sizes to the transmission. The steering and handling however may be another story.