tyre/wheel diameters?

I've always been told that its a no-no. The diffs do not like running for a long period with different rotational diameters. Like going round a perpetual bend :). However, as an emergency spare to limp to the next servo with, I don't really see a great problem.

Are you sure they are different diameters. Stand them side by side and check. I wouldn't think that 15 or 16 inch rims would make any difference cos you can fit the same overall size tyre to either rim. I have two sets of wheels also and I only look at the overall mounted diameter. If you have a flat you could mount the differing wheel to your front axle then UNLOCK your hubs and there shouldn't be a problem. Use the same diameter tyre across an axle with the diff operational. It might mean you have to change two wheels at once though!!!!!! Not good if you need 4x4. Go to a wrecker and get some more 15" wheels and remanufactured tyres for spares. A trip like this is a major (read expensive) undertaking and it would be shame to stuff it up for the sake of a couple of hundred dollars.

Glen a point to think about, toyotas all came out with 16 rims so if you are running 15,s your speedo will be out as well as fuel consumption will not be good, you will find that the 15,s are probibly nissan which have a different offset to toyota rims. My suggestion would be to replace all the 15,s for toyota 16,s and keep the split rims as spares.You can do a lot of damage to the centre diff if you have constant 4wd with different dia wheels Col.

Glenn, if you want to compare your tyre circumferences check out this site which tells you how to interpret the tyre numbering systems.
http://www.dunloptire.com/tt101_sizing.html Regards, Gordon

definitive answer - Preferably all your tyres should be same over all diameter but consider what happens when you have 4 new tyres with 16mm of tread and you put an old spare with 3mm of tread - there is 1/2 " difference in overall size and Im sure it happens a lot.
Main issues are:- limited slip diffs which will not like 2 different diameter tyres though I have no personal experience of any major problems but it does depend on how far you drive that way. Just expect extra wear of the clutches in the diff. Most limited slip diffs dont work properly anyway for a variety of reasons (often old or incorrect oils) Nissan Patrols seem to have the best LSDs on the market so would be most prone to problems.
6 Stud 15 and 16 " rims fit both nissans and toyotas and heaps of other 4wds most tyre websites give you the overall diameter of different size tyres and their no. of rotations per mile or kms. BF Goodrichs site is excellent.
As for steering it will destabilise your truck for steering and under hard braking you may not get an even stop. However If you drive like you are carrying a pallet of grog on the back (ie carefully)Im sure you will survive long enough to fix the flat tyres.
I think you have enough info to answer your question.
kezza

ps Oh by the way if you need a couple of spares I have 2 x Unused(spares) New BF Goodrich Mud Terrains 31x10.50R15LTs on 15"x8"rims (78.5cm or 30.9" diameter they do 673 revolutions per mile @45mph) If you have these type of tyres you get far less punctures anyway(touch wood) Im advertising them in the Trader on this site.