I have heard it said, here and elsewhere that if you don't run exactly the same tires all round , then you risk damage to the transmission, specifically the bevel gear. Now what is the explanation for that? Surely there is a centre (front to back) differential as well as both side to side diffs, that permit unequal motion on the wheels. And of course has to allow for a single wheel lockup on braking, or slipping on ice, without damage.
So what is the real cause of broken bevel gears?
I'm anxious to know as I have one in my sites to buy.

I saw something here in this thread that should definitely be corrected.

The larger tires MUST GO ON THE FRONT AXLE, not the rear. Owners manual and TSB states that if you cannot replace all four tires put the "one or two" new tires on the front.

Rotation to keep the tires within 1/16 to max 1/8 of an inch of wear is recommended - every 5K miles or so b/c the front tires wear quicker.

Normally something like 90% of the power goes to the front wheels in this system.

If the front tires are smaller, they rotate faster which the viscous clutch "interprets" as front wheel spin, increasing the power delivered to the rear wheels.

For some reason the continual over-delivery of power to the rear on dry surfaces cooks the transfer case.

I bought a '98 V70 XC in October and went through a bunch of research about this tire thing. I needed to match two tires that had about 5000 miles on them. Discount Tire has a policy of installing new tires on the rear axle only. I sent them this TSB.

As the omnipresent Klaus said elsewhere (I believe) it may be a good idea to carry a can or two of fix a flat and a compressor in the spare tire area. And or join AAA. Flatbed towing only unless you disconnect the drive shaft!!

This is something I pulled from another post here:

Volvo TSB 4460005 calls out some of the issues with the older
AWD system (new models, S60 and 2003+ use Haldax AWD which
is different).

If you can live with them and know the previous owner followed
the guidelines then the AWD might be OK for you. Here is some of
the info from the TSB (the rest goes on to explain how
to service and inspect the AWD components and includes images so
I did not cuta nd paste it):

Wheels/Tires - AWD Information & Function Test
850 1997/S70/V70/V70 XC 1998-2000/V70 XC 2001-
1997-

Section
4

Group
46

No.
0005

Year
01

Month
01

Vehicles involved:
AWD/XC

AWD Wheels/Tires, Information and function test

Background
This service bulletin contains information to ensure that the all-wheel drive (AWD) system is not damaged when servicing wheels and tires. It also contains information about checking the freewheel and viscous coupling for the all-wheel drive (AWD) system.

Wheels and Tires, Information and function test

Information about wheels and tires and function test

Note! To prevent deterioration in the function of the power transmission system on All-wheel drive (AWD) cars, it is important to follow the instructions below.

1
All Wheel Drive vehicles have special requirements for tires and wheels. It is very important that the tire replacement guidelines below are followed. Failure to do so can result in damage to the AWD components (angle gear, viscous coupling/freewheel unit).

Always drive on tires of identical brand, size, construction (radial), tread pattern, load-, speed-, traction-, temperature-, and treadwear rating.

Never drive on mixed tires, except for brief periods when the temporary spare tire is in use.

Always use properly inflated tires of correct dimensions. Tire size and inflation pressures are shown on the tire pressure label located inside the fuel filler door.

Caution! Failure to always drive on properly inflated, identical tires of correct dimensions may result in a circumference difference between tires on the front and rear axles. This will cause excessive tire wear and may damage the transmission and all wheel drive system.

2
Replacing the tires

When tire replacement is necessary, Volvo strongly recommends replacing all four tires at the same time with identical tires as explained above. Failure to do so can result in circumference differences that may damage the transmission and all wheel drive system.

If only one or two tires are replaced, the new tire(s) must be identical to the tires with which the car was built, and must be mounted on the FRONT AXLE ONLY! Failure to do this may damage the transmission and all wheel drive system.

3
Spare wheel Temporary spare

The temporary spare tire is for temporary, low-speed, short-distance use only Do not drive on the temporary spare at speeds above 50 mph (80km/h) or for distances greater than 50 miles (80 km) or transmission and all wheel drive system damage may result.

4
Snow chains all-wheel drive (AWD)

On all-wheel drive cars, (AWD), snow chains must only be mounted on the front wheels. Only snow chains intended for all-wheel drive (AWD) cars can be used. Never install snow chains on a temporary spare tire.

Thanks for all your input.
I have avoided the problem by today buying a very nice '00 V70 FWD with 358,000 km on it.
As I read your responses, it would seem to me that the AWD system is not simply a simple mechanical arrangement of 3 differentials, which would pose none of the issues mentioned, but its a system that actively interferes with the traction using limited slip clutches or whatever.
Incidentally, I talked to the local Volvo dealership mechanics, who also told me the same, but said they had never encountered broken bevel gears, just the seals.

Now to try and sell my '92 740....

I don't know that I broke the gears as much as the unit was ruined and it had to be replaced. My apologies if I left you with the impression that we had actually broken the physical gears.

You can have slightly larger tires on the rear than the front. Smaller rear tires will confuse the ABS into thinking the rears are slipping (rotating faster) and try to compensate.

The bevel gear (transfer case) adds and removes power depending on wheel speed for each wheel. If the rear wheel(s) are spinning, more energy is routed to the rear wheels.

Different tire sizes, like putting a spare on the right rear, will cause the bevel gear to implode after 50 miles. It will over heat the fluid and start cooking parts and seals.

My 1998 V70 had AWD and never gave me problems up to 188K miles.
--
My name is Klaus and I am a V♂lv♂holic

Had a 2000 v70 xc, broke bevel gear after wife drove on space saver spare for 30 mi or so. Unavoidable but expensive. Time, place and distances were not conducive to getting to tire shop for repair or replacement of tire. You might just as well have no spare tire as have the space saver. My 2004 xc70 no longer has that type of awd system. Don't know specifically when Volvo changed over to different system.

Do you own an XC? What does the owner's manual say?

Almost all, if not all AWD (and any other vehicle that is more than one wheel drive) have specs about what differences in tire diameter or circumference that they will tolerate without too much damage to the drivetrain.

The exact process for each drive system varies, but the results are the same - do not use different circumference tires on a AWD (4WD) drive train.

Subaru says 1/4 of an inch.

While differentials accommodate wheels turning at different speeds, there is a cost.

The cost is mechanical wear and heat. You wear out parts in the drivetrain and the heat damages the fluids and parts. You then have early failure.

Actually, newer vehicles are much better than older AWD drivetrains. Most older AWD (4WD) trucks had transfer boxes and hubs to disconnect as many parts as possible because of wear and heat just driving around.

Tire Rack has a simple explanation (basically the same as mine) and what you can do if you just need one tire. A new tire can be shaved to match your other tires.

If I had a flat on the drive axle of my vehicle, I would move a full sized tire to the drive axle and put the space saver on a non drive wheel position.
--
'96 855R,'64 PV544 driver, '67 P1800 basket case, '72 Yamaha Rd400, '68 Honda 350-4, '12 XC70, the first 5 are mine, heh, heh, 525,000 miles put on 10 bricks James A Sousa