In response to your post, I reviewed my owner’s manual for one of our cars that came with a compact spare. It says quote: “The compact spare is made to perform well at speeds up to 65 mph for distances up to 3000 miles so you can finish your trip and have the full size (flat) tire repaired or replaced at your convenience. Of course it is best to replace the spare with a full size tire as soon as possible. The spare tire will last longer and be in good shape in case it is needed again.” Unquote
It says nothing about moving the compact spare to the rear of a front driver. That would not be acceptable and neither would it be acceptable to move the compact spare from the rear to the front of a rear driver by the side of a road, especially the driver side with only one jack.
After following Car Talk for several years, I have not heard of an instance where a compact spare ruined a differential. Think how much a differential needs to work on a winding road. A differential has gears and rolling element or plain bearings for the spider gears that are just like other gears and rolling element or plain bearings that work constantly while you are going down a straight road. Your engine has plain bearings. I seriously doubt that you could wear out a differential even if you tried to do that.
The problem here as is so often the case, owner’s manuals do not go into detail in the interest of simplicity and for keeping the manual as thin as possible which is not the case now with all of the lawyerproofing boilerplate. When information is lacking, people have a tendency to fill in the gaps with worrysome thoughts and fearmongering from others.
"I seriously doubt you could wear out a differential if you tried "
Using tires of different diameters is not an immediate impact act that causes the differential to instantaneously fail. But, you are in the minority of you think it has no affect on the wear to he point it will eventually fail much earlier. The ring and pinion gears operate continuously as the car moves. The other gears responsible for allowing the wheels to rotate at different speeds are not made to run continuously. That’s why you are cautioned not to drive at higher speeds with tires of unequal diameter, because of the heat and the breakdown of the lubrication. If a differential has been missused, the differential fluid will show it along with more shedding of metal particles. Will it fail? Maybe not with the first owner in 100k as they are unusually sturdy in solid axle rwd cars.
Imho, In front drive cars, the differentials are no where near as sturdy and are in closer proximity with autos and may share lubricant with a manual transmission. A transmission or related failure may be more likely then. Don’t bet your life on something else not failing. If it’s the differential between the front and rear of an Awd car, there are plenty of instances of failure in mileage of fewer then 50 k. But, if your hypothesis makes sense to you, that’s fine. Don’t worry obout tires of unequal diameter. Maybe you know more about them then the engineers who made them.
The"even if you tried" comment I will take as just an exaggeration. Otherwise, we could just drain your differential completely to prove your hypothesis wrong.
dagosa: During my time with cars I have not encountered nor ever heard of a worn out differential in normal, non abusive use. Possibly you have seen or heard of one.
@WhatWho
Oh for sure. It happened to an acquaintance who like me, spent some time mudding every year. I don’t call it abusive because Jeeps are suppose to do that. I upped the service in my truck, he didn’t. It started to get real noisy after an extended trip and he was presented with quite a repair bill. He chose to dump it. Granted, it was 10 years old but the mileage was less then 150k. He did nothing out of the ordinary other then fail to service. It wasn’t like he tried to destroy the thing. To say you could not wear one out is giving total disregard to the fwd trans axle which shares transmission and differential space in many cars. In a fwd car, isn’t like it never happens or people don’t have trans axle related problems including the teansmission because they failed to do what they were told. You also disregard the center differential in Awd cars and trucks… They start acting up and not functioning properly before they actually fail even with good service when they age.
You can’t shuffle the cards after the hand is dealt when I was responding to the comment. " you can’t wear one out if you tried" and now say…“in normal use” which presumes normal service. Under those circumstances…no ! But you can sure fail to maintain under heavy use, towing and off roading and have plenty of problems. Now, if you trade every 5 to 8 years with fewer then 100k, no too there. As far as you not having heard of a compact spare causing one, I haven’t in the crowd I run with because they all have AAA cards or are smart enough to do it right. I bet if you did see some dumb butt doing 80 mph on a small diameter wheel on a fwd car you probably didn’t follow him home. So anyone saying they had not heard of it happening in that case means little.
My 2010 Honda limits speed using the spare to 50 mph with no mileage limit (but with the usual “fix flat tire asap, and buy a new spare when it’s worn out” caveats). Also, there’s no position requirement (it’s FWD), I guess because the spare is only 1/2" smaller in diameter. Having said that, if I were at my brother’s house (400 miles away) and for whatever reason had to drive it home, I’d make sure the spare was on the rear.
Last November was the first auto show I can recall where a minority of cars on the floors had spares of any kind. Maybe a third did. The rest were about evenly split between high end models with run flats and economy cars with a can of leak fix stuff and a compressor.
The cars still had a well that looked big enough for a normal wheel and tire, filled with a plastic tray for the repair tools and whatever else you might want to put there. A few had first aid kits and reflector signs. Dealer-installed options, I’m sure, but a nice use of the space. With rising mpg requirements, that’s some of the easiest weight to cut. I sure was glad our 2013 Elantra came with a donut when it suffered sidewall damage taking a friend to catch a flight to London. The tire had a 50 MPH limit but no specific distance limit, just to repair when wear bars showing. I really like Hyundai.
on a small diameter wheel on a fwd car you probably didn’t follow him home. So anyone saying they had not heard of it happening in that case means little.