My wife’s mechanic replaced all 4 tires ($869) on her MDX claiming that it was a “requirement” because of the VTM-4 (all wheel drive). Allegedly, he’d had trouble replacing less than 4 because the “computer and sensors” that process each tire’s RPMs is so sensitive. The car’s manual says that the VTM-4 doesn’t even go in to action unless the car is going BELOW 18 miles/hour.
To clarify … One tire needed replacing because of a slow leak.
Time to read your owner’s manual carefully.
For a Subaru, what was done was in accordance to Subaru’s recommendations. Don’t know if Acura has similar parameters or not. Your owner’s manual should indicate a tolerance in either tread depth or wheel circumference that should be adhered to. You need to find out what that tolerance is. You don’t give us any indication of number of miles the old tires had on them, so a judgement of four new vs one new shaved down to proper circumference cannot be made.
In your case of one tire with a slow leak, consideration should be given to having a new exact replacement tire shaved down to the appropriate circumference. This would achieve the same effect of buying four new tires, as far as the AWD system is concerned.
VTM-4 is used at all speeds, it can only be manually locked at below 18 mph, my Ridgeline has the same system and yes it is recommended that all four be replaced at the same time. The rear differential on these vehicles are extremely expensive.
When car companies and sales people recite the virtues of AWD, they don’t tell you about the downside. The downside is more expensive maintenance due to extra components, 1 to 3 less mpg, and the need to have all 4 tires match (same brand, same tread, same wear). That makes tire rotations more critical too.
If you need AWD it is great. But many cars have it and are rarely in a situation where the AWD is crucial to safety. If you have AWD matching all 4 tires is part of the equation, and an expensive part.
Is that true of 4wd cars too?
You should check the owner’s manual for your particular vehicle.
In general; 4WD which you can run in 2WD mode, and then shift into 4WD need to have matching tires on the front axle and matching tires on the rear axle. The means replacing in pairs, not all 4.
The AWD system in the MDX operates at all speeds, and there is no manual overide to lock it into 2WD.
He replaced all four tires…because of a slow leak in one?
What was the reason the slow leak couldn’t be repaired?
Thanks to everyone for their replies - very helpful.
Since my first posting I’ve had a long “heart-toheart” talk with out (independent) service manager, after he looked at my wife’s tires.
- It appears that the tires did need to be replaced, independent of the MDX’s requirement.
- The consensus I’ve gather is that replacing the (“magic”) rear differential is expensive, as was observed by one of the responders above.
- In contrast to any car I drive with an ABS (Anti-lock Braking System) - something I notice whenever I brake quickly or hard, I’ve never noticed the VTM-4 (Variable Torque Management) kicking in on the MDX, and neither my wife nor I have had occasion to use the manual (less than 18 mph) lock on the VTM-4. Obviously this is the good news, but it’s involvement in tires, shocks, etc. is then unexpected.
- I had no idea about any of this when my wife bought the MDX - that the rear differential was so “delicate”, that the tires had to be replaced pretty much as a 4-some, etc. It’s definitely an added cost of driving the MDX, as someone else observed above.
- The owner’s manual is not definitive on this subject, although there’s some hand-waving about tire replacement and balancing.
- This all may be complicated by the shock absorbers gradually wearing out, as they tend to do.
- The MDX is the most expensive car I’ve ever had to service, although nothing has failed - catastrophically - yet.
It’s Honda’s luxury brand here in the US, so service will be more expensive, but the car should be more reliable than some other brands.
VTM-4 is pretty much used all the time. View the document in the following link for an explanation of how it works, courtesy of Gary Flint the engineer from Honda.
EDIT: And yes the explanation is for a Ridgeline but it’s my understanding that the system in the MDX is identical.