Different tire sizes affect car’s systems

Hey there, time traveller!
This article was published 08/05/2009 (6284 days ago), so information in it may no longer be current.

QUESTION: I have a 2000 Grand Prix with P205/70 15-inch tires (all worn out). A friend has offered me four P205/65R15 tires and rims with 30,000 kilometres on them for a good deal. Is there any problem changing from a 70 to a 65 series? His car was written off. It was a Grand AM.

ANSWER: The difference in aspect ratio (70 to 65 series), which is a ratio of the tire sidewall to the width of the tread, means that the P205/65-15 tires will be slightly smaller in diameter than the original tires. Although there are slight variances between tire brands, we can assume that there is about a half-inch difference in diameter between the two tire sizes. By calculating the difference in circumference, there is less than two per cent difference in rotating speed.

The systems affected by changes in tire size are ABS braking, stability control (if the car has it), transmission shift-speed control, and of course the speedometer. The tire speed difference is minimal, so it will have little affect on other systems, but the speedometer will read slightly fast by about two per cent if you use the P205/65R15 tires and fuel economy will be slightly less.

 

QUESTION: I have a 2003 Escape, 3.0 V6, auto, AWD. The problem is with the brakes (non ABS). When it reaches -25 C outside, the brake pedal freezes and there are no brakes until the engine warms up for about 10 minutes. When it is warm, the pedal is soft and although there are brakes, they don’t seem to be as hard as they should be. I can’t lock up the brakes at 60 km/h. The front pads and rotors were recently replaced, and the rear drums and shoes have about 25 per cent wear. My mechanic says Ford and Mazda Tributes have a common problem and that the booster is defective, so that was replaced with no difference. I had the brakes bled, master cylinder replaced, all with no difference. The booster vacuum check valve was also replaced. I even had the booster replaced again thinking the replacement was defective, and still no difference. Are you aware of any inherent problems, or have any clues? Any information is much appreciated.

ANSWER: Your brake pedal symptoms are a classic example of water freezing in the brake-booster assembly. All it takes is a single drop of moisture in the control-valve portion of the booster to cause the problem. Even though you have replaced the brake booster twice already, I think it may need replacing again. The real problem is, why are you getting moisture into the booster?

While it is possible to get moisture into the brake booster from inside the car, especially if you have a lot of humidity inside the vehicle on a winter day, it is uncommon for this to happen. The only other source of moisture is through the vacuum hose from the engine to the brake booster.

A faulty Positive Crankcase Ventilation (PCV) system can cause moisture to build up in the engine crankcase. Cold winter temperatures, short trips and idling the engine accelerate this problem. When there is moisture in the crankcase, it will become water vapour and can condense into liquid in colder hoses and fittings such as the brake booster vacuum supply, where it enters the brake booster.

Changing oil more often and changing the PCV valve and possibly hose will help reduce water in the crankcase. You should also check the routing on the vacuum hose to the brake booster and position it so it slopes downward towards the engine. This will allow moisture to drain back into the engine instead of being pulled into the booster. I have used fuel filters installed in the booster vacuum hose to help reduce water getting into the booster, but the connections must be secure and it needs to still be routed so water can drain back into the engine.

As for the braking performance, this isn’t related to the booster problem. It sounds like you have air in the system and it needs to be bled again. Manually bleeding the brake hydraulic system often doesn’t remove all the air. Instead, you should have the system pressure bled, where brake fluid is forced through the system with about 25 PSI. This will get the air out and provide a firm brake pedal.

Jim Kerr is an experienced mechanic, instructor of automotive technology, freelance journalist and member of the Automobile Journalists’ Association of Canada.

kerr.jim@sasktel.net