Question: I have a 1986 Ford Ranger pickup truck with a five-speed manual transmission. I am thinking about replacing the original P205-70-R-14 tires with larger "all terrain" 27x8.5-R14 tires. There seems to be plenty of space in the wheel wells to accommodate the larger tires. Other than slightly affecting the speedometer, could the larger tire diameter have any other adverse effects on the truck?--L.H.
Answer: You ask a very good question, because a lot of small-truck owners are souping up the look of their vehicles with massively oversized tires, and they are ignoring one potential danger.
When you increase the diameter of tire beyond what it was originally designed to have, you are degrading the effectiveness of the brakes. That's because the larger tire rotates at a lower speed, but has a greater amount of torque than the brakes are designed for.
If the size difference is only minor, it should not make a great deal of difference. But if you are increasing the size substantially, you can expect a potentially critical reduction in your braking capability.
The only way to know for sure how much of a reduction you will create is to measure the circumference of the original-equipment tire and of the new tire you are going to buy. Then calculate the percentage increase and figure that you will be reducing the effectiveness of your brakes by at least that much.
As an afterthought, I'd like to add that vehicle manufacturers sweat to get every inch of braking improvement they can. If you add a few feet to your stopping distance, it may not seem like much, until the time you stop three feet past somebody else's rear bumper.
Q: I own a 1983 Honda Accord. When I back out in the morning, my whole car seems to vibrate. I left it at the dealer overnight, but they said it didn't do it for them. It's true that it doesn't always happen. Any ideas?--M.H.
A: The car may have clutch chatter or the engine mountings may be lose. Either problem can be checked out by a competent mechanic. If they are in good order, I wouldn't worry about a small amount of vibration on a cold start.
Q: I have a 1979 Pinto with an oil leak around the fuel pump. How can that be fixed? They tell me all Fords have that problem.--H.Z.
A: That should be a relatively easy repair if you are handy and want to do it yourself. The problem is likely to be a bad gasket.
First, extinguish any nearby flames, don't smoke and do the work when the engine is cool, because you may get some gasoline leakage from the hoses.
The fuel pump is held on by several bolts that you'll need to remove with a socket wrench of the proper size. You may not have to remove the fuel lines, and if you can avoid it I suggest you do. But if the metal fuel line to the carburetor is rigid, you may want to remove it, because it may break and then you'll have a second thing to fix.
Once the bolts are off, gently pull the pump away from the engine block. You'll have to scrape the old gasket off the engine block and off the mating surface on the fuel pump. Be careful not to nick the metal surface, or you could create a place where oil will leak.
Apply a gasket cement to both surfaces and a new gasket to the engine block. Be sure to align the holes for the bolts properly, so the gasket lies flat when you reattach the pump. Now you're ready to slide the pump back onto the engine. Tighten the bolts with the socket wrench, but don't use all your strength or you may strip the threads.
Ralph Vartabedian cannot answer mail personally but will respond in this column to automotive questions of general interest. Do not telephone. Write to Your Wheels, You section, The Times, Times Mirror Square, Los Angeles 90053.