Tire sizes


#1

My Ford Taurus currently has P205/65R-15 tires. I need new tires and have 2 P205/75R-15 tires available. Can I safely use them?


#2

Your speedometer will be off a little bit. As long as your tires don’t rub the fender well, they should be alright.
http://www.tirerack.com will have answers, as well as tires for sale.


#3

It’s mostly a matter of clearance, but your speedometer and odometer will be off. Fords in the past had tires that were often too small for big loads, and owners mounted tires one or two sizes larger. It gave increased safety and longer tire life. Imagine a 4000 lb full size car of the sixties with puny 14" wheels and barely adequate tires. The high perfomance models had larger tires so they could handle the power. Still, I remember going through a set of 7.00X13 tires once a year on my Valiant with a 273V8 engine.


#4

According to this tire dimensions site, those tires will not be safe to use: http://www.1010tires.com/TireSizeCalculator.asp

If you input your current/stock tire size, and then put the 205/75/15 in “tire size 1” and click on compare, it will show that the new tires have a diameter difference of 5.95%, and the most (according to them) you should deviate from the original size is 3%, or you risk brake failure. Not sure on the technical part of why this is true, but perhaps going from a diameter of 25.49 inches to 27.10 could mean the tire will be too big on the wheel and maybe would slip in an emergency braking situation. But that’s just me speculating.

Of course, diameter is directly related to radius and according to carbibles.com (a great site):

"-Rolling Radius-
The important thing that you need to keep in consideration is rolling radius. This is so devastatingly important that I’ll mention it in bold again:rolling radius!. This is the distance in mm from the centre of the wheel to the edge of the tread when it’s unladen. If this changes because you’ve mismatched your new wheels and tyres, then your speedo will lose accuracy and the fuel consumption might go up. The latter reason is because the manufacturer built the engine/gearbox combo for a specific rolling radius. Mess with this and the whole thing could start to fall down around you.
It’s worth pointing out that the actual radius the manufacturers use for speedo calculation is the ‘dynamic’ or the ‘laden’ radius of the wheel at the recommended inflation pressure and ‘normal’ loading. Obviously though, this value is entirely dependent on the unladen rolling radius. "

Hope that helps.


#5

Shop around some more. Find the correct size. Don’t let them sell you anything other than the correct size. This could be a safety issue.


#6

Look on the sticker for acceptable sizes too. You may be able to use them anyway but it will make you feel better if you know for sure. Measure the circumference of your current tires. Sometimes the sizes printed on the tires are just plain wrong (not wrong, but not the same from one manufacturer to another).


#7

There is one problem not mentioned. You said you have two tyres available. That means you are going to have different size tyres on the front and back, or worse left and right. That MAY mean that one set will have more traction than the other. It may not seem like that should be a problem, but if you hit the brakes hard in an emergency situation and if the front tyres have more traction than the back, you will quickly find yourself sliding back end first.

FWD, RWD or AWD you always want the best traction on the back. In this case, it is not easy to determine which ones are going to give you the most traction or what pressure to use to try and even out the difference.

My best guess is the larger tyres will give the best traction and should be on the back, that may not be the case.

I would pass on those misfits and get four new tyres.


#8

They may rub on the on the strut on the inside of the tire when cornering. I had a Taurus that a previous owner had installed oversized tires on, but I don’t recall the size. That’s what it did. Not safe.

Also figure that the engineer who designed your Taurus’ suspension system probably knew what he was doing, whether the guy who designed the transmission did or not. Changing tire sizes messes with that design, and will effect the handling as well as the speedometer reading.