GINCSC

Good Morning Guys,
What is the correct factory Tire Size for a 71 convert?
The car I just bought has 245/60/14 on all 4.
I thought they should be 70 series am I correct?
what would the difference in handling and mpg be between 60 and 70?

Thanks and all have a great day.
Gary

jaunty75

The '71 Cutlass Owner's Manual provides an answer to this question:





What engine does your car have? Does it have air conditioning?


Assuming it has a 350 and does not have A/C since it's a convertible, you're in the top row, so the standard tire is F78 x 14. The closest to that today is P205/75R14. According to the chart above, G70 x 14 was optional, and that corresponds to P225/70R14.

These come from this chart:

http://www.tirerack.com/tires/tirete...sionchart.html

According to this chart, the tires on your car now, 245/60/14, are the correct size for a 60-series tire.


If your car does have A/C and/or a 455, then you need the equivalent of G78 x 14, which is P215/75R14

GINCSC

GREAT Information! Thanks

The car has a 350 rocket with AC.
Do you Think it would handle or get better MPG if I changed it to the 70 series tires.
I'm still going through the car to work out all the bugs. The tires have great tread but are 10 years old so I'm looking to replace them as well as a big list of things (don't tell my wife. LOL)

Runs very smooth and sounds strong but will not rip the rubber off the tire. (YET)

joe_padavano

The chart on Tire Rack is not a list of "correct" sizes, only the closest available. Diameter is what really matters.

205/75-14 - 26.1"
215/75-14 - 26.7"
225/70-14 - 26.4"

245/60-14 - 25.5"

The bottom line is that the 245/60-14s, while the largest 60-series 14" tire currently available, are somewhat smaller in diameter than the recommended factory sizes. This is a matter of personal preference, but I've always felt that they leave too much daylight between the tire and the wheel opening.

jaunty75

If you go with the optional G70 x 14 equivalent (225/70/14) over the standard G78 x 14 equivalent (215/75/14), you will have a tire with a slightly smaller sidewall height (0.15"), slightly larger width (about 0.5"), slightly smaller diameter (0.3"), and a smaller circumference (about 0.9"), giving you more revolutions per mile (764 vs 755). Thus, your speedometer will read about 1/2 mph too high at 65 mph. In other words, when the speedometer says 65, you're really going about 64.5, a negligible difference, in my opinion.

As far as MPG difference, I can't believe it would be noticeable, but with the greater width of the 70 tire, that's more rubber in contact with the road, creating more friction and perhaps lowering mileage slightly. But that's just my guess.

You may find that the tire you choose will be based more on what's available than what you want. 14" tires have fallen out of favor in recent years, and manufacturers are making them in fewer sizes, especially if you want a white wall.

When I needed new tires for my '67 Delta 88, I wanted 215/75/14 for them, but the tires shop didn't have that size. I put on 215/70/14 instead, and they've been fine. I put only about 500 to 1000 miles per year on the car, so mpg is not a concern.

As far as handling, using my same argument, with greater road contact area with the 70 tire, perhaps you would see slightly better handling if you were ever able to compare them side by side.

hookem horns

225/70/14 is about as close as you can get to stock in a modern radial for 14" wheels and still not too hard to find. Some folks like the shorter/wider 60 series you have now. Don't worry about mileage differences with either, but maybe compare your speedo to GPS to see how accurate it reads. I'll bet new tires of either size will ride/handle better than your current 10+ year old rubber.

GINCSC

So would you guys go with a 70 series or 60 series. Was was looking at these

http://www.treaddepot.com/tire/90000...-24BoCplfw_wcB

Good price for a 2 or 3 times a week driver

joe_padavano

I'd go with the 225/70 if you don't want to upgrade to a 15" wheel. The difference in section width from 225/70 to 245/60 is about 3/4".

GINCSC

Thanks so much for the help.

Have a great day

Gary