Tire-buying advice for '92 Mercury Cougar

[kalrith]

Well, it's time to buy new tires for my '92 Mercury Cougar. The stock size for the car is 205/70-15. When I bought the car, it had 205/65-15 on there, and that's what I put on there the last time. Unless the original owners had the speedo calibrated, it's off by 3.05%, or reads 61.8 mph when actually going 60 mph.

So, my first question is how to tell if the speedo is off. I don't own a gps, so that's not an option. If the speedo was recalibrated, then I'll definitely stick with the same tire size.

My second question is what tire size I should buy. Should I go back to the original 205/70-15 even though they're higher profile? Another option might be 215/65-15, which is only 1.11% off, and this size is specifically mentioned on the placard on the doorframe. I'm just not sure if my rims are wide enough for this size.

Thanks!

 

[radioouman]

I wouldn't worry about the speedo being off slightly. At 100 mph you are off 3.05 mph at most. You can get that kind of error just by the width of your speedometer needle.

 

[kalrith]

It's digital, so the width of the needle wouldn't come into play. I'm actually more concerned with the impact that the decrease in diameter (25.49" with the 205/65-15 compared to 26.29" stock) would do to the suspension, gearing, etc.

Edit: changed circumference to diameter as that seemed more appropriate.

 

[sdifox]

go on tirerack and it should be able to pull the stock tire size for ya.

 

[radioouman]

Originally posted by: kalrith
It's digital, so the width of the needle wouldn't come into play. I'm actually more concerned with the impact that the decrease in diameter (25.49" with the 205/65-15 compared to 26.29" stock) would do to the suspension, gearing, etc.

Edit: changed circumference to diameter as that seemed more appropriate.

You'll never notice the difference in suspension. The new tires will be stickier than the old ones and it is possible that you'll find a tire with a stiffer or softer sidewall, but the difference in diameter will have virtually no impact on suspension.

Gearing is gearing. You can change the gear ratio in the rear differential to offset huge changes in tire diameter, but no one would do that for a speedometer that is off by 3%.

 

[desy]

Actually I used my handheld GPS to tell me how fast I'm going
Used it on 4 vehicles, one speedo dead on, one out 1 mph, one 5 mph, one 6 mph

 

[kalrith]

Originally posted by: sdifox
go on tirerack and it should be able to pull the stock tire size for ya.

As mentioned in the OP, the stock tire size is definitely 205/70-15. I'm just questioning whether I should buy that size, the 205/65-15 that's on there now, or 215/65-15 that would be close to the stock diameter but might be too wide for my rims (I don't know how to tell this).

Originally posted by: desy
Actually I used my handheld GPS to tell me how fast I'm going
Used it on 4 vehicles, one speedo dead on, one out 1 mph, one 5 mph, one 6 mph

This was my first thought, but unfortunately I don't have a GPS. Maybe my father-in-law can loan me his.

 

[Vette73]

problem is tires are not exactly their stated size. Take 2 tires from different compnaies and you can see many are slightly different.

So even if you run the factory size it may still be off a little. That and as tires wear down they get smaller so that is another thing. and on and on...