How fast is your car?
How fast is your friends car on the other side of the country?

The NHRA uses a correction factor to compensate for altitude for stock/super stock classes.
Many automotive tests also use this factor when giving 1/4 miles stats. This way you could run a car at sea level and have an identical car running at 6000 feet. The car running at 6000 feet will be considerably slower but by using the conversion factor you would see they run the same. Just because someone says they have a car that run's low 13's doesn't mean much against yours unless they are on an equal playing field.
If you really want an accurate conversion, then do your calculations with density altitude. A car running on a high density altitude at sea level will be slower than than same car running on a low density altitude at 3000 feet elevation.

Race City has an altitude of around 3500 feet.
NHRA Altitude corrected index in Calgary is S/ST = 11.27, S/G = 10.24, S/C = 9.20

Comp, Super Gas, Super Comp and Super Street conversion

Convert to Sea Level
ET- .9669
MPH - 1.0315
Convert to Altitude
ET- 1.0342
MPH - .9694

Stock, SuperStock conversion

Convert to Sea Level
ET - .9575
MPH - 1.0451

Convert to Altitude
ET - 1.0444
MPH - .9568

To convert your Quarter Mile ET and MPH to sea level times, multiply applicable factor times your ET or MPH.
Altitude above Sea Level (in feet) Elapsed Time Factor MPH Factor
1200 .9874 1.0129
1300 .9861 1.0143
1400 .9848 1.0157
1500 .9835 1.0171
1600 .9822 1.0185
1700 .9809 1.0199
1800 .9796 1.0213
1900 .9783 1.0227
2000 .9770 1.0241
2100 .9757 1.0255
2200 .9744 1.0269
2300 .9731 1.0283
2400 .9718 1.0297
2500 .9705 1.0311
2600 .9692 1.0325
2700 .9679 1.0339
2800 .9666 1.0353
2900 .9653 1.0367
3000 .9640 1.0381
3100 .9627 1.0395
3200 .9614 1.0409
3300 .9601 1.0423
3400 .9588 1.0437
3500 .9575 1.0451
3600 .9562 1.0465
3700 .9549 1.0479
3800 .9536 1.0493
3900 .9523 1.0507
4000 .9510 1.0521
4100 .9497 1.0535
4200 .9484 1.0549
4300 .9471 1.0563
4400 .9458 1.0577
4500 .9445 1.0591
4600 .9432 1.0605
4700 .9419 1.0619
4800 .9406 1.0633
4900 .9393 1.0647
5000 .9380 1.0661
5100 .9367 1.0675
5200 .9354 1.0689
5300 .9341 1.0703
5400 .9328 1.0717
5500 .9315 1.0731
5600 .9302 1.0745
5700 .9289 1.0759
5800 .9276 1.0773
5900 .9263 1.0787
6000 .9250 1.0801