Overclocking SD cards on the Raspberry Pi

Warning: Overclocking SD cards can corrupt your data and possibly damage your SD card!

If you want better SD card speed and have an UHS-1/3-capable SD card, you can make the Pi run it with more than the standard of 50 Mhz. I tried this with a Rasperry Pi 0 (the non-W variant) and a Verbatim 8GB SD-HC Class 10 micro SD card (Model #44081) and settled at 80 MHz to be safe.

Some results using Jeff Gerlings benchmark suite:

SD clock in MHz	Read speed in MB/s
50 (default)	21.62
80	26.51
100	34.64

SD clock in MHz	Copy speed in MB/s
50 (default)	14.4
80	18.9
100	17.2

SD clock in MHz	write	rewrite		read	reread	random read	random write
50 (default)	1173	1257		5198	5085	4565	666
80	1132	1262		5764	5765	4969	699
100	1203	1293		5907	5756	5113	688

Enable SD card overclocking by editing /boot/cofig.txt and add:

# Overclock SD card. Check with "sudo cat /sys/kernel/debug/mmc0/ios"
dtparam=sd_overclock=80

Make sure your overclock value is a fraction of your Pi’s core frequency (e.g. Rasperry Pi 0 core clock = 400MHz -> 400 / 5 = 80 Mhz). You can find the default core frequencies (“core_freq”) here.
You can print the current SD card hardware state with “sudo cat /sys/kernel/debug/mmc0/ios”:

clock:        50000000 Hz
actual clock:    100000000 Hz
vdd:        21 (3.3 ~ 3.4 V)
bus mode:    2 (push-pull)
chip select:    0 (don't care)
power mode:    2 (on)
bus width:    2 (4 bits)
timing spec:    2 (sd high-speed)
signal voltage:    0 (3.30 V)
driver type:    0 (driver type B)