Yes but you need to keep the same rolling circumference.
You can use a website like this
https://tiresize.com/tyre-size-calculator/
on the comparison side of there app, enter your current tire size, then you can play with the second tire size to keep get as close as possible to the same overall size of the wheel. as an example
you have a 195/65/15 tire currently then a 185/70/14 would be about as close as you could possibly get to the same circumference. a -0.4% difference.
And if you had a super accurate set of dials in the car that read exactly 10mph at 10mph 20 mph at 20mph and so on.
When the dash reads 70mph, you would actually be doing 69.7mph you can play with that tool and see how it works.
One consideration is that many newer cars cannot go smaller than 14/15 inch wheels because the brakes and calipers are much larger than in the past and the wheels won't fit over the brakes. Diesel cars tend to have bigger brakes, so before down sizing look at the gap between the brake caliper and the inside of the wheel, to make sure there is a big enough gap.
Also Steel wheels are thinner than alloy wheels so if down sizing from steel wheels to alloy, the alloy wheels might not fit where as 14 inch steel wheels would.