I think visibility would be good except for a couple angles where the sun bounces off the back window. You could wrap it in clear plastic and put it on the back of the car too.
Anyway, not that I needed a distraction, but I couldn't help digging into this a bit more
I've always thought it would be kind of cool, and definately get peoples attention to see 999MPG popping up on the back of a car while someone is EOCing.
a possible layout:
notes for this layout:
22 controllable segments (MPG is one segment, rest is 4 LED groups)
38 x4 LEDs = 142 LEDs
3 numbers, each enabled by a transistor, scan style
7 segments per number, 7 transistors
transistors for MPG
mouser parts list:
quantity, part
142, 604-WP7113SRD/D 100: $0.06 1.85v 20ma RED LED
30, 610-2N3906 25: $0.08 transistor //not accurate, probablye need a few NPNs too.
1, 556-ATMEGA168-20PU 1: $4.11 atmega168
1, 559-FOX160-20-LF 1: $0.47 crystal
1, 511-L7808CP 1: $0.41 8 volt regulator, 1 amp
4, 512-1N456A 1: $0.06 Diodes (drops 8v to 5v for CPU)
1, 647-UVR1E331MPD1TD 1: $0.12 330UF 25V capacitor
total $16.27 for quantity 1, not including shipping. But a fair bit of soldering and getting a bootloader and program on the chip.
for comparison, the displays a few posts previous go for $400+.
Also note that I've seen regulations about what color lights can be where on a vehicle, so RED is probably the most defendable color for such a display, it is just a fancy taillight then.