85th percentile

Row\Col	A	B	C	D	E	F	G	H	I	J	K	L	M	N	O	P	Q	R	S	T	U	V	W	X	Y
2																							Pctile	85%	X2: Input
3																							Speed	46.4	X3: =PERCENTILE($A$5:$A$40, PERCENTRANK($X$5:$X$40, X2, 3))
4	MPH	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20			Helper
5	60																					0		100%	X5: =SUM(V5:V$40) / $V$42
6	59																					0		100%
7	58																					0		100%
8	57																					0		100%
9	56																					0		100%
10	55	X																				1		100%
11	54																					0		99%
12	53																					0		99%
13	52																					0		99%
14	51	X																				1		99%
15	50	X	X																			2		98%
16	49	X	X	X	X																	4		96%
17	48	X	X	X																		3		92%
18	47	X	X	X	X	X	X	X														7		89%
19	46	X	X	X	X	X	X	X	X	X	X	X	X	X								13		82%
20	45	X	X	X	X	X	X	X	X	X	X	X										11		69%
21	44	X	X	X	X	X	X	X	X	X	X	X	X	X	X	X	X	X				17		58%
22	43	X	X	X	X	X	X	X	X	X	X	X										11		41%
23	42	X	X	X	X	X	X	X														7		30%
24	41	X	X	X	X																	4		23%
25	40	X	X	X																		3		19%
26	39	X	X	X																		3		16%
27	38	X	X	X	X																	4		13%
28	37	X	X	X																		3		9%
29	36	X	x																			2		6%
30	35	X	X	X																		3		4%
31	34	X																				1		1%
32	33																					0		0%
33	32																					0		0%
34	31																					0		0%
35	30																					0		0%
36	29																					0		0%
37	28																					0		0%
38	27																					0		0%
39	26																					0		0%
40	25																					0		0%
41
42																					TOTAL	100

Is this the "raw" data, or did your survey output a "list" of car speeds: (34, 35, 35, 35, 36, 36,...)? If your original data are in a list, then it should be as easy as using one of the PERCENTILE() functions (PERCENTILE.EXC(), PERCENTILE.INC(), PERCENTILE() depending on your specific needs) on the original list.

If the histogram in the spreadsheet is the raw data, then I would suggest extracting the data into such a list and then using the appropriate percentile function. As tedious as it is, the easiest and fastest way might be to hand enter the list from the chart. Then, for future surveys, note that it is easier to process the data if the date are recorded in a list.

if I used the formula that shg suggested(which looks great btw) then it would round down to 46.

No; 46.4 is the interpolated result between 46 and 47.

If you wanted to return 47, you could use (referenced to my example),

=INDEX(A5:A40, MATCH(X2, X5:X40, -1))

You're welcome.