Mishna B"M 5, 10:
"אוֹמֵר אָדָם לַחֲבֵרוֹ, נַכֵּשׁ עִמִּי וַאֲנַכֵּשׁ עִמָּךְ, עֲדֹר עִמִּי וְאֶעְדֹּר עִמָּךְ,
וְלֹא יֹאמַר לוֹ נַכֵּשׁ עִמִּי וְאֶעְדֹּר עִמָּךְ, עֲדֹר עִמִּי וַאֲנַכֵּשׁ עִמָּךְ."
"One may say to his fellow, “Weed with me and I will weed with you,” or “Hoe with and I will hoe with you.” But one may not say to him, “Weed with me and I will hoe with you,” or, “Hoe with me and I will weed with you."
Because one work (service) is definitely harder than the other it is considered a Ribis.
And so is ruled in Shu"A Yo"D 160:
"לא יעשה מלאכה לחבירו על מנת שחבירו יעשה עמו אח"כ מלאכה שהיא יותר כבדה
ואפי' לעשות עמו אותה מלאכה עצמה אסור אם הוא בזמן שהיא יותר כבדה כגון שזה מנכש"
However, from the interpreters it seems to apply ONLY when either:
- the two works/services are not of the same type
- the two works/services are of the same type, but one service is knowingly and considerably bigger or harder or more expensive.
But as the Reyshah of the Mishnah says, if the work is "more or less" the same, as in "נַכֵּשׁ עִמִּי וַאֲנַכֵּשׁ עִמָּךְ", it is not considered a Ribbis.
For example, driving for 15 or 20 min is considered the same service, while driving for 30 or 60 min is definitely not.
Also, it only applies on a single agreement - you do X and I'll do Y, but if you agree in general that you exchange cars no matter what the commutes are, sometimes shorter and sometimes farther Ribbis will not apply.