It takes 8 gl of water per vehicle, your calculation of 25-30 cars with a 90 gl tank is WAY off. 90 gl tank would give you 11.25 washes. Keep in mind that a gl of water weighs 8 lbs so 90 gl tank x 8=720 lbs, 150 gl tank x 8= 1200 lbs. Something to think about when figuring your total towing weight.
[/COLOR]I can make 3 grand per month or a bit less with school and summer way more and the loan is from my mom not a bank[/QUOTE].
Don't expect to make this starting out. You would need a lot of complete details a month to make this. You want to insure your business in case you damage a customers vehicle. Look at garage keepers insurance.
Good luck.