Quick question, is there any way for you to measure the gasoline, fuel, or oil content in your tank? rather than using a rod to measure it?

In addition to my post found this online "Fuel Level Sensor" will this help too? 

If you are trying to get an idea of how much fuel you have left and don't have a fuel gauge, the typical way to do it is to fill your tank, reset your trip meter, and then run until you have to switch to reserve.

Then, you fuel gauge becomes your trip meter after each fill-up.

If you want to know the actual volume of fuel in the tank, a google search and/or filling it up and then emptying it into a measured container will do.

As far as oil level goes . . . why do you want to know the oil level?  Is this a two-stroke that mixes as it goes?

Can you give us some more info about that?

Hi, Ryan Thanks for your reply. It's actually my brother's small tank truck. It's delivering fuel, gasoline, oil to the different gasoline station and he just wants to know how to measure it.I know my question is general or broad but Thank you for the suggestion though. I really appreciate it. 

I've seen jerrycans that have a float on the inside with a piece of metal and a magnet on the outside that shows the level of the float. Not sure that would work well for a vehicle fuel tank, but the float idea seems like the start of a good method.

Hi Ben, About the float? I'm not really sure how it works. Do you have an idea how it works or how it can measure the fuel, oil, gasoline inside?
I googled it but really not getting the point, sorry. Can you enlighten me?.
I'd appreciate.

I was thinking that you could use a float to gauge the height of the liquid in the tank, and you could use that height to estimate the volume of liquid remaining. It probably wouldn't be any more accurate than using a rod. The specific implementation would probably depend on the tank you were trying to measure.

I was in the business of petro transport for many years and am intimately familiar with tanks and trucks. The tanks contents are typically measured by Isengard a measuring stick and a chart relative to that tank. Simple as that.

Don’t know that word that ended up in the middle of my reply.
It should say
USING a stick. A yard stick would do in a smaller tank. The sticks we used were probably ten feet long. 

Not sure if this information will be helpful to your situation, but here are a few commercial options I found that looks like they would apply to small tanks.

http://www.flowtechinstruments.in/float-and-board-type-...
This float and board type seems to have a float that can move up and down within the tank on a guide line. The float is connected by a string to an indicator on the board. As the level in the tank goes down, the indicator is pulled up toward the top of the board. You then just need to mark the board with the different volumes.

http://us.magnetrol.com/Technologies_Individual.aspx?id...
This magnetic style has a float that sits inside the tank and triggers the gauge outside the tank.

https://www.amazon.co.uk/Indicator-Liquid-cylinder-dete...
This kind relies on a temperature differential, since the portion with liquid will cool faster than the portion with air. Depending on the size of the tank, you might need to use multiple strips. 

https://www.gemssensors.com/level/single-point-level-sw...
In this style, you have a float that when raised actuates an electrical switch. the float would be installed at a fixed point so that the switch would indicate if the level is above or below a given threshold.

http://www.fluidswitch.com/multi-level-switches.php
You can mount multiple floats/switches on the same rod to monitor multiple thresholds.