Is it more cost effecient to put medium grade fuel or regular in an SUV?

I have a 4door Jeep Wrangler Unlimited Sahara and have recently been putting medium grade fuel in it because I heard it is better for the car and lasts longer (which is a lease so I don't even own it). Is this true or is regular fuel fine?

Is it more cost effecient to put medium grade fuel or regular in an SUV?
Use 87 octane regular





The octane rating of gasoline tells you how much the fuel can be compressed before it spontaneously ignites. When gas ignites by compression rather than because of the spark from the spark plug, it causes knocking in the engine. Knocking can damage an engine, so it is not something you want to have happening. Lower-octane gas (like "regular" 87-octane gasoline) can handle the least amount of compression before igniting.





The compression ratio of your engine determines the octane rating of the gas you must use in the car. One way to increase the horsepower of an engine of a given displacement is to increase its compression ratio. So a "high-performance engine" has a higher compression ratio and requires higher-octane fuel. The advantage of a high compression ratio is that it gives your engine a higher horsepower rating for a given engine weight -- that is what makes the engine "high performance." The disadvantage is that the gasoline for your engine costs more.





You most likely do not have a high compression engine so you do not require high octane premium fuel. All you are doing is giving the big oil companies even more money. I think that they already make enough money now.





See link below for mor information.
Reply:All engine produced by manufacturers will state in their warranty the "acceptable" grade of fuel. Most times it's 86 octane minimum (which as far as I know is the lowest grade you can get these days). Depending on driving habits and conditions, you might see an increase in economy using a higher grade of fuel. It depends on the computer controlling the engine. If a computer detects preignition (spark knock), it retards timing to keep the engine from breaking parts. The result of that is lower fuel economy. Higher octane fuel is less prone to pre-ignite, which would cause the engine to not retard timing AS MUCH. The only way to know for sure in the real world is test it.





Run the tank to almost empty. Fill with the highest octane you can find. Mark you mileage. Drive until almost empty again. Mark your mileage. Fill with lowest grade of fuel. Drive to almost empty again. Compare results. Not perfect, but if you only see a 1 or 2 mpg increase with higher octane, it's probably most cost effective to use the lower grade fuel.