I bought a 1998 mustang convertible which had no owners manual. It has a 3.8 v6. What grade of gas do I use?

regular 87 octane

87

FOR THE SMALL PRICE DIFFERENCE, 5-10 A GALLON. GO FOR THE HIGHER OCTANE GAS. YOUR ENGINE WILL RUN BETTER, LONGER. AND WILL ACTUALLY COST YOU LESS $$ IN THE LONG RUN.

Use 87, on a low compression engine like the 3.8, the high octane go go juice will do nothing except waste your money.

that engine has 9:1 comp ratio, so its pretty borderline between high and low ratio, a good rule of thumb is 10 octane points for every 1.00 ratio, so 9:1 comp would run BEST on 91

A v6 use 87. The 91 isn't going to do anything for that small engine. You could put it in every now and then to clean out the system alittle. It's a waste of your money. I have a2001 bullit gt and I use 87 it runs great

i use premium on all my cars economy and muscle because im not just paying for higher octane fuel, im paying for higher quality fuel with much less risk of running into a bad batch of regular grade....a gas station down here got sued over 7 cars getting fuel and breaking down just miles down the street, i just dont buy cheap fuel