Octane reduction for 1998 & 1999 ?
 

According to the US government mpg site, 1998 & 1999 318tis take regular, not premium gas, while getting about the same mpg. Does anyone know why there is a difference for those model years? For the owners of 98s & 99s, what do the owners manuals/gas fill stickers recommend? Do you have any issues running regular on a stock engine?

Thanks!
Michael

As far as I know our cars take 89 octane.
When I had my M44 I found mileage went down with 93.

My 98 says to use 93 octane but I always used 89 and got excellent mpg.

My 95 will run great on 87, makes power and gets great mileage till the outside air temps get above 70. Then I need 89, in 100 + degree weather its happiest with 92.

YMMV

I wonder if they changed that during the year. I had an early build 98 and am pretty sure it was 89. But who knows? That was a long, long time ago.

From owners manual my 99 says 89 octane for US spec car. It also says that on the gas cap, if premium was required it would say it on the instrument cluster.

My 1998 owner's manual says minimum of 90 Anti-Knock Index (AKI) and I think USA using the Motor Octane Number (MON). The equivalent for 90 AKI to MON is 85, meaning it can use the regular unleaded (87).

...Unless you are using some sort of aftermarket chip that requires higher octane.

Says 89 in my manual. I ran 87 on my stock motor with no issues. If the gas is so bad it causes "ping" the knock detectors will pick up that noise and the DME will retard timing until it runs ping free. You most likely won't ever hear a ping or know that your car is retarding timing.