My stock 87 GT gets about 25 city/highway as long as I don't flog it, while a week of hard driving and short trips while I was on vacation netted about 14mpg. Last year, I took a 300 mile trip at a fairly steady 60 mph with a 2.73 gear, 6* advance & no tuneups in years - it averaged 29.5 on the way up on a hot day, and 31 on the way back at night on fairly tall tires. Thats not bad, considering that my son's empty 4 cyl Nissan pickup only got 22 while following me up, and 16 on the way back carrying 500# in the bed. Just took the same trip again with fresh plugs/cap/wires, 10* advance, freshly serviced 19# injectors, a K&N and Flowmasters - it got 27 up and down. On my next long cross country trip I'll try backing the timing down a couple degrees and see if the mileage improves. I haven't had any issues with running 87 Octane fuel, even at 10*, and I just borescoped the cylinders - there's no sign of detonation or carbon buildup. I think jet coat/header wrap, a quality 175* t-stat, and a good distilled water + WaterWetter mix really aid in scavenging excess heat away from the combustion chamber and let you run a little more timing before pinking sets in under load.

 

what do you mean 10* timing? over stock or 10* before TDC? stock is 10* advanced so of course you wont have detonation with 87 sence thats how it was made to run from the factory

either way, i wish i had your fuel economy

 

i would advance it to 12*

 

I can't believe your gas milage was that good with the timing retarded so badley! Run about 12-14* timing and your gas milage will be even better. You car must have been a dog with the timing set back like that.

 

damn, my mustang gets like 12 mpg if I'm lucky. Even my 95 integra barely gets 27 mpg on the freeway. I don't drive hard either most of the time.

 

mine gets 19 combo driving

 

Mine averages 25-27 with 2.73's as well.

 

I guess I really can't expect really great MPG with 4.10's and with engine work, but even stock with 2.73's I think my very best was like 17 MPG.

 

i wish i had his too i only get like 15 in city with 373 i can only imagine what its going to be once i get my cam in......oh yeah my timing is at 15 is that too high?

 

thats witha 331