is there a big difference in gas mileage between the 305 tpi and the 350 tpi. keep in mind that both are stock, from fore to aft.

 

I would think it has mostly to do with gearing, 305's are geared higher than 350's so there is going to be a loss due to that fact. I got 25.5mpg with my 305 TPI with 2.73 rear on the highway and my best with a 350 TPI with 3.27 rear is 20mpg. Im sure if they both had the same gearing the difference would be minimal.

 

thanks for that info, bonz

 

I got 28-29mpg with my 305TPI on a trip to the Michigan dunes and back. I didn't have to touch the brakes once the entire time until I got near my house...it was a real nice drive at a constant 70mph. Slowest I had coasted down to throughout the drive was about 50mph in a construction zone. This was with SLP 1 3/4" headers, 2.73, 26" tires, MSD plug wires, somewhat fresh tuneup.

With my head/cam 355 "LT1" I got 19mpg on a 300-mile drive in a horrible tune with 3.23 gears, bad alignment, no highway fuel/spark modes, and no torque converter clutch lockup. I'd assume with all of that fixed and a good tune, it should also get mid-upper 20s for highway mileage.

I don't think there's a big difference in terms of fuel mileage. The 350 might be 15% bigger in displacement, but it also makes more torque so it doesn't have to work quite as hard as a 305 to keep the car moving.