Right now I'm thinking about a 347 425hp crate engine from Prestige Motorsports. It would be backed up with a T-5 and a 9 inch 3:55 rear end. Now I'm wondering what this beast would be like to live with. The car is going to be used as basically a weekend cruiser with an occasional trip to the track. I'm wondering what it would be like on the occasional short trip up or down the interstate to this or that event. Premium fuel is a given but does anyone have actual experience with mileage numbers cruising in 5th gear at 60 or 65 mph? Could I expect 12-15 miles per gallon? More? Less?
If you want a 4oo+ horse engine...Toss the idea of fuel economy out the window. Especially in stop and go traffic. Highway may be better with the right gear combo.A safe guess...12 in the city...15 to 17 on the highway. You may get 20/23 on the hwy with the right fuel metering. Depending on cam profile and compression ratio, the engine is going to be choppy idled and ill tempered in traffic. Make certain you have a very good cooling system as well. It will certainly be a head turner while your sitting at traffic lights. Sounds like fun!!! My Sprint with a 351 and a prety radicle cam and 10.5 to 1 copression ratio is just on the edge of being liveable on the street. I do have an Aod and 3.80 gear though so that helps (A better converter would be nice too) it gets 10 to 12 mpg in traffic but will pull down 21+ mpg on the hwy at 65 mph with the OD. and the Quick fuel 650 carb on it right now. You have to start considering carb choice and where that carb transitions from the idle circuit to the main metering circuit rpm wise with an over drive trans like the T-5. If you only turn 1800 rpm at 60 mph a holly will still be in the idle ckt. This means rich mixture and poor fuel usage in hwy driving situations. In traffic though, you will never be in OD so its a minor issue there. Lots to consider beside this but...400+ hp in a street car is allways too much fun to have to worry about fuel consumption if you ask me.
Fuel mileage will have a lot to do with how your engine builder achieved 425 HP. The compression ratio, cam timing, cylinder head flow and induction system will all figure into it. There are many ways to get there. All 425 HP 347's aren't created equal.
I second that, roller cam, roller rockers, fuel injection, exhaust, etc will free up hp instead of throwing gobs of cam until u get the hp. Then making the gearing is right to use the power at the right time is essential.
480hp/347 t5 and 350 gears got 22mph highway and with aid got 21mph. Never checked milage city driving. Always thought that was fun driving time.
347 EFI roller, with ~425hp, t-5, 4.11 gears, on 26" tires was getting 17-18 mpg doing 70-80 mph driving 1200 miles to Vegas. Just keep the pedal mashing to a minimum. It gets better gas mileage than my daily driven Jeep Wrangler.
The thing no one ever considers is that it only requires approx 35-40Hp to move a Maverick/Comet down the highway 60-65 MPH... As long as the cam doesn't have a obscene amount of over lap and you aren't running 85-90 mph, mileage should be at least be in the low 20s...
