What should I expect...

...to get gas milage wise?

In the past driving normally for the entire tank:

306 fuel injection: about 21 MPG.
Changed from 3.08 to 3.73 rear end: down to about 18~19 MPG.

In the present trying to drive normally:

331 with a "close to properly tuned" Edelbrock 650cfm American Thunder carb, still with a 3.73 rear end. By "close to properly tuned", I mean the idle mixture screws are adjusted properly, there is no off idle stumble, no major hesitation during cruise or power modes, and since I'm testing MPG, there is no WOT.

I know what I'm getting... I'm just curious what you all would EXPECT to get (or are used to getting).



<font size=-1>[ This Message was edited by: 89Trooper on 2/17/06 9:09am ]</font>

I think you should still be at or near your 18-19mpg range. I'm a bit surprised that your mileage went down when swapping from the 3.08's to the 3.73 gears. My experience is that most 5.0's will stay the same or actually improve their mileage with a gear swap.

It did stay the same until I changed the speedo gear in the transmission to the proper 3.73 gear! The odometer was spinning too fast giving me more miles than I actually was driving (thus a higher MPG calculation). I would think that it has to drop... spinning the engine at higher RPMs in any given gear sucks more gas.

my guess would be somewhere around 13 to 15 at best. am i close?

I want some more estimates or real life situations without leading anybody. I'll post tomorrow morning what I'm getting.

<TABLE BORDER=0 ALIGN=CENTER WIDTH=85%><TR><TD><font size=-1>Quote:</font><HR></TD></TR><TR><TD><FONT SIZE=-1><BLOCKQUOTE>
On 2006-02-16 18:33, 89Trooper wrote:
it has to drop... spinning the engine at higher RPMs in any given gear sucks more gas.
</BLOCKQUOTE></FONT></TD></TR><TR><TD><HR></TD></TR></TABLE>This is not necessarily true.If you are lugging the engine it may get worse mileage than at a slightly higher rpm where it can cruise easily...I installed a 3 speed overdrive in my old pickup that only had 3:00 rear end gears.When cruising on the highway it would actually get worse mpg in overdrive than in 3rd because the engine had to work harder to pull the tall gearing.That is why trucks with the overdrive usually had 3.55 gears.......Some specs on the 331 would be helpful in guessing the mileage.
[addsig]

I have a 300 rwhp 302 in my 67 mustang with a T5 (.68OD) and 3.80 gears. The carb is a properly tuned 650 DP on an Edelbrock RPM manifold. I get 19-20 mpg on the freeway crusing 65-75 mph. I would expect you to get about the same.

<font size=-1>[ This Message was edited by: ujt389 on 2/17/06 11:43am ]</font>

Going to a lower gear will decrease gas mileage on the highway and increase in town mileage.

I would estimate that you're getting about 15mpg because a carbed motor isn't as fuel efficient as EFI. I HAVE to be close on that......

16-18 is what I would say. Since an engine is more efficient at a bit higher speeds, (mostly because of valve overlap/extended duration cams) ifyou lug it down where you're losing fuel out the exhaust pipe/not producing much power you will lose mileage, as frdnut said.

You want your cruise speed to be right in the meat of the torque curve - as it starts to rise. Your engine will be making more power with less gas, and you wont be lugging the engine while trying to maintain speed up grades and against wind.

I'd say that a cruising RPM of 2100 is the bare minimum that you want to be turning with that combo. preferably somewhere around 2500.

Get your carb dialed in (at the track or at the dyno) for peak power, then lean it out a little and cut back a little timing for better street mileage (or if you can lean it out and maintain the advance on the timing without detonation, go for it) You can keep tweaking the carb and timing for optimal mileage. Just keep track of what you changed and what kind of mileage you're getting in similar situations. I bet you can get 18 if you keep messing with it.

Well, unfortunately, my drive to work is alot of city driving and there are alot of lights (mostly red, it seems). Last time I figured MPG, it was 13. This time it looks like I might get 14, maybe 15. I only drive the car to work one time a week, if that, and the weekly car show I go to is only 3 miles away. So it can take up to 3 weeks to get a MPG figure.