I was able to get better MPGs with a hi-compression 360 than I could ever get out of a 9.5Scr 225 with an automatic; go figure. OK I'll admit it,I had a double overdrive,lol.
One of the problems with a 225 is you still have to get up to cruising speed and that takes a lot of throttle in an F-body(what I had). Another thing is trying to give her enough ignition timing in overdrive.
If she's got 2.45s in the back, and a trans with 3.09-1.67-1.00-.73od, then consider how you're gonna get up to cruising speed.You're probably gonna run it up to 2800 or so and then shift into second, right? Ok how fast are you going and what rpm will you be doing? Well with 26.5 inch tires,You'd be doing 29mph@2800 ..Whoa! So now you shift into second, and accelerate from 1505 to 2800, which would get you 62mph,and you shift straight into overdrive and creep up to 1474rpm@65. Notice Direct was never used.
Now to get to 65 in a reasonable period of time running at these pretty LOW rpms, is gonna take a considerable amount of throttle, in a part of the powerband where the engine is not very efficient, and hasn't got a lot of power in the first place. I mean check it out; in the getting to 65mph, the engine will run from say; 1200 to 2800, then from 1505 to 2800, then a drop to 1474.So that is gonna take a very specialized combination of parts to make any kind of power.
Next lets talk about cruising at 1474rpm. To cruise at 65 takes something like 45/50hp. How much throttle is it gonna take for the 225 to make 50 hp at 1474 rpm? 50hp converts to 117 ftlbs at 1475 rpm. That's a lotta ftlbs for this little engine. It's gonna take a lotta throttle. She'll be into the mains for sure. So your gonna have your hands full on the tune, to satisfy the lean running on cruise, yet make some power when it's needed.
But that's not the end of it;
Let's talk about ignition timing. How are you gonna build a curve to satisfy BOTH the power-timing and the cruise timing? You cannot.
And if you can't give the engine the timing it wants, then the engine is gonna want more throttle opening than necessary to achieve a certain loaded rpm, and also to maintain it. And that just plain wastes fuel.
So the conclusion is; don't try this with 2.45s like I did,..............lol.
And we haven't even discussed city driving......
Go back to the top of the page; see where it says 29mph at 2800 in first. Well on the shift into Second, the Rs will drop to 1505. That's not gonna work. Firstly, you're gonna be slipping the clutch out to over 10/12 mph, then more or less flooring it to get up into traffic, and then creeping along at 29mph =1505rpm, in second gear. I wonder if the slanty can even maintain that speed in the low-speed circuit, so again, she will be on the mains.
Conclusion; don't try this with 2.45s.
Ok so what gear should you use? I'm gonna guess 2.94s or so.These will get you some acceleration capability, yet cruise at 65=1770. Now you can cruise 30mph in second at 1868, and that will work. And now you can begin to build a timing curve to satisfy both the power timing and the cruise timing, cuz now the V-can is gonna work, cuz the throttle is gonna be closed far enough to be in it's working range.And then you can lean the carb out.
Conclusion: IMO 2.94s are the minimum rear gear to use.And that would be with an engine of close to 9.5Scr, and a proper cam for this application. Less compression is gonna require more throttle opening, that's just the way it is.
I tried this with a SuperSix intake and 2bbl carb. I chose that carb cuz of it's tiny venturies and multiple signal boosters. At 9.5Scr I could not get any detonation with "normal" timing curves. Getting sufficient cruise-timing was impossible, so I just made the best of it. I used the 2.45s, and an A999 with ratios of 2.74-1.54-1.00 and a loc-up TC.
It did pretty good on the hiway, but was a slug on take-off,lol.And it was a DEAD car around town. I will never use 2.45s behind a slanty again.
I also had some pretty tall tires on her, over 28", but I don't recall exactly;maybe 255/75-15s. 65mph was around 1900.
This car never made the numbers I was hoping for. Yet I drove it for over 21 years, and it's parked outside right now, ready to go.
I did way better with a Hi-compression,small-cam, 360, geared for 65=1590 And no it wasn't cheap. It was,however, the funnest iteration of the three I have tried. In case someone is wondering; the trans to make that 1590@65 was the same one OP has but with a GVOD behind it, and 3.55s. That was 3.55x.71x.78= a final drive of 1.966. She easily pulled that gear down to 55mph@1350rpm.
