BB Gearing ?

Hello,

I am most likely going to be putting more miles on my "hot rod" than I had initially planned. That is a good thing for me I guess

It is a 74 Dart Sport with a mild 440, SD intake, 750 or Theromquad on it, Headers, 3" exhaust. The cam is a MP .450-455. Car runs well. It has the factory HS convertor in it.

Initially, I was building a mild engine, but I built the car for a better engine in the future. It does have 4.10's in it now. It will likely see a better set of heads and maybe a slightly bigger cam but that is about it.

I need to drive a minimum of 40 miles each time I start it pretty much. Obviously 4.10's aren't going to cut it at 60-65 mph on a regular basis. At least for me they aren't. I have driven it a few miles with them and it was getting old already

I am looking at a compromise gear. Something I can get mid-teens out of for MPG, and still run 13 something at the strip.

Tire size is likely going to be 255/70/15 to match my rim size.

If you were compromising, and wanted decent driveability with a relatively mild engine, and some performance, what ratio would you run? I'd rather err on the side of conservative.

I am leaning towards 3.23's.

Also, what am I likely giving up in the quarter dropping from a 4.10 to a 3.23?

Thanks, ROB

Try a 3.55sg. Not so stiff you can't highway travel with it.

I had 3.55's in my old 340 swinger. They worked fine. I could drive about 70 no problem and got 17mpg even w/ a 750 DP.
Your 440 should run well into the 13's w/ 3.55's. That cam you have would probably like less than 4.10's So you might see an improvement. I'm guessing it quits at about 5200 and you are probably over 6 at the stripe.

You know when I built it, I started with a slightly bigger cam in another engine. I never really drove it, but once I ran the numbers, I could see the same thing. Running out of RPM wouldn't be good in the quarter!

Years ago, with a 302 F..d Mustang I ran 2.79's and then 3.55's. It was night and day with a smaller engine. It did have some real short small rear tires though so it wasn't too great going down the road though. They were probably 3-4 inches shorter though.

Running out of RPM with 29" tires, 3.23 OR 3.55 and mid 13's ain't gonna happen.
Either will work. 3.23's will be more comfortable on the road and give an MPG or 2.

A 255x70x15 tire it really tall, about 29" so it will help reduce your RPM at speed.
With 3.55:1 gears, 60 MPH would be about 2,500 RPM, and 70 MPH should be under 3,000 RPM (may vary a bit because of converter slippage and actual tire size.)
going from 3.55 to 3.23 is a 9% change, so your rpms would be around 250-300 RPM less.
Using a 255x60x15 tire (about 27" tall), and 3.55:1 gears, 70 MPH is about 3,150 RPM.

I meant with the 4.10's and running out.

I agree, I don't think it would be close to an issue with the 3... gears.

I guess it comes down to 3.23's vs. 3.55's.

It may be helpful to know that with the exception of a trip or two a year, all of my driving will be done on 55 MPH posted roads. Everybody drives 63-67 but I wouldn't get run off the road at 60ish.

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I meant with the 4.10's and running out.

I agree, I don't think it would be close to an issue with the 3... gears.

I guess it comes down to 3.23's vs. 3.55's.

It may be helpful to know that with the exception of a trip or two a year, all of my driving will be done on 55 MPH posted roads. Everybody drives 63-67 but I wouldn't get run off the road at 60ish.

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Based on that (a 60-65 crusing speed) and 29 inch tires, I would opt for the 3.55's