3.90 YES or NO?

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Marcelo Dart BR

Marcelo Dart BR

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I have a 318 with a MP 284/512 solid cam. It has a Performer RPM intake, Holley 650 double pumper, 9.8 compression and headers. The tires are 255/60r15. It's a street car, and I almost don't drive it on highways.

As you may think I don't have lots of low end torque, and the engine starts to run nice at about 2500 rpm. My rear end is a 3.15, and with the big tires it's not the best choice. A friend of mine has a 3.90 in his stock 318 and he proposed a rear end swap.

Do you guys think it's a good idea?

Thanks
 
3.90 would be a noticeable difference. I've got 4.10's in the Duster with comparable sized tires and it goes down the highway okay. I wouldn't want to drive it to Vegas, but it's fine for short runs and it really gets the power to the ground.
With 3.90's @ 60 mph you will be running just over 3000 rpms with those tires.
3.15's should be just under 2500 rpm's @ 60 mph now.
 
Damn a 3.55. They are for people who are scared to go fast. Throw the 3.91 in it. I'm gonna put 4.30s in mine.
 
I got 3.90's also and like them but traffic in places go 80 mph now on freeways. Its the only place were 3.15-3.21 gears are better since 10 mph over 80 is 90 mph. You get places quick if you can do it for an hour.

For most people 3.90's are a great all around gear. Car Craft in 1970 said 4.10 gears and I think it was 27 inch tires were the best all around gear....this is how that BS 55 mph came about, it saved lots of gas--only to be used up by SUV's later on.
 
I got 3.91s in the Duster right now. they are great all around gear set.
Going to pull them out for a trade. Going back to my orignal 3.23s for the
street driving. Trading the 3.91s for a set of 4.56s for racing.
Thats a Wow factor for putting your head back.
 
Yeah, I had 4.56's in a big block Challenger and it did more than pull your head back... I had to move the seat forward to reach the shifter with the G's that it would put on my body. With 4.56 ratio you'd be running 3600 rpm's @ 60 mph.
Even on the freeway @ 55mph, mashing down the throttle will launch the car. Fun when the Camaro's blow by laughing... they stop laughing when their rear bumper never gets past your front bumper before you dust 'em... that's without even downshifting.
 
tranniedoc said:
Yeah, I had 4.56's in a big block Challenger and it did more than pull your head back... I had to move the seat forward to reach the shifter with the G's that it would put on my body. With 4.56 ratio you'd be running 3600 rpm's @ 60 mph.
Even on the freeway @ 55mph, mashing down the throttle will launch the car. Fun when the Camaro's blow by laughing... they stop laughing when their rear bumper never gets past your front bumper before you dust 'em... that's without even downshifting.
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4.56 with a 518 trans is a great combination.
With a 727 it's a nightmare on the highway.

Now that I am using a 727, I wouldnt go lower than 3.90.
But that's just me.
 
I agree with POS Dakota. Non-OD and low gear makes for a lousy street car.

Hell, even with the 3.91 it gets old spinning it to 2800 at 40-ish.

I put 3.55 in the Chally (which has a 318), and that's plenty for a cruiser. A 318 is only gonna have so much umph anyway.

I think a converter would help the car more than the gear with fewer side effects.

Steve
 
POS Dakota said:
Yeah, then put 44 inch mudboggers on it and go hit the swamp.:grin:
I had them and absolutely hated them. I would only go that low with an overdrive transmission.
Click to expand...

27" tire, solid roller 318, 3500 stall.......... 60mph was about 4100rpm, and the motor loved it. However, it DID get less than 5MPG, on 100LL....... Not very efficient for todays prices.
 
dodgetkboy78 said:
27" tire, solid roller 318, 3500 stall.......... 60mph was about 4100rpm, and the motor loved it. However, it DID get less than 5MPG, on 100LL....... Not very efficient for todays prices.
Click to expand...

Definitely fun for an exclusive weekend cruiser, but damn it on the highway.

I used to just laugh in 3rd gear with my 518 and 4.88s how terrible life woudl be if I didnt have another gear. haha
 
LXguy said:
I agree with POS Dakota. Non-OD and low gear makes for a lousy street car.

Hell, even with the 3.91 it gets old spinning it to 2800 at 40-ish.

I put 3.55 in the Chally (which has a 318), and that's plenty for a cruiser. A 318 is only gonna have so much umph anyway.

I think a converter would help the car more than the gear with fewer side effects.

Steve
Click to expand...

Expect it feels right and 2nd gear goes when needed--fast passes on 2 lane roads. Plus its easier to move forward in 3rd gear and make the lights before they turn red. My gas mileage stinks but my driving record shines...and only hit one car last season, ha ha..wasn't my fault she didn't look far down the road. Was able to pop the front bumper back out and pocket most the $500 I got from her insurance company, lol

A decent semi quiet exhaust is a must. The punks tell me it needs louder mufflers when they look at it idling...driving thou its perfect.
 
Hey guys!
We made the swap today. I'm almost dead right now, we had some problems with the brakes and we worked on it from 10 AM to 11 PM.

BUT

Now the Dart is really awesome, it's way way faster. I'm really happy with that.

Tanks!
 
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