gearing ??

[Big J]

I need some help. I have a 1970 challenger 383 650 carb ,auto with ac. Little bigger cam than stock. I have a 391 gear now 8 .75 741 case... Seems like I always need gas. looking for a good gear to cruise or drive down the Highway.
Thanks .

 

[70chall440]

Well the gears certainly are not helping but these were never 20 mpg cars (usually) and given your engine/cam situation you are probably not going to get a whole lot better.

That said, I'd go with 3.55s, they are my favorite usually. Good low end but not horrible for the highway.

 

[Big J]

Thank you !

 

[Chryco Psycho]

There is so much to this .
Where does the cam start to make power , what converter lock up speed do you have ?
If the converter is unlocked at normal cruising speed you will make a lot of extra heat in the trans from slippage .
Use an online calculator to make sure the change puts you into the range you want

 

[Challenger RTA]

Personally I like the 3:23 gear with stock tire size. Drove to navy bases about 275 mile,on just less then a tank of gas.I would have about a 1/8 tank left as long as I didn't kick in the four barrel. four barrel would kick in on some grades running 60-65+. It amazed me how the thermo quad performed. I had a 68 and 67 mustang that would pass any thing but a gas station.same trip four tanks of gas.high lift cam 650 dbl pumper 302 auto

 

[Big J]

There is so much to this .
Where does the cam start to make power , what converter lock up speed do you have ?
If the converter is unlocked at normal cruising speed you will make a lot of extra heat in the trans from slippage .
Use an online calculator to make sure the change puts you into the range you want

Thank you for this update.

 

[Big J]

Personally I like the 3:23 gear with stock tire size. Drove to navy bases about 275 mile,on just less then a tank of gas.I would have about a 1/8 tank left as long as I didn't kick in the four barrel. four barrel would kick in on some grades running 60-65+. It amazed me how the thermo quad performed. I had a 68 and 67 mustang that would pass any thing but a gas station.same trip four tanks of gas.high lift cam 650 dbl pumper 302 auto

 

[Big J]

Thank you .Also for the ratio chart.
Jeff

 

[moparleo]

Seems like I always need gas. ? What is your MPG ? My 72 340 Challenger got 11-14 mpg no matter how I drove it.

 

[Big J]

Seems like I always need gas. ? What is your MPG ? My 72 340 Challenger got 11-14 mpg no matter how I drove it.

I get about 10 mpg

 

[Challenger RTA]

I would fill up at start of trip squeeze in 16-18 gal,fill tube filled. Drive the dead of night and cool. 90% highway and turnpike I would get 15-17 mpg lucky if I got 18. stock 340 4spd 3:23 G 60 15. I Know the 71 thermo quad was different then 72 and up, if that made a difference?

 

[Racer Dave]

All this talk about gearing but you're overlooking the obvious - get a bigger fuel tank!

As was said earlier, these cars never got good - or even decent - mileage compared to modern cars. Have you considered an overdrive? They're pretty expensive and it might take a long time before the fuel savings offset the cost - except here in California where an overdrive costs about the same as a couple tanks of gas A higher (lower numerically) gear ratio might help a little, but you sacrifice performance. I just look at it like this: being able to drive a muscle car gives a sense of gratification that you just can't get any other way, so I'm willing to pay the price.

 

[fasjac]

All this talk about gearing but you're overlooking the obvious - get a bigger fuel tank!

As was said earlier, these cars never got good - or even decent - mileage compared to modern cars. Have you considered an overdrive? They're pretty expensive and it might take a long time before the fuel savings offset the cost - except here in California where an overdrive costs about the same as a couple tanks of gas A higher (lower numerically) gear ratio might help a little, but you sacrifice performance. I just look at it like this: being able to drive a muscle car gives a sense of gratification that you just can't get any other way, so I'm willing to pay the price.

That’s what I like about having 5th in mine. It’s a Tremec, best of both. Has 3.91 gears. Launches in first like 4.88’s and fifth is .64 to 1. The downfall is the price. I’d still say it’s the best upgrade I’ve ever done to muscle car.