What is a good "all purpose" gear ratio?

[69redrunner]

My 69 RR has a factory 3:91 rear end gear and a 4 speed trans. Great for acceleration, lousy for highway travel and fuel consumption. It's turning 5k rpm @ 55-60mph.

I'd like to try to find another 3rd member to put in it, but what ratio would be good? 3:55? Lower yet?

Anybody with 4 speed and a known ratio know what they are turning for rpm @ highway speeds?

Thanks

 

[Cranky]

Your tach is wrong if it's showing 5k at 55-60mph.....but if you'd like to run the freeways and let the engine loaf, go with a 2.76. You could run 130 @ about 4800 or so.

 

[shag766]

this is going back in the memory banks but if i remember right one of my old vets with 4 speed, years ago would run 3-3500 rpm at 55-60 mph with 4;11. i like 3;55 myself. a little torquie but reasonable.

 

[70rr-brian]

this is going back in the memory banks but if i remember right one of my old vets with 4 speed, years ago would run 3-3500 rpm at 55-60 mph with 4;11. i like 3;55 myself. a little torquie but reasonable.

I will second that. The 3.23 is a little slow off the line and general acceration. The 3.91 is just too many rpm's on the highway. The 3.55 is perfect IMHO.

And 5k at 60mph is like someone else said a broken tach or you have 4.xx gears.

Use this to help: http://vexer.com/automotive-tools/speed-rpm-calculator

 

[howiedodat]

I would hope the brother would know what that engine song would be like at 5k rpm's. Perhaps you think you have a 3.91 and don't ? I turn 3k at 60 with 3.55's w/ my torqueflite.

 

[Cranky]

There are all kinds of internet calculators on this....and they are pretty darn accurate.

 

[Meep-Meep]

I like nothing lower than a 3.23. If you do more HWY driving then go higher with a 2.94. I've spent a lot of years driving a 68 Mustang GT 4 spd with 3.00 and it was great.

 

[69redrunner]

Fender tag calls out a 3:91, previous owner says it's 3:91.
I don't believe the tach is wrong, its definately turning some high rpm's at highway speeds.

I don't want a dog, but I'd like to bring the revs down and give the motor a chance at a longer life. The original 383 didn't make it, it has a 440 in it now.

 

[696pack]

What size tires and wheels are you running? If you have low profile/small overall diameter this may be causing your RPM problem.

My last 66 Corvette with 215/70X15" with 3:73 gears ran 4000 rpms at 80mph so either your tac is off or the above possible wheel/tire size.

 

[Al K]

696Pack beat me to it;you must have a small diameter tire,and some other issue. Not to be a smart ***,but are you sure that's a 3.91 in there? 5K rpm at 60 is just wrong! I calculated a 3.91 ratio with 29 inch tall tires and came up with something in the mid 3K range at 60 mph. There are a couple of good online calculators for this stuff.

That being said,I really like my 3.55's with a 29" tire. Good acceleration in any gear,and mileage..well,it's a big block. No such thing as "mileage". I do have a 3.91 r&p,and an extra case,and a Sure-Grip carrier and...that presents a dilemna. Build the next motor for the 3.91's or the 3.55's?

 

[1badgtx]

I,m runnin a 3.55...Seems to be pretty good all around gear set...
Petty Blue 67 gTX

 

[6packMIKE]

I have 275-60-15's with a 3:91 gear and run about 3500 rpms at 70MPH on the freeway, so 5k at 60 sounds more like 4:56 gears or more.....if the tach is right? What do you read at idle on the tach?

anyway, 3:55 is a good all around gear.

 

[Crunchy_Frog]

Don't ever take what the previous owner (of anything) states as gospel. Trust, but verify.

 

[68GTX4SPEED]

With rear tires 225-50-14 and 5.7 in the rear 5000rpm will be 59.76 mph
http://vexer.com/automotive-tools/speed-rpm-calculator

 

[hunt2elk]

I am running 3200 rpm @ 60 mph with 3.91 and 28" tall tires. You either have short tires, lower gears, or your tach is off.

 

[SUPERSTOCKRACER]

JACK UP THE REAR. and put in nuetral and turn your wheel around 1 time and count how many times your u joint goes around.

 

[shag766]

JACK UP THE REAR. and put in nuetral and turn your wheel around 1 time and count how many times your u joint goes around.

X2

thats an old one i have seen before:iamwithstupid:

it will get you close

 

[ga66mopar]

3.23 for me.

 

[Scatransit]

My 3:91 car is about 2300-2500 at 50-55. 3:55's are a great all around gear, but you will miss the 3:91's!

 

[quick63]

SSRacer has your answer. Jack it up and do the math. I use to street race and I have run everything from a 3.55 to 4.88 on the street. A 4.10 was the best street/strip gear on my 4 speed GTX. Basically you trade top end for bottom end. Run the talllest and widest tire you can.