[Tom Vaught]

Originally Posted by indymanjoe

A lot of my driving is in the metro det area. with spirited drives to the UP..

Tell them indymanjoe, that I speak truth, kemosabe, since you have been there and done that.

Tom V.

Got a ticket coming back from the bridge one time, State Cop said he had chased me for several miles with his light on.

I told him Ambulances have one light, police cars have two lights where I grew up. He wrote me for 5 over and daylight driving and it was 10 pm and I was at least 4 times over that number. Nice guy.

[will slow gto]

Originally Posted by MUSLCAH

My 3:23 posi with turbo 400....67 GTO....cruises at 65 mph ...2800 rpm...27 inch tire 255 60 15

I think you’ve found the sweet spot right there!

[lust4speed]

We always put our club member with the 3.55's in the front of the group so we wouldn't be the cause of him spinning his engine to death. He naturally fell into the 63-65 mph cadence and while boring for those of us with overdrives, it was acceptable.

At the beginning of this year he swapped out the Muncie for a Tremec. Now we put him at the back of the pack because his new comfort zone with the same vibrations and noise is upwards of 85.

[Formulajones]

Funny how that changes for some.

Just the other weekend we drove my sons Mustang to Phoenix, 3.73's with a 5 speed and .63 overdrive.

Of course it'll go 85 if you want to and be comfortable.

I still found myself just putting it on 70, hit the cruise button, and just ride. Plenty fast enough on the 75mph highways here, with other lanes to choose from it works fine, plus it's way less nerve racking. Ironically you still come up on other cars going even slower while others go around doing 85+.
Don't think you'll get away from that no matter what the speed limit is.

[Half-Inch Stud]

Not kidding: unbelievable slingshot feeling to hit WOT at 70 MPH with the 469 held in 4L80 OD.

Factory 4L80 gasoline Converter delivers the goods. 90+ happens quick. Other cars got nuthin on it.

[Skip Fix]

I think my 4 speed Impala came with a 3.36 originally.

[SRR]

Originally Posted by slowbird

Just curious how people like 3.55 gears on the highway. What size tire are you running and what rpm you turning on highway?

My 72 455HO GTO had 3.55 gears with a 4 speed. I hated it on the highway, 3000 rpm @ 70 MPH and would heat up.

Like many here I have become very used to OD vehicles. My 70 Skylark has 3.31 gears and a TH400 and even that car is no fun on the Highway. I once had a 70 Chevelle with a 307, 200R4 and 3.23 gears. It was an awesome highway car that I used as a daily driver. 3000 rpm at 90 MPH.

[69Fbird455]

Have a 3.55 12 bolt in my 69 Firebird with a TKO600 (0.64 OD). Rear tires are 26.5" tall. 2000 rpms at 70 mph. Perfect.

[Ponchy]

Originally Posted by slowbird

Just curious how people like 3.55 gears on the highway. What size tire are you running and what rpm you turning on highway?

You can do some research on your own, with just a simple calculator, if you know the forumla.
It's pretty easy and I've been using it to bench race for years (something to do when I'm taking a dump )
First you have to figure out effective gear ratio.
If top gear is 1:1, then effective is simply the ratio of the drive axle, but if you're using an OD tranny, then you have to remember to include the OD ratio (OD ratio times drive axle ratio).
So, if you have 3.55 gears in the rear end and you're using, say, a T5 with a .76:1 top gear, then you need to multiply 3.55 by .76, which will give you 2.698.
That is the effective gear ratio you need to use.
Consiquently, if you want to end up with a 3.55:1 effective gear ratio, using the same tranny (and OD ratio), just divide the two numbers, and you'll find you'll need to use a drive axle ratio of 4.671:1.
...ANYWAY...
You simply take the speed you want to drive at, multiply it by the effective drive gear ratio, multiply that by 336 and divide by the height of the tires in inches.

The resulting number will be engine rpm.

You can either go out with a tape measure and measure the diameter of your tires, or you can calculate using the tire's size.
To calculate tire height using the tire's listed size, take the first number, divide by 25.4, then multiply by the second number (its a percentage, so you'll either need to hit the percentage button instead of equals, or simply put the decimal point in front of that number), multiply that number by 2, then add in the last number.
So if you're running 245/60-15's, then you'd divide 245 by 25.4 and multiply that by either 60% or .60. Take that number and multiply by 2 then add the 15.
The end result will be the calculated height of the tire.
...or you can grab a tape measure and just measure the height of the tires (not always convenient, so why I explained how to calculate it on a calculator).

So, in practice it would look something like this....

Let's say you're Firebird has a Muncie 4-speed and 3.55 gears and the wheels are shod with 245/60-14's.
You want to know engine rpm @ 70 mph.

Since the Muncie has a 1:1 top gear ratio, we only need to deal with the drive axle ratio, which is 3.55.
In this case, this will be the effective gear ratio used in the formula.

Input the following numbers into your calculator, as they're written here....

70 x 3.55 --> x 336 --> / 25.6

The number you end with will be the rpm the engine is turning while you're driving @ 70 mph.

Now that you know that, you can take your car out on the highway and drive it at the rpm you just calculated and decide for yourself if that "comfortable" to you or not.

Hope you found this helpful.
Feel free to ask for further explanation, if there's something here you don't understand.


Ponchy

[Tom Vaught]

Originally Posted by Ponchy

So, if you have 3.55 gears in the rear end and you're using, say, a T5 with a .76:1 top gear, then you need to multiply 3.55 by .76, which will give you 2.698.
That is the effective gear ratio you need to use.
Ponchy

Your calculation above, assuming a 3.55 gear in the rear and the overdrive
ratio .76 to arrive at approximately a 2.75 effective ratio rear axle.

I have 5 gears but my final drive is the approximate 2.698 ratio above assuming the T5 trans and the 3.55 rear gear. EXCEPT my NASH 5 speed Trans is a lot stronger vs a basic T5 trans.

At the end of the day if the two vehicles had the same tire/wheel on our cars
we would both be doing the same engine rpm at 75 mph going down the highway.

Tom V.

[Steve C.]

Gears - Tire - RPM Calculators

http://www.wallaceracing.com/Calculators.htm


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[PunchT37]

I run 3.55`s in my car. I also have a set of 3.08 gears. It is tempting to put those in as I am also spoiled to newer vehicles. But, it`s not just simple math to determine rpm`s at speed. It`s behavior at speed. I run a 306 seat duration cam. 246 @ 050. One doesn`t want to lug this engine combo down.


Newer cars behave better not just because of od transmissions, but, efi with computer timing control, AND, variable cam timing found on almost all new engines. Just a thought while driving your newer vehicle.

[72LuxuryLeMansLa.]

Originally Posted by PunchT37

I run 3.55`s in my car. I also have a set of 3.08 gears. It is tempting to put those in as I am also spoiled to newer vehicles. But, it`s not just simple math to determine rpm`s at speed. It`s behavior at speed. I run a 306 seat duration cam. 246 @ 050. One doesn`t want to lug this engine combo down.


Newer cars behave better not just because of od transmissions, but, efi with computer timing control, AND, variable cam timing found on almost all new engines. Just a thought while driving your newer vehicle.

My thought process leads me to think that if you want it to behave like a new car, drive a new car. If you want to drive a hot rod , drive a hot rod.....But I'm a simple man....

[i82much]

Originally Posted by 72LuxuryLeMansLa.

My thought process leads me to think that if you want it to behave like a new car, drive a new car. If you want to drive a hot rod , drive a hot rod.....But I'm a simple man....

that’s where i’m at. i jump in the silverado most of the time and love the gto for what it is when i take it out.

[Ben M.]

Originally Posted by SRR

I once had a 70 Chevelle with a 307, 200R4 and 3.23 gears. It was an awesome highway car that I used as a daily driver. 3000 rpm at 90 MPH.

This is what sold me on overdrive. Driving on I-8 and it's doing maybe 2200RPM and I ask how fast we're doing and you said "70" and you truly couldn't tell.

[Ponchy]

Originally Posted by PunchT37

I run 3.55`s in my car. I also have a set of 3.08 gears. It is tempting to put those in as I am also spoiled to newer vehicles. But, it`s not just simple math to determine rpm`s at speed. It`s behavior at speed. I run a 306 seat duration cam. 246 @ 050. One doesn`t want to lug this engine combo down.


Newer cars behave better not just because of od transmissions, but, efi with computer timing control, AND, variable cam timing found on almost all new engines. Just a thought while driving your newer vehicle.

Good post.
You can use the formula I explained above, to determine what gear ratio you'd need to use to work best with your own engine's power curve.
The OP mentioned a 70 mph cruise speed, so one could tailor the effective gear ratio to match that cruise speed to the peak torque RPM the engine creates in whatever form it is.
Now this creates an interesting variant to the formula I mentioned above, because you'd actually have to run it backwards in order to figure this out.
So, let's say the peak torque of your engine happens at 2400 rpm and you're running those 25.6" tires and you want to hit that rpm @ 70 mph.
All you need to do is to start with the RPM and work back to the gear ratio, doing the opposite operations you did before.
...SO...
In this case you'd multiply 2400 by 25.6, then divide that number by 336, then divide that number by 70.
The end result will be the effective gear ratio your car will require in order for you to do this.
Keep in mind, we're dealing with an engine with a pretty broad torque band, so you don't necessarily have to hit it "spot on".
"Close enough" should work fine (of course, you're going to strive to hit it as close as possible).

Ponchy

[Steve C.]

Not many Pontiac engine combinations will have a peak torque rpm at 2600 !
A stock '67 GTO with a 360-horse 400 might have a peak torque closer to 3600 rpm. That might be taken from factory gross ratings of a stock engine. Getting a true torque peak from a chassis dyno may show slightly different numbers, given the same engine. The popular Pontiac combination mentioned in my post number 28 makes peak torque at 4500 rpm and one might consider 4.56 gears for it. Fun to consider, but gearing for the torque rpm might be a bit of fantasy, hard on fuel economy and engine wear


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[Tom Vaught]

Originally Posted by Steve C.

Not many Pontiac engine combinations will have a peak torque rpm at 2600 !
A stock '67 GTO with a 360-horse 400 might have a peak torque closer to 3600 rpm. That might be taken from factory gross ratings of a stock engine. Getting a true torque peak from a chassis dyno may show slightly different numbers, given the same engine. The popular Pontiac combination mentioned in my post number 28 makes peak torque at 4500 rpm and might consider 4.56 gears. Fun to consider, but gearing for the torque rpm might be a bit of fantasy, hard on fuel economy and engine wear .

Very true Steve.

Tom V.

[Formulajones]

Yeah our little DZ 302 is very high strung and makes peak torque at 4600 rpm, but it still works perfectly fine with 3.55's out back and a TKO600 with a .64 overdrive. It'll cruise along at 1800-2000 rpm easily and climb the mountain grades around here without the need to downshift.

Most all of these muscle car engines are more capable of low rpm cruising than people give them credit for. Pontiacs especially.

[Steve C.]

That stock '67 GTO with peak torque at 3600 rpm, if gearing for torque = 4.29 gear in theory with a TH400 trans and 28" tires at 70 mph.
http://www.wallaceracing.com/calc-rgr.php


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