Rear End Ratio question

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Tjhoward84

Tjhoward84

1969 Barracuda Convertible
Joined
Mar 19, 2014
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Location
Bothell
I have a Magnum crate 360 mated with a TorqueFlite 727 transmission. The rear end is a Dana 60 from Strang Engineering with a 3:54 to 1 ratio. In 3rd gear when the car is doing 60 mph the engine is turning at 3,000 rpm. This seem way to high of rpm to me.

Do I need a taller rear end gear?

Can I change the shift points in the trans?

Thanks for the advice.
 
Depends on your rear tire size. What is it?

How do you know you are doing 60?
 
Yes, need to know tire size. If you had a normal 26" tall tire and were truly
doing 60 mph at 3,000 rpm I'd say you had a 3.91 gear in your Dana.
 
Is your speedometer accurate? You can download a speed app for your phone that will tell you exactly how fast you are going. What kind of converter is in the car, how much slippage is in it?
 
Tjhoward84 said:
245/40-18s

I just have the speedo calibrated and it's pretty close to accurate.
Click to expand...

According to this site

http://www.audiworld.com/forums/tt-mk2-discussion-120/tire-diameter-chart-2704424/

Those tires are 25.7" in dia

There are tons and tons of mph / gear/ speed / rpm calculators on the 'net

"Wallace" is just one
http://www.wallaceracing.com/Calculators.htm

http://www.wallaceracing.com/gear-speed.php

Which generates this chart

and that means "something is wrong." Either you don't have a 3.54, or the tires are different, or the speedo is wrong, or you have a converter with a VERY high stall

Cheapest easiest way in my opinion, in this day, is to use a GPS for speed.
 

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Tire diameter is 25.72.

The shop told me the ratio was 3.54 to 1. I have not verified that number.

3.91 gears would definitely explain the 60 mph at 3,000 rpm.
 
d55dave said:
Is your speedometer accurate? You can download a speed app for your phone that will tell you exactly how fast you are going. What kind of converter is in the car, how much slippage is in it?
Click to expand...

The converter is a stock unit with a 1,400 rpm stall.
 
67Dart273 said:
According to this site

http://www.audiworld.com/forums/tt-mk2-discussion-120/tire-diameter-chart-2704424/

Those tires are 25.7" in dia

There are tons and tons of mph / gear/ speed / rpm calculators on the 'net

"Wallace" is just one
http://www.wallaceracing.com/Calculators.htm

http://www.wallaceracing.com/gear-speed.php

Which generates this chart

and that means "something is wrong." Either you don't have a 3.54, or the tires are different, or the speedo is wrong, or you have a converter with a VERY high stall

Cheapest easiest way in my opinion, in this day, is to use a GPS for speed.
Click to expand...

Thanks for the info. That chart says at 3,000 rpm, the speed should be 64 mph. This would lead me to believe that I'm not too far off and it's running correctly or I'm missing something.

The tires are definitely 245/40-18s that are 25.72 inches tall so we can rule that out.

I haven't verified the rear end gear ratio. I am just going with what the shop told me.

My goal is to be able to cruise at highway speeds (60-70mph) around 1,700-2,000 rpm but no loose any of the low end torque the car has.

Any thoughts???
 
You still didn't tell us why you seem to think the speedo is accurate. Heck the TACH could be off. Something is certainly "wrong" LOL
 
67Dart273 said:
You still didn't tell us why you seem to think the speedo is accurate. Heck the TACH could be off. Something is certainly "wrong" LOL
Click to expand...

Why do you say there is something wrong based on the Wallace table? It states the car should be going ~65mph at 3,000 rpm. I'm right in that range, in my opinion. I could be a few miles per hour over 60 at 3,000. To me, it just feels that at 60+ in cruise mode, I'm used to cars at 1,700-ish rpm. This is really the first time I've had the car on the freeway to those speeds. I guess I have to re-adjust my thinking.

The speed was just calibrated at the transmission shop based on the tire size and rear end gear ratio. I put a 36 toothed speedo gear in place of the 29 toothed gear that was originally in the trans. I'm sure the gauge (45 years old - the car is a 1969 Barracuda Convertible) is not 100% accurate. Next time I take it out, I will download a GPS app on my phone and determine how far off the speedo is.

Yes, the tac could be off but I'm really basing my concern on the sound of the engine and not 100% on the tac.

I appreciate all the feedback. Keep it coming.

Here is picture of the car.
 

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abodyjoe said:
sure its not a 3.73?

http://www.bigblockdart.com/forum/speedtire.php
Click to expand...

I just measured it for myself 30 minutes ago. It's not 3.73. It's closer to 3.54 than anything.

I am so used to driving cars with 6 speed automatics with overdrive that this just caught me by surprise.

Guess I'll have to get used to it or change the rear end gears to something like a 3.23 or taller.
 
Tjhoward84 said:
I just measured it for myself 30 minutes ago. It's not 3.73. It's closer to 3.54 than anything.

I am so used to driving cars with 6 speed automatics with overdrive that this just catch me by surprise.

Guess I'll have to get used to it or change the rear end gears to something like a 3.23 or taller.
Click to expand...

...or get a Gear Vendors unit, or put in an automatic OD transmission...
 
3:23 not bad but I think im going to gear Vendors , just depends on how much im going to drive my '66. MT
 
3.54 is the highest gear you can get for a 60. If you don't like the rpms you will have to get an overdrive.
 
Frankly you can get a LOT taller tire under there and it would look better with them
 
67Dart273 said:
Frankly you can get a LOT taller tire under there and it would look better with them
Click to expand...

I agree, X2 - put some properly sized tires on there and it will drop the highway revs, and - this is only my opinion so take it as you want - it will look better (BTW, it does look nice now).
 
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