Did I do the math right on computing rear end ratio?

[GreatScat1965]

Edouard,

Hi all, as previously mentioned I've been loosely following this post. Not to throw a monkey wrench in all the calculations, but since the transmission doesn't have a tow/haul feature(Computer Controlled Logic) aren't you supposed to tow in Drive not OverDrive? I was always told the transmissions would overheat and/or damage the converter towing in OD. At least that is what it said in the owner's manuals for my early GM cars and trucks. I'd consider consulting your trans builder and inquire with them.

Jerry

[elagache]

Dear Jerry and mid-60s Buick owners with towing aspirations,

Hi all, as previously mentioned I've been loosely following this post. Not to throw a monkey wrench in all the calculations, but since the transmission doesn't have a tow/haul feature(Computer Controlled Logic) aren't you supposed to tow in Drive not OverDrive?

I've also been very cautious about the transmission.  It is a TH200-4R hardened by California Performance Transmission and it has its own transmission cooler.  In actual practice I would pay careful attention to how the car is performing and downshift to 3rd whenever there is any concern about overloading the transmission.  Even so, when the road is flat it should be possible to stay in overdrive and save some gas.

Cheers, Edouard 

[elagache]

Dear mid-60s Buick owners who are handy with figures,

I sent my analysis on the rear end ratio to Mike Tomaszewski at TA-Performance and got this response.

The 3.08 will not be noisy at all, and I wouldn't think the 3.23 would be much more noticeable. If the trans has a over drive gear in it I would myself be putting in the 3.23

So I thought the matter over and realized I would have made an important mistake if I stuck with a 2.78:1 rear end ratio.  Biquette has smaller tires right now.  Her new taller tires would force the engine to spin even more slowly than it does now.  I'm greedy but not that greedy.  So basically, I need to jump at least one notch to 3.08:1.  So this has been considered a safe choice for a long time, so I could have just stopped there, but I looked into a few more things.  As it turns out, with the taller tires, the engine RPM will only increase by 7% with a 3.08:1 rear end ratio.  So basically guaranteed to be the same noise level.  I can't complain about that!  Moreover that small change has some significant benefits.  If I redo the comparison graph between the 2018 Tahoe, 1965 Sportwagon, Biquette in 3rd, 1965 Electra, and 1970 Estate wagon, Biquette compares much more favorably with the full-sized cars:



That certainly should be enough torque to handle a class-III load!  I did one more comparison.  I used the horsepower graph to estimate the horsepower in overdrive and in 3rd:



I think this graph settles any question of attempting to tow in overdrive.  The engine isn't producing enough power even with the 3.23:1 rear end ratio.  However in 3rd it is a different story.  Even with the 3.08:1 ratio, the engine is over 200 horsepower cruising at all highway speeds.  So if the need came to pass a truck even with the trailer, there is the horsepower to at least make the attempt.

So I think this matter is settled!  3.08:1! 

Thanks for your support!

Cheers, Edouard 

[elagache]

Dear mid-60s Buick owners who want their car's to run "just right!"

This morning on the Hemming's Blog, Thomas A. DeMauro offers his own thoughts on what rear end ratio to choose:

https://www.hemmings.com/blog/2018/07/19/gearing-for-go-or-no-do-you-prefer-high-revs-or-low-revs/

Nothing surprising here, but another quick read with a bit of food for thought.

Cheers, Edouard