Overdrive Transmission Options

[RamRunner]

2.94ratio x 70mph x 336=69,148.8 divided by 26.5 tire diameter = 2609rpm multiplied by .70 overdrive ratio = 1826rpm

I'm no Einstein so please help me to see where my calculations are off

Randy's ring and pinion calculators have the same numbers, "Check out the big brains on Brad" (Sam Jackson Pulp Fiction)

 

[mopar 3 B]

2.94ratio x 70mph x 336=69,148.8 divided by 26.5 tire diameter = 2609rpm multiplied by .70 overdrive ratio = 1826rpm

I'm no Einstein so please help me to see where my calculations are off

2600 rpm, 2.94 rear ratio, 26.5" dia. tire, 1:1 trans gear = 63 mph
1826 rpm, 2.94 rear ratio, 26.5" dia. tire, .7:1 trans gear = 70 mph
numbers above are per calc's from Tremec Tool Box Vehicle Speed Calculator.

You are correct there is either something wrong with Tremec's calculator or I am not entering the information incorrectly. I went back to my old math formulas and came up with your same figures. Tremec was off on the 1:1 speed, but ok in OD.
Also interesting that at the 2600 rpms in OD you would be going 99.6 mph if the 318 has enough power to pull it.
We must have to watch OD ratios or you were rounding because the 47RH uses a .69:1 OD.

 

[Greg Ducato]

Thanks, I wonder how Tremec came to that first figure? I did round that OD up to .70 because the retrofits that use the 4L60E and 700R4 have a.70 ratio in overdrive and I didn't want to break it down by trans. Advantage Mopar!

 

[mopar 3 B]

Glad you questioned it. Don't know how many use the app but it could save some one a greater legal embarrassment now knowing it doesn't always work.

 

[chrger1967]

I agree with Greg. Leave it. From a financial point of view, how long will it take to payback what you spent by only getting 2 or 3 mile per gallon more than what you currently get? Is this your daily driver? Do you put 15,000 to 20,000 miles a year on it? If you were to drive 15,000 miles per year @15mpg and @$2.50/gal, it will cost you $2500 in gas. Same conditions @18mpg, it will cost you $2082.50, saving only $417.50 per year, not worth it in my opinion. My dad wanted to do the same conversion to his Dart and he only drives it 3,000 to 4,000 per year and once I showed him this comparison, he said never, he'd rather do something else to the car. Just my opinion.

Thats a shame. I put a 4-speed automatic transmission in a 65 Impala S.S I had years ago, and it was an easy swap. I just had to cut the driveshaft and buy a different transmission cross member and use a cable setup for the kickdown. And what a difference that one gear made.