Might have screwed up--------

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DesertRat

DesertRat

Leading edge boomer
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Just picked up my rebuilt 8 1/4 from RR&P. Had built with 3.73 R & P & new Trac-Loc unit. I think the road RPM will be too high with my 904 transmission. Plan on 26" tall tires and the calculators show almost 3500 RPM @ 70 MPH. Now I am thinking what to do, AOD auto or 833 OD manual. Maybe just do nothing and keep it in town:)
DR:error:
 
Just drive 55. Problem solved. Calculate the difference in travelling 100 miles at 70 and then 100 miles at 55. You ain't losin much time at ALL. I rarely drive over 55 anywhere I go. As long as you keep engine RPM at or under 2K, that's when you will get about the best mileage.
 
DesertRat said:
Plan on 26" tall tires and the calculators show almost 3500 RPM @ 70 MPH. Now I am thinking what to do, AOD auto or 833 OD manual.
DR:error:
Click to expand...

Why are you limiting yourself to 26" tall tire? A taller tire will lower the RPMs as well, for a heck of a lot cheaper than a transmission.
 
Look at the GM 200R4. Mopar To Ya has a thread on installing one in his dart.
 
I agree that a taller tire is the easiest and cheapest solution- you can calculate the rpm to determine height
 
I have 26" skins on my Duster. I wouldn't be able to get anything taller than that up front. A taller tire in the back might depend upon what stance he is looking for with the car.
C
 
60 mph's with 27" tall tire 2785 rpm's not bad , if you can get a 28" tall tire like 255r65/15 you would drop to 2600 rpm's @ 60 mph
 
Thanks guys! I think 26" will be about the max for the rear but i will check further. I saw the thread to change to an AOD transmission but I am committed to my original 904 with master rebuild and Transgo-2 kit. By researching the forums it looks like I will need a 37-38 tooth transmission speedometer drive gear for 26-27" tires. Still checking things out-------:burnout:
 
Hmmm....calculator I just used said with a 26" tall tire, 1.00 final drive in trans, 3.73 gears at 70 you will be turning 3374 rpm. Same setup with a .75 final drive in the trans will yield 2531 rpm. To get to that rpm with a taller tire it will take a 35" tall tire....if you plan on putting a taller tire on it to kill the gear ratio care to guess what you are going to do to the effects of the 3.73 while accelerating? In order to get to the same rpm by slowing down, according to this calculator you will need to go 52 mph. No way I would be able to cruise at 52 mph while folks around me are going 70.....To me that is just going to be an accident waiting to happen...Good Luck with that....

Why are folks so hesitant to adapt any sort of technology beyond 1970 into these old cars? I get it that it is not cheap....then just buzz the thing 3400 rpm and live with the decreased fuel economy and the possibility of increased engine wear...not to mention living with the annoyance of 3400 rpm....
 
Actually the final rpm is higher than that Inkjunkie. Your calculation is based on a 1.0 final drive ratio (standard transmission) but with an automatic and non-lockup converter according to the calculator on Randy's Ring & Pinion the rpm is a bit higher. I am actually happy with the build so far and look forward to taking it to the strip in Spokane and getting a timing slip to hang in my shop. Hard to say what I will wind up with. These builds keep evolving and I could wind up with a manual transmission with OD. No tires bigger than 26-27" though!:coffee2:
 
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