Got a 67 300 'vert with 440 TNT and auto. Car came with 3.23:1 rear axel ratio. A few years ago I had the diff sent out to be rebuilt. The vendor screwed up the backlash on the ring and pinion. The diff was VERY noised but I lived with it until I had money to fix. The diff was sent out again to a different vendor. He said the 3.23's were worn kinda funny and suggested a new ring/pinion. I went with 3.55:1 this time.
I was a bit worried that the rpm might be too high on the high way. I was also worried about how it would affect mpg (which is pretty bad anyway).. Anyone know of an rpm calculator for my new combo. I searched on the web and got a calculator that said the rpm would be around 5600 at 70 mph. I HOPE not!!!
Joined: Mon Oct 10 2005, 08:43PM
Location: Calgary, Alberta
Posts: 978
Don't know what size you are running, but my 235/70-15s are 28" tall and 743 Revs per mile, so at 60 mph with a 3.55 it works out to 743x3.55= 2640. That would then translate to just under 3100 at 70.
Ok, after I stepped away from the computer for a bit I realized that I put my Wheel diameter (15") into the calculator instead of the total tire diameter. So, thanks 300 rag, my tire size is 235/75-15 so the revs should be pretty similar to yours.
I did check fuel mileage once and I'm getting about the same mileage as I did with the 3.23's. Not a real good test but mileage should be in the same ball park.
Joined: Sat Aug 19 2006, 05:03PM
Location: Ontario, Canada
Posts: 2919
Short answer: Whatever your engine RPM was at 70MPH with the 3.23 gears, the 3.55s will be about 10% higher. 3.55/3.23 = 1.10
Longer answer: Assuming your car has 225/75R15 tires, those roll 744 rev/mi. So per mile your engine turns over 744 x 3.55 = 2641 rev/mi. (If your tires are a different size, look up their revolutions/mile from a tire website and recalculate using that number.) 70 mi/hr = 1.167 mi/min 2641 x 1.167 = 3081 rev/min
It should actually be a bit higher because that doesn't include torque converter slippage.
Remember that your speedometer and odometer will now read 10% fast unless you change the gear in the transmission accordingly.
EDIT: Peter answered while I was typing. I'm glad our calculations agree. A 235/70R15 has the same overall diamater as a 225/75R15.
Joined: Tue Nov 07 2006, 11:35AM
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67Chrysler300 wrote ...
I was a bit worried that the rpm might be too high on the high way. I was also worried about how it would affect mpg (which is pretty bad anyway).. Anyone know of an rpm calculator for my new combo. I searched on the web and got a calculator that said the rpm would be around 5600 at 70 mph. I HOPE not!!!
Thanks! Very helpful. Only thing I can't believe is the estimated 1/4 time. The calculator says 13.07 and 104mph. I seriously doubt a big Chrysler can do that with a 440 hp
Joined: Thu Nov 24 2005, 07:29PM
Location: So Cal.
Posts: 1111
67Chrysler300 wrote ...
Thanks! Very helpful. Only thing I can't believe is the estimated 1/4 time. The calculator says 13.07 and 104mph. I seriously doubt a big Chrysler can do that with a 440 hp
You're right- it won't.
My '68 300 with a "TNT Spec" (I swapped cams and manifolds on a stock 440 to TNT stuff) did a 15.8 after I adjusted the kickdown linkage to make the transmission shift earlier. My first run was a low 16.x and I could feel the trans holding on too long. I also had 2.56 gears but I know that 3.55's won't cut nearly three seconds off of your time!
I have 3.55's and a 4-speed now but the same 440 engine. It feels quicker but I also have a '65 Bonneville that does 12.9 at 107 mph (with 3.23's) that I know would kick my 300's butt all up and down the strip.