RE: Tire Sizes and how to calculate

[mikelj@xxxxxxx]

Here is a discussion thread that has the formula when I was trying to determine the rear end gear ratio of my '64 Polara 500, from Don Dulmage.  Just replace the factors to solve for tire size (diameter) for a given RPM.

Mike in New Mexico

 

From: "Don Dulmage" <big-d@xxxxxxxxxxxx> 
To: 1962to1965mopars@xxxxxxxxxx 
Sent: Wednesday, February 25, 2009 5:42:54 AM GMT -08:00 US/Canada Pacific 
Subject: Re: Rear-end gear ratios 
 
 
Why not go for a drive. take the RPM and the MPh and just do the math. I 
would have someone veryiythe speedo witha neewer car. I get My wife to check 
ine with her PT cruiser. I tell her to drive at 50 MPH which she will do 
exactly and then follow and check mine. I correct my speedo error mentaly 
After that read the tach . Do the math for the rear tire rollout. Work that 
back into the fifty MPH which will give you axle rpm per hour . divide that 
by sixty for axel RPM per minute. Divide that into engine RPM and Bingo! 
minus a tu iny bit of torqueconverter slip you will have the number. Since 
gear ratios are only available in steps it is a simple matter then to figure 
out which one you have. 
I know it seems like a bigdeal since we mostlyhate math but in fact a matter 
of minutes will do it. Certainly less time than to jack up the car. 
Don 
ie 28 inch tire rolls 87.92 inches per revolution. or 7.3266 feet 
50 MPH is 50 X 5280 which is 264000 feet per hour divided by 60 which is 
4400 feet per minute. 
sincethetire rolls 7.3266 feet per revolution and 50MP is 4400 feet per 
minute then 4400/7.3266= 600Axle RPM 
if the engine is turning 2500 RPM at 50 then  2500/600= 4.1666666666666666 
gear ratio. Since the nearestpossible gearratio is 4.30 then we know this 
car has 4.30s. It could not be 4.10s because you cannont invent speedonly 
ose a bit to driveline inefficiency. 
Now i have only had one coffee today so my math may need correction but the 
idea is right. It is based on the same way we figure Gear ratio and speed 
for racing except just working from the other end. 
----- Original Message ----- 
Don 
> 
> mikelj@xxxxxxx wrote: 
>> 
>> 
>> 
>> I am curious what the gear ratio is for the rear-end of my '64 Dodge 
>> Polara 500.  I can't find anything about that on the window sticker, 
>> build-sheet, tag, etc.  It seems like it is around 3:00, and I am sure 
>> they didn't have many choices in the mundane 383 2 bbl. TorqueFlight 
>> setup.  Anyone know? 
> 





  


 

> To: 1962to1965mopars@xxxxxxxxxx
> From: 64Polara500@xxxxxxxxx
> Subject: Tire Sizes and how to calculate
> Date: Tue, 9 Jun 2009 22:58:03 -0500
> 
> Does anyone know how to calculate final tire size when trying to keep
> the RPM's down? What is the typical RPM range I should try to keep it
> in and at what speeds? I was figuring about 2800 RPM at about 60-65
> mph.....We have a 3.91 Sure Grip rear...Want to get the right tires
> the first time.... :) Is there a formula?
> 
> Also was the typical final gear on the A727 1.00??
> 
> Thanks all!
> 
> Layne
> '64 Polara 500
> '08 Charger
> '08 Ram (yeah it's got a Hemi)

--
[Non-text portions of this message have been removed]


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