Figuring out my engine RPM
I guess I should thank my junior high school algebra teacher for this
story. So Thank You Mr. Kumbe, even though I did think you were quite a
After about 7 years I finally pulled my '66 Chevelle out of my
brother's garage (well, he is actually co-owner but that's another
story). A bad radiator and a slipping Powerglide brought it to rest and
it was finally time to get it going again. With a new cooling system
installed I brought the car over
to Foothill Transmission in
for a transmission swap. A TH-350 with a shift kit and 1800 stall
converter was installed and I was good to go! It felt like a totally
different car. After I drove it around for a while I remembered I never
quite figured out what my engine RPM's were (crusing at 60MPH) since
this car doesn't have a tachometer (yet).
I found the following formula on the internet:
MPH = RPM x Tire Diameter (in inches)
Rearend Gear Ratio x 336
After 10 minutes of head scratching and almost just forgetting about it
my mathematical sense re-awakened. I hope my algebra is correct. Here
assuming 3.08 Gears in my '66 Chevelle cruising at 60MPH
x = RPM
60 = x * Tire Diameter (in inches)
1034.88 (3.08 x 336)
Now I needed to do some algebra to isolate the RPM variable. --->
multiplied both sides by 1034.88
this brings the formula to:
62092.8 = x * Tire Diameter (in inches)
then, dividing variable "Tire Diameter" on both sides
brings the formula to:
_______ = x
Tire Diameter <--- now I have my adjusted formula
Oh wait, now I had to calculate how tall my tires were. This is the “Tire Diameter” in the formula. Of course you could just take a measuring tape to your tires but you should understand the math behind tire sizes.
In my case :
My Chevelle with 14" rims and 215/70 R14 tires:
215/70 R14 is the tire size as printed on my tires. Most people get very confused here. Here is the breakdown :
215 is the tire width in millimeters. The 70 means each sidewall (top and bottom) is 70% of the width of the tire, or 215 millimeters in my case. R just means Radial and 14 means a 14 inch tire and rim.
Converting sidewall mumbo-jumbo to height in inches: That’s 70% of 215 millimeters (150.5 mm) multiplied by 2 for each sidewall (301 mm), converted to inches, then adding in the rim diameter (14 in).
Hint: If you visit http://www.google.com and type in 301 mm to inches, for example, Google will automatically calculate the conversion for you.
Once again, in my case:
(combined sidewall height + rim) --->
11.85 + 14 = 25.85 inch tall tire
Now I finally plugged it in the formula:
_______ = 2402 RPM
Now I know what RPM my 350 engine is cruising at!
So now I was curious how a different tire size would affect my RPM's...
I decided on three possible tires sizes using 16" rims.
Here are the calculations:
With rear wheel 245/50 R16 tire
9.65 + 16 = 25.65 inch tall tire
_______ = 2421 RPM
With rear wheel 265/50 R16 tire
10.43 + 16 = 26.43 inch tall tire
_______ = 2349 RPM
With rear wheel 295/50 R16 tire
11.61 + 16 = 27.61 inch tall tire
_______ = 2249 RPM
I guess you could say that 17 years later I finally had fun with algebra. I am such a fuckin’ geek!