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

strange fellow!



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 Pasadena

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

goes! --->



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 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!