Engine Compression Ratio (CR) Calculator

Enter the measurement designation in either inches, 1, or millimeters, 2. Enter the cylinder bore diameter. Enter the piston stroke length. Click on Calculate Displacement. The value will be returned in CR, Compression Ratio from ALL input values excluding Total Volume. You can also calculate Total Volume from bore, stroke, and CR. This returned value for volume should be measured with the piston at top dead center, with all valves closed and with the head gasket installed, or, Calculate Combustion Chamber Volume from ALL input values except Combustion Chamber Volume and Total Volume by entering the required values and clicking on the proper Calculate button. To do another, click the Clear Values button and then enter a new set of values. Calculated accuracy of current Javascript enabled browsers is generally 16 places.

Enter Bore/Stroke Designation Type: 1 = Inches 2 = Millimeters   
Enter Cylinder Bore Size   
Enter Piston Stroke Length   
Enter Head Gasket Bore Diameter
Enter Compressed Head Gasket Thickness
Enter Combustion Chamber Volume In CCs
Enter Piston Dome Volume In CCs Negative For Dished Pistons (Use '-')
Enter Piston Deck Clearance Negative If ABOVE Deck (Use '-') :
Calculated Engine Compression Ratio  :1
Total Displacement Volume  cc
Version 1.2.1

Gear Ratio Calculator

This calculator will get you back to a ratio that's closer to your original. Enter the values on the left and press Compute.

 Find new Gear Ratio with new tire size 
Old Tire Diameter: 
New Tire Diameter: 
Current Gear Ratio:  New Gear Ratio:

Ever wonder how far off your speedometer is with your new bigger tires? This uses a simple ratio calculation, so the info will only approximate your corrected speed and depends on the accuracy of tire size diameters (assuming nothing but tire size has changed). 

 Speedometer converter 
Original Tire Diameter: 
New Tire Diameter: 
Speedometer Reading (MPH):  Corrected Speed (MPH):

Ever wonder what your low-low-low gear ratio is, your "crawl ratio"? You'll need to do a little research by finding out the transfer case and transmission ratios first. Enter your values on the left side, press Compute and your answer will be on the right. 

  Find Overall Crawl Ratio 

Transmission Ratio:        

Transfer Case Ratio: 

Axle Ratio: 

Overall Ratio:

I've spend a bunch of time trying to WAG (wild ass guess) my actual MPH when going down the road. By knowing your gear ratios and tire size you'll get a more accurate idea here. Enter the value on the left, press Compute. 

  Find MPH 
RPM:    Tire Diameter: Axle Ratio:      
Transmission Ratio:                   TransCas Ratio:


I suppose if you don't have a tachometer you could use this form, otherwise it's only useful to find out how accurate your Tach is. Enter the values on the left, press Compute. 

  Find RPM 


Axle Ratio:                

Transmission Ratio:  

Transfer Case Ratio:        

Tire Diameter:      


This formula is a nice way to get the tire diameter of those metric tires that are common on just about everything stock. For example a 265/75R16 would be around 31.6 inches tall. Enter the numbers into this form in the order you read them from the tire and press Compute. 

  Convert Metric tire to Standard 
Example: 195/75R15
Tire Diameter:

Credit for this page goes to Mark Medina. If you would like to see Marks page go here