Car and Auto Calculators
These calculators cover the numbers drivers and car enthusiasts look up most. Thinking about bigger wheels or a different tire? Compare two sizes in the usual 225/65R17 format to see the change in diameter, circumference and revolutions per mile, and what your speedometer will show against your real speed. Changing gears or axle ratio? Enter the transmission gear, final drive and tire diameter to get engine RPM at any speed, with a table from 30 to 80 mph.
The horsepower and torque calculator solves for horsepower, torque or RPM from the other two using the standard 5,252 formula, and the oil change calculator works out your next service by mileage and by date from the oil you use, how hard you drive and how many miles you cover a month. Each tool runs in your browser with nothing to install. The results come from standard formulas and typical intervals, so your owner's manual and a proper speedometer recalibration after a tire change have the final word.
4 free tools in this category. All run in your browser with no signup.
Tire size and speedometer error
Enter your current tire and the one you are considering to see the size difference and your real speed when the speedometer reads 60 mph. The calculator flags any change of more than 3%.
Gear ratio, RPM and horsepower
Find cruising RPM for a gear and axle ratio, or convert between horsepower, torque and RPM with HP = torque x RPM / 5,252, including kilowatts and newton meters.
Maintenance and running costs
Know when the next oil change is due in miles and on the calendar. For trip fuel costs and car payments, see the fuel cost calculator and the auto loan calculator.
Frequently asked questions
How much will bigger tires throw off my speedometer?
By the same percentage as the change in the tire's overall diameter. Going from 225/65R17 to 245/60R18 makes the tire about 3.7% taller, so when the speedometer shows 60 mph you are really doing about 62.2 mph, and the odometer undercounts your miles by the same amount. The tire size calculator flags any change over 3%, a common rule of thumb for staying close to the original size.
How do I calculate engine RPM at a given speed?
RPM = speed in mph x gear ratio x final drive ratio x 336 / tire diameter in inches. For example, 70 mph in a 0.75 overdrive gear with a 3.73 axle ratio and 28.5 inch tires works out to about 2,309 RPM. A numerically higher axle ratio or a smaller tire raises the RPM at the same speed.
Why do horsepower and torque curves cross at 5,252 RPM?
Horsepower equals torque in lb-ft times RPM divided by 5,252. That constant comes from the definition of one horsepower, 33,000 foot-pounds of work per minute, divided by 2 times pi, so at exactly 5,252 RPM the two numbers are equal. Below that speed the torque figure is higher, and above it the horsepower figure is higher.
How often should I change my oil?
Follow your owner's manual or the car's oil life monitor first. As a rough guide, the oil change calculator starts from 3,000 miles for conventional oil, 5,000 for a synthetic blend and 7,500 to 15,000 for full and extended-life synthetics, then shortens the interval by a quarter for severe driving such as towing, stop-and-go traffic or dusty roads.