Fitment guide
Speedometer gear calculation for an LS swap: the formula, worked out
Tire diameter and rear gear in, drive and driven tooth count out. The math behind the calculator, with an example you can check by hand.
Short answer
Driven gear teeth = (20,168 ÷ tire diameter in inches) × rear axle ratio × drive gear teeth ÷ 1,001. For a 26-inch tire, 3.73 gears and a 15-tooth drive gear: 20,168 ÷ 26 = 775.7 tire revs per mile, × 3.73 × 15 = 43,400, ÷ 1,001 = 43.4, so a 43-tooth driven gear.
What is the speedometer gear formula?
Driven gear teeth = (20,168 ÷ tire diameter) × rear axle ratio × drive gear teeth ÷ 1,001
- 20,168 ÷ tire diameter is tire revolutions per mile. 20,168 is the inches in a mile (63,360) divided by pi.
- × rear axle ratio turns tire revolutions into driveshaft (output shaft) revolutions.
- × drive gear teeth counts the teeth that pass the driven gear per mile.
- ÷ 1,001 is the cable revolutions per mile a GM mechanical speedometer is built to see.
This is the same formula TCI and Summit Racing publish (TCI, Summit), and it's what our speedometer gear calculator runs.
What's a worked example?
| Step | Example A: 26 in. tire, 3.73 gear, 15T drive | Example B: 28 in. tire, 3.55 gear, 15T drive |
|---|---|---|
| Tire revs per mile (20,168 ÷ tire) | 775.7 | 720.3 |
| × axle ratio | 2,893.3 | 2,557.0 |
| × drive gear teeth | 43,400 | 38,355 |
| ÷ 1,001 = driven teeth (exact) | 43.36 | 38.32 |
| Driven gear to use | 43T | 38T |
Driven gears only come in whole teeth, so you round to the nearest one. The calculator also shows every drive gear option with the same tire and ratio, so you can pick the combination with the smallest error.
How do I measure tire diameter for the speedometer?
Measure it loaded, on the ground, not off the sidewall or the size printed on it. A loaded tire squats at the contact patch, and the published size is rarely the rolling size. For a better number, Hot Rod recommends the rolling-circumference method: chalk the tire and the ground, roll the car one full revolution, measure between the marks and divide by pi (Hot Rod). If you land between sizes, go bigger and call us.
Does the transmission's overdrive change the speedometer gears?
No. The speedometer drive gear is on the output shaft, behind all the transmission's gearing, so the 4L60E's 0.70 and the 4L80E's 0.75 overdrive don't enter the formula. Only tire diameter, rear axle ratio and the drive gear tooth count do. That's why we ask for three numbers when you order: tire diameter, rear axle ratio and transmission model.
How do I check the speedometer after install?
Mechanical speedometers are most accurate at 60 mph. Check against a GPS or a measured mile at a steady 60 (Hot Rod). If the odometer is right but the needle is off, the problem is inside the speedometer head, not the gears.
What if I change tires or rear gears later?
Run the numbers again. A new tire size or rear ratio changes the driven gear count. Call the shop with the new numbers and we'll work out the gears.
Which kits ship with gears cut to my numbers?
Our tail housings and speed kits ship with gears cut to your calculated tooth count: the 4L60E / 4L65E / 4L70E speed kit ($850), 4L60E tail housing kit ($850), 4L80E tail housing kit ($900) and 4L80E speedometer kit ($900). New to the idea? Start with how to keep your mechanical speedometer with an LS swap.
Frequently asked questions
What is the formula for speedometer gears?
Driven gear teeth = (20,168 ÷ tire diameter in inches) × rear axle ratio × drive gear teeth ÷ 1,001. Round to the nearest whole tooth.
What does 1,001 mean in the speedometer gear formula?
It is the number of cable revolutions per mile a GM mechanical speedometer is calibrated for. When the driven gear turns the cable 1,001 times per mile, the gauge reads true.
Does the 4L60E or 4L80E overdrive ratio affect speedometer gearing?
No. The speedometer drive gear sits on the output shaft after all the transmission's gearing, so only tire diameter, rear axle ratio and the drive gear tooth count matter.
How should I measure tire diameter for speedometer gears?
Measure the tire loaded, on the ground, rather than using the sidewall size. For more accuracy, roll the car one tire revolution, measure the distance and divide by pi.
Talk to the shop
Every housing and speed kit is machined in Caddo Mills and ships with gears cut to your tire diameter and rear axle ratio. Questions about your combination? Call 903-527-2006, Monday to Friday, 7am–3pm Central, or email [email protected].
Shop the kitsGear calculatorSources
- TCI: Speedometer gear calculator and formula
- Summit Racing: What speedo gears should I buy?
- Hot Rod: How to calibrate GM mechanical speedometers
Kit details come from Southwest Fabrication's own product listings. Outside technical figures are linked above; check them against your exact parts.
