Speedometer Gear computations for the Various TurboCoupe Rear End and Tire Combinations


This table shows the driven gear which will yield the most accurate speedometer for a given tire size, rear end, and drive gear. Turbo Coupes came with the six tooth drive gear, but the transmission may have a seven tooth gear as a result of a swap with a Mustang. Negative error percentages indicate that indicated speed is lower than actual speed.

All computations are based on the assumption that the speedometers are calibrated for 1000 sensor rotations = 1 mile. If the speedometers are calibrated differently, let me know and I will re-compute the table.
Rear Gear Ratio Tire Width Aspect Ratio Wheel Diameter Tire Height Tire Circumference Driven Gear needed with six tooth drive gear Driven Gear needed with seven tooth drive gear Error with six tooth drive gear Error with seven tooth drive gear
3.45 205 55 14 22.9 71.9 18 21 1.49% 1.49%
3.45 205 60 14 23.7 74.4 18 21 -1.99% -1.99%
3.45 205 70 14 25.3 79.5 17 19 -2.88% 1.44%
3.45 205 75 14 26.1 82.0 16 19 0.08% -1.71%
3.45 215 55 14 23.3 73.2 18 21 -0.38% -0.38%
3.45 215 60 14 24.2 75.9 17 20 1.76% 0.93%
3.45 215 70 14 25.9 81.2 16 19 1.06% -0.71%
3.45 215 75 14 26.7 83.9 16 18 -2.18% 1.47%
3.45 225 55 14 23.7 74.6 18 21 -2.24% -2.24%
3.45 225 60 14 24.6 77.4 17 20 -0.16% -1.00%
3.45 225 70 14 26.4 82.9 16 18 -1.05% 2.56%
3.45 225 75 14 27.3 85.7 15 18 2.09% -0.71%
3.45 235 55 14 24.2 76.0 17 20 1.68% 0.85%
3.45 235 60 14 25.1 78.9 17 19 -2.08% 2.21%
3.45 235 70 14 27.0 84.7 16 18 -3.16% 0.52%
3.45 235 75 14 27.9 87.6 15 17 -0.03% 2.82%
3.45 245 55 14 24.6 77.3 17 20 -0.08% -0.92%
3.45 245 60 14 25.6 80.3 16 19 2.11% 0.37%
3.45 245 70 14 27.5 86.4 15 18 1.31% -1.51%
3.45 245 75 14 28.5 89.4 15 17 -2.15% 0.77%
3.45 255 55 14 25.0 78.7 17 19 -1.84% 2.44%
3.45 255 60 14 26.0 81.8 16 19 0.30% -1.48%
3.45 255 70 14 28.1 88.1 15 17 -0.67% 2.21%
3.45 255 75 14 29.1 91.3 14 17 2.68% -1.29%
3.45 205 50 15 23.1 72.5 18 21 0.66% 0.66%
3.45 205 60 15 24.7 77.6 17 20 -0.39% -1.23%
3.45 205 70 15 26.3 82.6 16 19 -0.66% -2.46%
3.45 205 75 15 27.1 85.2 15 18 2.74% -0.04%
3.45 215 55 15 24.3 76.4 17 20 1.13% 0.30%
3.45 215 60 15 25.2 79.0 17 19 -2.31% 1.99%
3.45 215 70 15 26.9 84.4 16 18 -2.77% 0.90%
3.45 215 75 15 27.7 87.0 15 18 0.62% -2.22%
3.45 225 55 15 24.7 77.7 17 20 -0.63% -1.47%
3.45 225 60 15 25.6 80.5 16 19 1.90% 0.15%
3.45 225 70 15 27.4 86.1 15 18 1.68% -1.13%
3.45 225 75 15 28.3 88.9 15 17 -1.50% 1.40%
3.45 235 55 15 25.2 79.1 17 19 -2.39% 1.91%
3.45 235 60 15 26.1 82.0 16 19 0.09% -1.69%
3.45 235 70 15 28.0 87.8 15 17 -0.30% 2.56%
3.45 235 75 15 28.9 90.7 14 17 3.29% -0.66%
3.45 245 55 15 25.6 80.5 16 19 1.98% 0.23%
3.45 245 60 15 26.6 83.5 16 18 -1.71% 1.92%
3.45 245 70 15 28.5 89.5 15 17 -2.28% 0.64%
3.45 245 75 15 29.5 92.6 14 17 1.31% -2.72%
3.45 255 55 15 26.0 81.8 16 19 0.32% -1.46%
3.45 255 60 15 27.0 85.0 15 18 2.95% 0.17%
3.45 255 70 15 29.1 91.3 14 17 2.69% -1.28%
3.45 255 75 15 30.1 94.4 14 16 -0.67% 1.39%
3.45 215 55 16 25.3 79.5 17 19 -2.93% 1.39%
3.45 215 60 16 26.2 82.2 16 19 -0.12% -1.90%
3.45 215 70 16 27.9 87.5 15 18 0.07% -2.79%
3.45 215 75 16 28.7 90.2 15 17 -2.97% -0.03%
3.45 225 55 16 25.7 80.9 16 19 1.47% -0.29%
3.45 225 60 16 26.6 83.7 16 18 -1.93% 1.71%
3.45 225 70 16 28.4 89.2 15 17 -1.91% 1.00%
3.45 225 75 16 29.3 92.0 14 17 1.92% -2.09%
3.45 235 55 16 26.2 82.2 16 19 -0.19% -1.98%
3.45 235 60 16 27.1 85.1 15 18 2.75% -0.03%
3.45 235 70 16 29.0 91.0 14 17 3.04% -0.92%
3.45 235 75 16 29.9 93.9 14 16 -0.06% 1.98%
3.45 245 55 16 26.6 83.6 16 18 -1.85% 1.79%
3.45 245 60 16 27.6 86.6 15 18 1.05% -1.77%
3.45 245 70 16 29.5 92.7 14 17 1.19% -2.84%
3.45 245 75 16 30.5 95.7 14 16 -2.04% 0.04%
3.45 255 55 16 27.0 85.0 15 18 2.96% 0.19%
3.45 255 60 16 28.0 88.1 15 17 -0.64% 2.23%
3.45 255 70 16 30.1 94.4 14 16 -0.66% 1.40%
3.45 255 75 16 31.1 97.6 13 16 3.41% -1.90%
3.55 205 55 14 22.9 71.9 19 22 -1.32% -0.56%
3.55 205 60 14 23.7 74.4 18 21 0.63% 0.63%
3.55 205 70 14 25.3 79.5 17 20 -0.25% -1.09%
3.55 205 75 14 26.1 82.0 16 19 2.64% 0.90%
3.55 215 55 14 23.3 73.2 18 21 2.20% 2.20%
3.55 215 60 14 24.2 75.9 18 21 -1.35% -1.35%
3.55 215 70 14 25.9 81.2 17 19 -2.43% 1.87%
3.55 215 75 14 26.7 83.9 16 19 0.44% -1.34%
3.55 225 55 14 23.7 74.6 18 21 0.38% 0.38%
3.55 225 60 14 24.6 77.4 17 20 2.41% 1.59%
3.55 225 70 14 26.4 82.9 16 19 1.54% -0.22%
3.55 225 75 14 27.3 85.7 16 18 -1.76% 1.87%
3.55 235 55 14 24.2 76.0 18 21 -1.44% -1.44%
3.55 235 60 14 25.1 78.9 17 20 0.53% -0.30%
3.55 235 70 14 27.0 84.7 16 19 -0.52% -2.31%
3.55 235 75 14 27.9 87.6 15 18 2.53% -0.25%
3.55 245 55 14 24.6 77.3 17 20 2.48% 1.66%
3.55 245 60 14 25.6 80.3 17 20 -1.34% -2.19%
3.55 245 70 14 27.5 86.4 16 18 -2.57% 1.09%
3.55 245 75 14 28.5 89.4 15 18 0.47% -2.38%
3.55 255 55 14 25.0 78.7 17 20 0.77% -0.07%
3.55 255 60 14 26.0 81.8 16 19 2.86% 1.13%
3.55 255 70 14 28.1 88.1 15 18 1.91% -0.89%
3.55 255 75 14 29.1 91.3 15 17 -1.60% 1.30%
3.55 205 50 15 23.1 72.5 19 22 -2.17% -1.40%
3.55 205 60 15 24.7 77.6 17 20 2.19% 1.37%
3.55 205 70 15 26.3 82.6 16 19 1.92% 0.17%
3.55 205 75 15 27.1 85.2 16 18 -1.09% 2.52%
3.55 215 55 15 24.3 76.4 18 21 -2.00% -2.00%
3.55 215 60 15 25.2 79.0 17 20 0.32% -0.52%
3.55 215 70 15 26.9 84.4 16 19 -0.13% -1.92%
3.55 215 75 15 27.7 87.0 15 18 3.16% 0.40%
3.55 225 55 15 24.7 77.7 17 20 1.95% 1.13%
3.55 225 60 15 25.6 80.5 17 20 -1.56% -2.41%
3.55 225 70 15 27.4 86.1 16 18 -2.19% 1.46%
3.55 225 75 15 28.3 88.9 15 18 1.10% -1.73%
3.55 235 55 15 25.2 79.1 17 20 0.24% -0.60%
3.55 235 60 15 26.1 82.0 16 19 2.66% 0.92%
3.55 235 70 15 28.0 87.8 15 18 2.27% -0.52%
3.55 235 75 15 28.9 90.7 15 17 -0.96% 1.92%
3.55 245 55 15 25.6 80.5 17 20 -1.48% -2.33%
3.55 245 60 15 26.6 83.5 16 19 0.89% -0.88%
3.55 245 70 15 28.5 89.5 15 18 0.34% -2.50%
3.55 245 75 15 29.5 92.6 15 17 -3.03% -0.09%
3.55 255 55 15 26.0 81.8 16 19 2.88% 1.14%
3.55 255 60 15 27.0 85.0 16 19 -0.87% -2.67%
3.55 255 70 15 29.1 91.3 15 17 -1.58% 1.32%
3.55 255 75 15 30.1 94.4 14 17 1.91% -2.09%
3.55 215 55 16 25.3 79.5 17 20 -0.29% -1.14%
3.55 215 60 16 26.2 82.2 16 19 2.45% 0.71%
3.55 215 70 16 27.9 87.5 15 18 2.63% -0.15%
3.55 215 75 16 28.7 90.2 15 17 -0.33% 2.54%
3.55 225 55 16 25.7 80.9 17 19 -2.01% 2.28%
3.55 225 60 16 26.6 83.7 16 19 0.69% -1.09%
3.55 225 70 16 28.4 89.2 15 18 0.70% -2.14%
3.55 225 75 16 29.3 92.0 15 17 -2.40% 0.53%
3.55 235 55 16 26.2 82.2 16 19 2.38% 0.63%
3.55 235 60 16 27.1 85.1 16 18 -1.07% 2.54%
3.55 235 70 16 29.0 91.0 15 17 -1.23% 1.67%
3.55 235 75 16 29.9 93.9 14 17 2.50% -1.48%
3.55 245 55 16 26.6 83.6 16 19 0.76% -1.01%
3.55 245 60 16 27.6 86.6 16 18 -2.84% 0.84%
3.55 245 70 16 29.5 92.7 15 17 -3.15% -0.21%
3.55 245 75 16 30.5 95.7 14 16 0.58% 2.61%
3.55 255 55 16 27.0 85.0 16 19 -0.85% -2.65%
3.55 255 60 16 28.0 88.1 15 18 1.94% -0.86%
3.55 255 70 16 30.1 94.4 14 17 1.93% -2.08%
3.55 255 75 16 31.1 97.6 14 16 -1.35% 0.72%
3.73 205 55 14 22.9 71.9 20 23 -1.45% 0.00%
3.73 205 60 14 23.7 74.4 19 22 0.23% 0.98%
3.73 205 70 14 25.3 79.5 18 21 -0.97% -0.97%
3.73 205 75 14 26.1 82.0 17 20 1.60% 0.78%
3.73 215 55 14 23.3 73.2 19 23 1.80% -1.89%
3.73 215 60 14 24.2 75.9 19 22 -1.76% -1.00%
3.73 215 70 14 25.9 81.2 17 20 2.57% 1.75%
3.73 215 75 14 26.7 83.9 17 20 -0.62% -1.47%
3.73 225 55 14 23.7 74.6 19 22 -0.02% 0.73%
3.73 225 60 14 24.6 77.4 18 21 1.71% 1.71%
3.73 225 70 14 26.4 82.9 17 20 0.49% -0.35%
3.73 225 75 14 27.3 85.7 17 19 -2.85% 1.47%
3.73 235 55 14 24.2 76.0 19 22 -1.85% -1.08%
3.73 235 60 14 25.1 78.9 18 21 -0.18% -0.18%
3.73 235 70 14 27.0 84.7 17 20 -1.59% -2.44%
3.73 235 75 14 27.9 87.6 16 19 1.11% -0.66%
3.73 245 55 14 24.6 77.3 18 21 1.78% 1.78%
3.73 245 60 14 25.6 80.3 18 21 -2.07% -2.07%
3.73 245 70 14 27.5 86.4 16 19 2.43% 0.69%
3.73 245 75 14 28.5 89.4 16 18 -0.99% 2.62%
3.73 255 55 14 25.0 78.7 18 21 0.06% 0.06%
3.73 255 60 14 26.0 81.8 17 20 1.82% 1.00%
3.73 255 70 14 28.1 88.1 16 19 0.48% -1.30%
3.73 255 75 14 29.1 91.3 16 18 -3.08% 0.60%
3.73 205 50 15 23.1 72.5 20 23 -2.30% -0.84%
3.73 205 60 15 24.7 77.6 18 21 1.49% 1.49%
3.73 205 70 15 26.3 82.6 17 20 0.87% 0.04%
3.73 205 75 15 27.1 85.2 17 19 -2.17% 2.13%
3.73 215 55 15 24.3 76.4 19 22 -2.41% -1.64%
3.73 215 60 15 25.2 79.0 18 21 -0.40% -0.40%
3.73 215 70 15 26.9 84.4 17 20 -1.20% -2.05%
3.73 215 75 15 27.7 87.0 16 19 1.75% -0.01%
3.73 225 55 15 24.7 77.7 18 21 1.25% 1.25%
3.73 225 60 15 25.6 80.5 18 21 -2.29% -2.29%
3.73 225 70 15 27.4 86.1 16 19 2.80% 1.06%
3.73 225 75 15 28.3 88.9 16 19 -0.35% -2.14%
3.73 235 55 15 25.2 79.1 18 21 -0.48% -0.48%
3.73 235 60 15 26.1 82.0 17 20 1.62% 0.79%
3.73 235 70 15 28.0 87.8 16 19 0.84% -0.93%
3.73 235 75 15 28.9 90.7 16 18 -2.44% 1.22%
3.73 245 55 15 25.6 80.5 18 21 -2.21% -2.21%
3.73 245 60 15 26.6 83.5 17 20 -0.16% -1.01%
3.73 245 70 15 28.5 89.5 16 18 -1.12% 2.50%
3.73 245 75 15 29.5 92.6 15 18 2.00% -0.80%
3.73 255 55 15 26.0 81.8 17 20 1.84% 1.01%
3.73 255 60 15 27.0 85.0 17 19 -1.94% 2.34%
3.73 255 70 15 29.1 91.3 16 18 -3.07% 0.61%
3.73 255 75 15 30.1 94.4 15 18 0.03% -2.82%
3.73 215 55 16 25.3 79.5 18 21 -1.01% -1.01%
3.73 215 60 16 26.2 82.2 17 20 1.41% 0.58%
3.73 215 70 16 27.9 87.5 16 19 1.20% -0.56%
3.73 215 75 16 28.7 90.2 16 18 -1.80% 1.84%
3.73 225 55 16 25.7 80.9 18 20 -2.74% 2.15%
3.73 225 60 16 26.6 83.7 17 20 -0.37% -1.22%
3.73 225 70 16 28.4 89.2 16 19 -0.75% -2.55%
3.73 225 75 16 29.3 92.0 15 18 2.60% -0.18%
3.73 235 55 16 26.2 82.2 17 20 1.33% 0.51%
3.73 235 60 16 27.1 85.1 17 19 -2.15% 2.14%
3.73 235 70 16 29.0 91.0 16 18 -2.71% 0.96%
3.73 235 75 16 29.9 93.9 15 18 0.63% -2.20%
3.73 245 55 16 26.6 83.6 17 20 -0.30% -1.14%
3.73 245 60 16 27.6 86.6 16 19 2.18% 0.43%
3.73 245 70 16 29.5 92.7 15 18 1.88% -0.92%
3.73 245 75 16 30.5 95.7 15 17 -1.33% 1.57%
3.73 255 55 16 27.0 85.0 17 19 -1.93% 2.35%
3.73 255 60 16 28.0 88.1 16 19 0.50% -1.27%
3.73 255 70 16 30.1 94.4 15 18 0.05% -2.81%
3.73 255 75 16 31.1 97.6 15 17 -3.29% -0.34%
4.10 205 55 14 22.9 71.9 22 25 -1.45% 1.19%
4.10 205 60 14 23.7 74.4 21 24 -0.25% 1.79%
4.10 205 70 14 25.3 79.5 20 23 -1.99% -0.53%
4.10 205 75 14 26.1 82.0 19 22 0.02% 0.78%
4.10 215 55 14 23.3 73.2 21 25 1.33% -0.68%
4.10 215 60 14 24.2 75.9 21 24 -2.25% -0.16%
4.10 215 70 14 25.9 81.2 19 22 1.00% 1.75%
4.10 215 75 14 26.7 83.9 19 22 -2.24% -1.47%
4.10 225 55 14 23.7 74.6 21 24 -0.50% 1.55%
4.10 225 60 14 24.6 77.4 20 24 0.71% -2.12%
4.10 225 70 14 26.4 82.9 19 22 -1.11% -0.35%
4.10 225 75 14 27.3 85.7 18 21 1.00% 1.00%
4.10 235 55 14 24.2 76.0 21 24 -2.34% -0.25%
4.10 235 60 14 25.1 78.9 20 23 -1.19% 0.25%
4.10 235 70 14 27.0 84.7 18 21 2.21% 2.21%
4.10 235 75 14 27.9 87.6 18 21 -1.14% -1.14%
4.10 245 55 14 24.6 77.3 20 24 0.79% -2.04%
4.10 245 60 14 25.6 80.3 19 23 2.06% -1.62%
4.10 245 70 14 27.5 86.4 18 21 0.21% 0.21%
4.10 245 75 14 28.5 89.4 17 20 2.45% 1.63%
4.10 255 55 14 25.0 78.7 20 23 -0.95% 0.49%
4.10 255 60 14 26.0 81.8 19 22 0.25% 1.00%
4.10 255 70 14 28.1 88.1 18 21 -1.79% -1.79%
4.10 255 75 14 29.1 91.3 17 20 0.43% -0.41%
4.10 205 50 15 23.1 72.5 22 25 -2.30% 0.35%
4.10 205 60 15 24.7 77.6 20 23 0.49% 1.91%
4.10 205 70 15 26.3 82.6 19 22 -0.72% 0.04%
4.10 205 75 15 27.1 85.2 18 21 1.66% 1.66%
4.10 215 55 15 24.3 76.4 20 24 2.00% -0.80%
4.10 215 60 15 25.2 79.0 20 23 -1.41% 0.04%
4.10 215 70 15 26.9 84.4 18 22 2.59% -2.05%
4.10 215 75 15 27.7 87.0 18 21 -0.49% -0.49%
4.10 225 55 15 24.7 77.7 20 23 0.25% 1.68%
4.10 225 60 15 25.6 80.5 19 23 1.85% -1.84%
4.10 225 70 15 27.4 86.1 18 21 0.59% 0.59%
4.10 225 75 15 28.3 88.9 18 20 -2.63% 2.26%
4.10 235 55 15 25.2 79.1 20 23 -1.49% -0.04%
4.10 235 60 15 26.1 82.0 19 22 0.04% 0.79%
4.10 235 70 15 28.0 87.8 18 21 -1.41% -1.41%
4.10 235 75 15 28.9 90.7 17 20 1.05% 0.22%
4.10 245 55 15 25.6 80.5 19 23 1.92% -1.76%
4.10 245 60 15 26.6 83.5 19 22 -1.77% -1.01%
4.10 245 70 15 28.5 89.5 17 20 2.33% 1.51%
4.10 245 75 15 29.5 92.6 17 20 -0.97% -1.82%
4.10 255 55 15 26.0 81.8 19 22 0.26% 1.01%
4.10 255 60 15 27.0 85.0 18 21 1.87% 1.87%
4.10 255 70 15 29.1 91.3 17 20 0.44% -0.39%
4.10 255 75 15 30.1 94.4 17 19 -3.00% 1.33%
4.10 215 55 16 25.3 79.5 20 23 -2.03% -0.58%
4.10 215 60 16 26.2 82.2 19 22 -0.17% 0.58%
4.10 215 70 16 27.9 87.5 18 21 -1.04% -1.04%
4.10 215 75 16 28.7 90.2 17 20 1.67% 0.84%
4.10 225 55 16 25.7 80.9 19 22 1.41% 2.15%
4.10 225 60 16 26.6 83.7 19 22 -1.98% -1.22%
4.10 225 70 16 28.4 89.2 17 20 2.68% 1.86%
4.10 225 75 16 29.3 92.0 17 20 -0.35% -1.20%
4.10 235 55 16 26.2 82.2 19 22 -0.25% 0.51%
4.10 235 60 16 27.1 85.1 18 21 1.67% 1.67%
4.10 235 70 16 29.0 91.0 17 20 0.79% -0.04%
4.10 235 75 16 29.9 93.9 17 19 -2.38% 1.93%
4.10 245 55 16 26.6 83.6 19 22 -1.91% -1.14%
4.10 245 60 16 27.6 86.6 18 21 -0.04% -0.04%
4.10 245 70 16 29.5 92.7 17 20 -1.09% -1.94%
4.10 245 75 16 30.5 95.7 16 19 1.74% -0.01%
4.10 255 55 16 27.0 85.0 18 21 1.89% 1.89%
4.10 255 60 16 28.0 88.1 18 21 -1.76% -1.76%
4.10 255 70 16 30.1 94.4 17 19 -2.98% 1.34%
4.10 255 75 16 31.1 97.6 16 19 -0.16% -1.95%

The presence of a tire size on this listing does not mean that the tire will have adequate clearance, or is even a good idea. Some tooth sizes listed may not actually be available.