April 20, 2022

Solving the Diff Ratio Mystery

The diff ratio used in the Red Rocket has been a bit of a mystery until now. The original Owner's Manual (which came with the car) has two hand-written notes which indicate a 3.7:1 ratio was fitted to the car, however the code 'AP' stamped on the diff housing indicates a 4.11:1 (refer to Corvette Rear Axle Codes & Ratios link below). The diff housing date code is 1-14-65 (14 January 1965), which suggests that the housing is original to the car.

When the car was at S R Performance in March 2022 they measured the diff ratio as 4.11:1. Using some simple maths, and knowing that the rear tyres are 28" tall (711mm), I calculated that at 3000rpm a 4.11:1 ratio would give a road speed of 98km/h. This is almost exactly what I see in the car, so it confirms the 4.11:1 diff.

5000rpm with a 4.11:1 diff would be 163km/h — just over 100mph.

For comparison, if the diff was changed to a 3.7:1, revs at 100km/h would drop by 300rpm to 2760 — this would probably deliver a decent fuel saving. 5000rpm would be 181km/h, almost 20km/h more than the 4.11:1.

I feel like the car is very happy cruising at around 3000rpm, and 100km/h comes up in under 6 seconds with the 4.11. Driving at 160km/h on Australian roads will send you to jail, so I think the 4.11 gears are a good choice for the L79 engine.

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