Mailbag: Tire Size, Gear Ratio & Stall Speed–and How They Relate

Got questions?

We’ve got the answers—the Summit Racing tech department tackles your automotive-related conundrums. This week, we’re nailing down the right combination of tire size, gear ratio, and torque converter stall speed for a Pro Street Plymouth.

C.C. Governors Island, NY

Q: I am building a 1948 Pro Street Plymouth Club Coupe for weekend cruising and a little racing action. The chassis is subframed with a 1981 Dodge Diplomat K member and narrowed rear rails. I’m running 36-inch ladder bars, coilovers, and a panhard sway bar. The car should weigh approximately 2,900 pounds when finished.

The powerplant is a 440. It has a .030-inch over police block, 9:1 TRW pistons, 454-casting heads with port work and a competition valve job, Mopar Performance camshaft (284-degree duration, .484-inch lift), Edelbrock Six Pack intake manifold with Holley carburetors, Mopar Performance vacuum advance distributor, and an MSD 6A ignition with a Blaster coil. Exhaust consists of 1 7/8-inch primary headers and 2 1/2-inch dual pipes with Flowmaster mufflers.

The transmission is a 727 Torqueflite that will be rebuilt with a B&M Shift Kit. The rear-end is a narrowed Dana 60 with a Trac-Loc posi.

Here are my questions:

1. What is the best rear-end gear for this application. I have 3.54 and 4.10 gear sets, but I can get a different one if you recommend it. I am thinking about using the 4.10 gear and a 2.76 first gear in the Torqueflite. Would this give me a good holeshot ability and a decent cruising rpm?
2. I have a set of Mickey Thompson 31 x 16.5 -15 tires, but I think they might be too tall. I am leaning towards smaller 29 x 15.5-15 tires. What do you think?
3. What stall speed torque converter should I be using? Some folks say 3,500 rpm is the absolute minimum; others say 2,800 rpm is OK. I am using a big transmission cooler regardless of the converter stall.

A: We’ll answer all three questions as a group since they are interdependent. First, let’s set up some guidelines:

• You’re running a big block, which means lots of torque.
• Your camshaft makes its best power between 2,500 and 5,500 rpm.
• Your car is fairly light at 2,900 pounds.

These three factors, plus transmission gear ratio, rear-end ratio, and tire size, affects the stall speed of a torque converter. Given that, we recommend a 2,500 rpm stall converter for the following reasons:

• You cam starts making power at 2,500, so you don’t need a converter that stalls any higher.
• The car will be primarily street-driven. Higher stall converters slip at rpms lower than their stall ratings, which creates a lot of heat.

A converter, regardless of its stall rating, can have different stall points depending on the application it is used in. Behind a big block like your 400, the converter will stall at a higher rpm. Here is where tire size and gear ratio come into play. Using this handy gear ratio calculator, we generated the following chart which shows engine speed at 60 miles-per-hour with the different combinations of the tire sizes and gear ratios you mentioned:

Tire diameter: 29-inch + Gear ratio: 3.54 = 2,460 rpm at 60 mph

Tire diameter: 29-inch + Gear ratio: 4.10 = 2,850 rpm at 60 mph

Tire diameter: 31-inch + Gear ratio: 3.54 = 2,300 rpm at 60 mph

Tire diameter: 31-inch + Gear ratio: 4.10 = 2,660 rpm at 60 mph

As you can see, using a 3.54 with either tire size puts you under the converter’s stall rating, which can burn up your transmission under continuous use. The 4.10 gear with 31-inch tires locks up your converter at high way speeds, which is what you want. The 4.10 gear with 29-inch tires will raise your engine rpm close to 3,000 rpm, which is a lot for cruising. Combining the 4.10 gear, 31-inch tires, and that low transmission first gear ratio should give you the launch you want without compromising streetability.