Ask Away! with Jeff Smith: How Do I Drive My Pro Street Car at Highway Speeds Without Damaging Gears?

I have a Pro Street car with a big-block Chevy, TH400, Dana 60 and 4.30 gears. I just bought this car a few months ago. The gear ratio is what the previous owner told me it had. At approximately 50 mph I’m at 3,000 rpm. I’m running a 31-inch tall tire. How fast can I go on the highway without damaging the rear gears? I usually stay at 45-50 mph. A friend of mine has clocked me with a GPS unit so I think the speed is pretty close. My friends think it’s a joke that I drive this slow. Can I go faster?

B.G.

Jeff Smith: Something appears to be off here.

Rather than guess, let’s apply some basic math to see if we can figure out what’s really happening. We’ll assume that your tachometer is correct and that your buddy’s GPS highway speed numbers are also accurate.

There are several other variables besides gear ratio relative to vehicle speed that affect engine rpm.

The most prominent is rear tire size. As the tire becomes larger in diameter, one revolution of the tire moves the car forward a greater distance. So right away, a 31-inch tall tire has a significant effect on highway rpm.

Another factor that few people take into account is torque converter slippage. You didn’t mention what torque converter you are using—we’ll assume with a deep gear and big rear tires, that it’s not a stock 11-inch converter but rather something like a 10-inch converter with a higher stall speed. This will affect engine speed at cruise compared to a late model car with a lockup torque converter.

A lockup converter uses a small clutch inside the converter to physically lock the transmission’s input shaft to the converter that is bolted to the flexplate. This prevents slippage, reduces engine speed, and lowers heat in the transmission. We’ll assume you don’t have a lockup converter and that your converter slippage at highway speeds with light throttle on level ground will be 200 to 300 rpm.

The following is the formula to determine mph given your information:

MPH = (RPM x Tire Dia.) / (Gear Ratio x 336)

MPH = (3,000 x 31) / 4.30 x 336)

MPH = 93,000 / 1,444.8

MPH = 64.3 mph

This is based on engine rpm, but with slippage that we’ll assume is around 200 rpm at speed, this means that the input shaft is really only seeing 2,800 rpm. This will reduce the speed. We’ll save the space of going through the math again and just tell you that 2,800 rpm is almost exactly 60 mph.

Since your buddy’s GPS tells you that at 3,000 rpm engine speed you are travelling 50 mph, we have a discrepancy that we need to figure out. Since your information is from the previous owner, it appears that perhaps that information is incorrect.

The best way to know what the real gear ratio is would be to jack the rear of the car, place jack stands in a safe location, and have a buddy rotate a rear tire one full revolution while you count the number of times the pinion rotates. This works best when the rear end has a posi or a locker so that both rear tires rotate in the same direction.

Our guess, given the information you’ve shared, is that if you were to do this that the pinion is likely going to rotate more like 5-plus turns.

The best way to know for sure is to use a slightly different formula to determine engine rpm based on the other variables—including changing the gear ratio. In this example, we’ve plugged in a speed of 50 mph, an rpm of 3,000 and a 5.13:1 rear gear ratio instead of the 4.30:1.

RPM = (MPH x Gear Ratio x 336) / Tire Dia.

RPM = 50 x 5.13 x 336) / 31

RPM = 86,184 / 31

RPM = 2,780

This equation assumes a locked-up converter or manual trans with a clutch so we would need to add roughly 200 rpm to this for a loose converter—making the engine speed nearly 3,000 rpm. This is almost exactly the engine rpm number you have given us so likely this is the real gear ratio in the car.

While your question was “how fast can I drive?”, we’ll assume that what you really were asking is what is the best way to increase your highway speed to perhaps 70 mph while either maintaining the same engine rpm or simultaneously reducing the engine speed. All of this is possible and there are several ways to approach this. A taller tire will help, but we did the math and going to a 33–inch-tall tire is only going to help reduce the rpm by roughly 170 rpm—so that won’t help much.

Before we go further, we’ll assume that the rearend is using normal street gears and not what are called Pro gears. The Pro gears are intended only for drag racing and are heat treated differently to absorb the shock of high-g launches like from a manual transmission car or an automatic with a trans brake. Pro gears won’t last long in a street car because they aren’t designed to be high-mileage gears like a normal street ring and pinion.

You could change the rear gear ratio to what you originally were told was in the car. This allows you to drive at roughly 64 mph at 3,200 rpm—including the slippage factor. The downside to this approach is that it would really hurt off-the-line acceleration since this reduces the gear ratio by 16 percent.

Depending on how you drive the car, this might be unacceptable. If you spend more time driving the car on the highway than you do running it at the track, then changing the gear ratio is the least-expensive way to go.

However, there is a way to maintain both the deep gear and be able to drive at a decent highway speed.

A 4L80E automatic overdrive transmission is the answer.

The 4L80E is essentially a TH400 with a 0.75:1 overdrive as fourth gear. This effectively reduces the rear gear ratio by 25 percent. Multiplying 5.13 x 0.75 = 3.84:1—which would become the effective rear gear ratio in overdrive. That computes to a highway cruising speed of 72 mph at 3,200 rpm, adding the 200 rpm for converter slippage.

Of course, buying a performance built 4L80E is significantly more expensive than a gear change, but it allows you the opportunity to have both the deep gear for drag-strip passes while still using overdrive to reduce engine speed on the highway.

Another way to go would be to retain your TH400 trans but add a Gear Vendors overdrive. The overdrive ratio is almost the same as the 4L80E at 0.78:1. So the effective gear ratio in overdrive would be 5.13 x 0.78 = 4.00:1. With this reduction, at the same vehicle speed, the engine would run 22 percent slower.

Hopefully this sheds a little light on your situation. There are plenty of options available depending upon how you intend to use the car.

But above all, have fun with it!