How to Select Gear Ratio Combos for Better Performance – Part 2

Click Here to Begin Slideshow In the last issue we examined several different engine power curves and shared info on over-gearing. What all that means is less can often prove better than more. Get a wee bit more complex and it’s easy to go too far. Let's compare a pair of hypothetical hot rods -- one is a traditionally styled car with Firestone 7.50X16s on the rear and the other a high tech with P335-30ZR18 BF Goodrich G-Force T/A tires. For the sake of comparison, let’s "test" both cars with conventional Turbo 350 transmissions (the high gear ratio is 1:1). In this comparison, we’ll assume that cars run a ZZ4 crate motor and they both carry 3.08:1 gears out back. Here are the basics: P335-30ZR18 7.50X16 ________________________________________________________________ Axle Ratio 3.08:1 3.08:1 Tire Diameter 25.90-inches 31.00-inches Final drive ratio - transmission 1.00:1 1.00:1 With this information, we compute the vehicle speeds at a specific RPM number. The simple formulas are as follows: Loaded Radius Of Tire: P335-30ZR18 7.50X16 ________________________________________________________________ Loaded radius = Tire Diameter 2 = 25.90 = 31.00 2 2 Loaded Radius = 12.95 15.5 The next step is to calculate the tire revolutions per mile for each car. The following formula is used when crunching the numbers: Tire Revolutions Per Mile: P335-30ZR18 7.50X16 ________________________________________________________________ Tire Revolutions = 10084 loaded radius of tire = 10084 = 10084 12.95 15.5 = 778.69 = 650.58 The above figures don’t mean much until they are plugged into the next formula, which is Cruising Speed. For the sake of comparison, let's assume that we'll use a figure of 2,500 RPM for a cruise speed. This occurs just before the fat part of the torque peak for the ZZ4 engine. The results are interesting. Cruising Speed: P335-30ZR18 7.50X16 ________________________________________________________________ Cruising Speed = RPM X 60 Gear Ratio X Tire Revolutions Per Mile = 2,500 X 60 = 2,500 X 60 3.08 X 778.69 3.08 X 650.58 = 150000 = 150000 2398.365 2003.786 = 62.54 MPH = 74.86 MPH As you can see, there’s a considerable difference in the speed of the two cars at a given RPM level. The only mechanical difference between the two is the rear tire diameter (height). What would happen if each car was fitted with an overdrive automatic? For our example, we’ll use a common 700R4. High gear on a 700R4 is 0.70:1. Given the final axle ratio of 3.08:1 in each car, the overall ratio drops to 2.156:1. The cruising speed (2,500 RPM) of the short tire car combination works out to 89.346 MPH while the tall tire combo cruises at 106.94 (!) MPH. Remember, that's with a 3.08:1 rear gear. If your car is equipped with an even taller two-series gear, then it becomes even worse. When the numbers are crunched, things get interesting. For example, our "big vintage tire" car engine speed works out to just 2,000 RPM at a vehicle speed of 60 miles per hour (using the 7.50X16 tire combination, the 3.08:1 ring and pinion and the Turbo 350 as a point of reference). Swap in an overdrive and the engine speed drops to a little over idle. When the math is done, the cruising speed works out to approximately 1,400 RPM at 60 MPH for the tall tire combination and a slightly better 1,675 RPM or so at 60 MPH for the short tire car. As you can easily see both combinations are just off idle with the 700R4. When you factor in the fat torque curve of the hypothetical LS from our first segment, you can see that the engine just might live peacefully with the combination. The 302 (with its much lower torque figure at a relatively high RPM level) using a similar gear combination won’t prove to be much fun to drive. That’s a good example where both performance and fuel economy will likely improve with a steeper gear. But how much gear is needed with the overdrive combination? That’s a good question, and one that isn’t easy to answer. Let’s plug two gear ratios (for each combination) into the formulas and see what happens. The idea here is to see the approximate engine speed at 60 MPH. For the “high tech”, short tire hot rod, we picked a 3.50:1 rear gear along with a 3.89:1 rear gear. The RPM at approximately 60 MPH worked out to 1,900 for the 3.50 (in overdrive) and 2,100 for the 3.89:1 combination in overdrive. For the hypothetical hotrod with the traditional tall tire setup, we selected a 4.10:1 gear along with a 4.56:1 rear gear. The RPM at approximately 60 MPH with the 4.10:1 gear worked out to 1,850 RPM while the 4.56:1 gear worked out to 2,075 RPM (both in overdrive). From these calculations, you can see just how much change in RPM an engine will experience with simple tire height and gear ratio changes. It’s important to understand that you're moving the engine into a higher RPM range. The basic premise when picking gears is to set up the entire combination (including transmission gears) and make it conform to the power band. With an overdrive, you can compromise somewhat – gearing the car for the 1:1 gear (which in an automatic is often 3rd gear, or 4th or 5th with an overdrive stick), and then you can take advantage of the overdrive to reduce the RPM on the highway. When fiddling with combinations, it’s all about reaching a happy medium. That’s not quite the end of it either; many overdrive automatics and a number of late model manual transmissions can create untold grief when blindly adding a considerably lower rear axle ratio. Why? That’s easy. To illustrate, first gear in 700R4 transmission is an extremely low 3.06:1. Compared to the older TH350 design (first gear is 2.52:1), this is a considerable difference, especially when you "launch" the car. The writer has considerable first hand experience with this dilemma. In the past, the writer swapped various ring and pinion sets in a 3,400 pound, 350-powered, 700R4-equipped Trans Am. With a nearly stock engine, the car turned into an undriveable mess with a 4.10:1 ratio in the back. The overall or combined first gear ratio consistently overwhelmed the short back tires (for the purposes of that old test series, it was equipped with stock size P245-50 tires). Given the extremely low overall first gear of the 700R4, it was impossible to feather the throttle when you really got on the gas. But there was more. We took the car to the track. The results proved equally disappointing: The car refused to pull hard at the big end of the track. It just nosed over when it shifted into high gear. The bottom line here is, automatic transmissions with very deep first gear ratios (the 700R4 has a 3.06:1 first gear) tend to melt tires in first gear, but at the same time, tend to have very big splits between the gears. Adding power (such as the big stroker combination) doesn’t help. It only makes the situation worse. In the end, it’s easy to see that mistakes in gearing can craft a truck load of trouble. The real key to optimum performance is to take the engine torque curve, the transmission gear ratios, rear axle ratio and tire diameters into consideration as a total package. Once you’ve done your math, it’s probably still a good idea to take Jenkins’ ancient advice and err on the conservative side. You’ll be happier. And so will your car.

How to Select Gear Ratio Combos for Better Performance - Part 2

Click Here to Begin Slideshow

In the last issue we examined several different engine power curves and shared info on over-gearing. What all that means is less can often prove better than more. Get a wee bit more complex and it’s easy to go too far. Let's compare a pair of hypothetical hot rods -- one is a traditionally styled car with Firestone 7.50X16s on the rear and the other a high tech with P335-30ZR18 BF Goodrich G-Force T/A tires. For the sake of comparison, let’s "test" both cars with conventional Turbo 350 transmissions (the high gear ratio is 1:1). In this comparison, we’ll assume that cars run a ZZ4 crate motor and they both carry 3.08:1 gears out back. Here are the basics:

P335-30ZR18 7.50X16
________________________________________________________________
Axle Ratio 3.08:1 3.08:1
Tire Diameter 25.90-inches 31.00-inches
Final drive ratio - transmission 1.00:1 1.00:1


With this information, we compute the vehicle speeds at a specific RPM number. The simple formulas are as follows:

Loaded Radius Of Tire:

P335-30ZR18 7.50X16
________________________________________________________________

Loaded radius = Tire Diameter
2

= 25.90 = 31.00
2 2


Loaded Radius = 12.95 15.5


The next step is to calculate the tire revolutions per mile for each car. The following formula is used when crunching the numbers:


Tire Revolutions Per Mile:

P335-30ZR18 7.50X16
________________________________________________________________

Tire Revolutions = 10084
loaded radius of tire


= 10084 = 10084
12.95 15.5

= 778.69 = 650.58

The above figures don’t mean much until they are plugged into the next formula, which is Cruising Speed. For the sake of comparison, let's assume that we'll use a figure of 2,500 RPM for a cruise speed. This occurs just before the fat part of the torque peak for the ZZ4 engine. The results are interesting.


Cruising Speed:

P335-30ZR18 7.50X16
________________________________________________________________

Cruising Speed = RPM X 60
Gear Ratio X Tire Revolutions Per Mile

= 2,500 X 60 = 2,500 X 60
3.08 X 778.69 3.08 X 650.58

= 150000 = 150000
2398.365 2003.786


= 62.54 MPH = 74.86 MPH

As you can see, there’s a considerable difference in the speed of the two cars at a given RPM level. The only mechanical difference between the two is the rear tire diameter (height). What would happen if each car was fitted with an overdrive automatic? For our example, we’ll use a common 700R4. High gear on a 700R4 is 0.70:1. Given the final axle ratio of 3.08:1 in each car, the overall ratio drops to 2.156:1. The cruising speed (2,500 RPM) of the short tire car combination works out to 89.346 MPH while the tall tire combo cruises at 106.94 (!) MPH. Remember, that's with a 3.08:1 rear gear. If your car is equipped with an even taller two-series gear, then it becomes even worse.

When the numbers are crunched, things get interesting. For example, our "big vintage tire" car engine speed works out to just 2,000 RPM at a vehicle speed of 60 miles per hour (using the 7.50X16 tire combination, the 3.08:1 ring and pinion and the Turbo 350 as a point of reference). Swap in an overdrive and the engine speed drops to a little over idle. When the math is done, the cruising speed works out to approximately 1,400 RPM at 60 MPH for the tall tire combination and a slightly better 1,675 RPM or so at 60 MPH for the short tire car.

As you can easily see both combinations are just off idle with the 700R4. When you factor in the fat torque curve of the hypothetical LS from our first segment, you can see that the engine just might live peacefully with the combination. The 302 (with its much lower torque figure at a relatively high RPM level) using a similar gear combination won’t prove to be much fun to drive. That’s a good example where both performance and fuel economy will likely improve with a steeper gear. But how much gear is needed with the overdrive combination?

That’s a good question, and one that isn’t easy to answer. Let’s plug two gear ratios (for each combination) into the formulas and see what happens. The idea here is to see the approximate engine speed at 60 MPH. For the “high tech”, short tire hot rod, we picked a 3.50:1 rear gear along with a 3.89:1 rear gear. The RPM at approximately 60 MPH worked out to 1,900 for the 3.50 (in overdrive) and 2,100 for the 3.89:1 combination in overdrive. For the hypothetical hotrod with the traditional tall tire setup, we selected a 4.10:1 gear along with a 4.56:1 rear gear. The RPM at approximately 60 MPH with the 4.10:1 gear worked out to 1,850 RPM while the 4.56:1 gear worked out to 2,075 RPM (both in overdrive). From these calculations, you can see just how much change in RPM an engine will experience with simple tire height and gear ratio changes. It’s important to understand that you're moving the engine into a higher RPM range. The basic premise when picking gears is to set up the entire combination (including transmission gears) and make it conform to the power band. With an overdrive, you can compromise somewhat – gearing the car for the 1:1 gear (which in an automatic is often 3rd gear, or 4th or 5th with an overdrive stick), and then you can take advantage of the overdrive to reduce the RPM on the highway. When fiddling with combinations, it’s all about reaching a happy medium.

That’s not quite the end of it either; many overdrive automatics and a number of late model manual transmissions can create untold grief when blindly adding a considerably lower rear axle ratio. Why? That’s easy. To illustrate, first gear in 700R4 transmission is an extremely low 3.06:1. Compared to the older TH350 design (first gear is 2.52:1), this is a considerable difference, especially when you "launch" the car. The writer has considerable first hand experience with this dilemma. In the past, the writer swapped various ring and pinion sets in a 3,400 pound, 350-powered, 700R4-equipped Trans Am. With a nearly stock engine, the car turned into an undriveable mess with a 4.10:1 ratio in the back. The overall or combined first gear ratio consistently overwhelmed the short back tires (for the purposes of that old test series, it was equipped with stock size P245-50 tires). Given the extremely low overall first gear of the 700R4, it was impossible to feather the throttle when you really got on the gas. But there was more. We took the car to the track. The results proved equally disappointing: The car refused to pull hard at the big end of the track. It just nosed over when it shifted into high gear. The bottom line here is, automatic transmissions with very deep first gear ratios (the 700R4 has a 3.06:1 first gear) tend to melt tires in first gear, but at the same time, tend to have very big splits between the gears. Adding power (such as the big stroker combination) doesn’t help. It only makes the situation worse.

In the end, it’s easy to see that mistakes in gearing can craft a truck load of trouble. The real key to optimum performance is to take the engine torque curve, the transmission gear ratios, rear axle ratio and tire diameters into consideration as a total package. Once you’ve done your math, it’s probably still a good idea to take Jenkins’ ancient advice and err on the conservative side. You’ll be happier. And so will your car.

You Have Choices!

Today, gearing choices are incredibly wide. So are the types of rear axles available (9-inch Ford shown here). Ford eight and nine-inch rear ends are common, as are quick-change setups, vintage banjo housings and of course, Chevys, Mopar, Dana and even vintage rears. When picking gears, remember specific ring and pinion sets are engineered for high shock loads (drag racing) while others are built for street use.

Manual or Automatic

Although the majority of hot rods are built with automatic transmissions, more and more are being built with stick shift gearboxes. And many are equipped with overdrive. This Richmond six speed has an overdrive ratio of 0.77:1.

Interior Choice

There’s a lot of gear ratio choice inside a transmission too. For example, the Richmond transmission has several different first gear sets available: 3.33:1, 3.06:1 and 2.89:1. Second gear ratio is 1.85:1; third gear ratio is 1.31:1; fourth is 1.00:1 and as pointed out earlier, fifth is overdriven at 0.77:1. Choices like this are available for many manual transmissons.

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