How to Select Gear Ratio Combos for Better Performance

Click Here to Begin Slideshow A long time ago, there wasn’t much to think about when it came to gear ratios. Bigger was better. Out back, you had to pack (at least) a set of 4:11’s to be somebody. The reality was, you didn’t have much choice either; for example, if you were a GM guy all you could get inside a close ratio Muncie was a 2.20:1 first gear set. GM Powerglides offered a bit of choice as they came in either a 1.76:1 or a 1.82:1 low gear format. Choices weren’t all that different in the Ford or Mopar camps either, although they had better (deeper) first gear ratios in their transmissions. It’s true you could mix and match certain transmissions with different engines, but that usually meant a mix of adapters, release bearings and other not-so-standard hardware. That was then. Today things are very different. From a mechanical perspective, ratio choices for both the rear axle and the gearbox are extensive. Equally important is the fact that modern tuning, particularly with electronically fuel injected engine combinations, takes the engine torque curve, the transmission gear ratios, torque converter slippage (and indirectly, the torque converter stall speed), rear axle ratio and tire roll out (diameter) dimensions into consideration as a total package. That’s not the end of it either; overdrive transmissions are commonplace. Tires are available in a zillion different combinations. When all of the factors are tallied, it’s easy to see the combinations can have a decisive effect upon how your car runs. As you can well imagine, there’s a veritable truckload of recipes. The mix can include the engine torque curve, the transmission gear ratios (along with the torque converter stall speed), the rear axle ratio and the overall diameter of the rear tires. Over the course of this short series, we'll take several hypothetical and real, common combinations and show the effects of the variables. We’ll show you how these variables intertwine and how you can crunch the numbers to work out the right ratios for your modified car. First, look closely at the rolling stock fitted to a modern high performance car. It will seldom be 26.5 inches in height. If you spin back the clock, tires with diameters two, three and even four inches bigger were the norm. Nostalgia hot rods tend to be great examples – they often roll on rear rubber that’s 30-inches tall or greater in height. Today, a street-strip car might wear set of P275-60 rear tires. They’re typically 28 inches tall. Changes in tire height from 26 to 28 to 30 inches aren’t minor changes in the overall combination. They can have a considerable effect when figuring rear axle ratio. We’ll dig into this deeper in the second part of our series. But before we get there, one of the first points to consider in the overall combination is the RPM of the engine at approximately 60 miles per hour. Just how fast is that engine under your hood turning? Not long ago, cars came with very short axle ratios (high 2-series ring and pinions were common). Coupled with 1:1 high gear (a typical pre-overdrive automatic or manual trans), the engine could be turning well below 2,000 RPM. Factor in an overdrive, as found on most later production vehicles and plenty of street rods, and the engine will be running just off idle. As you imagine, when you whack the throttle pedal at 60 MPH in an application such as that (2-series ring and pinion, overdrive automatic), the engine response will prove lackluster – particularly if the engine has been modified - we understand that an automatic will drop down a gear when this happens, or with a stick, you can drop down a gear or two, but that's not the point. Worse yet, the engine speed could easily be well beneath the torque curve, which often ends up translating as worse-than-expected fuel economy numbers. On the flipside of the coin is the combination that, for example, turns 3,500 RPM or more at 60 miles per hour. Performance is, to say the least, lively with a combination such as this. Sure you might live with it for a once-a-month hop to the local drag strip, but using such a car with regularity can get old real fast, and given today’s gasoline prices, it can get real expensive too. When it comes to selecting the right gear ratio for your hot rod or street machine, the real place to begin is the engine combination. For the sake of comparison, we decided to include a very lightly modified rebuilt Ford 302, a GM Performance Parts ZZ4 crate engine and a slightly hopped up 376 cubic inch LS small block. While these combinations might not exactly match the hot rod you have in the garage, the information proves thought-provoking. By the way, these aren’t real engines, simply hypothetical combinations. The torque figures for each look like this: Torque Chart 302 ZZ4 LS RPM Torque - Lb-Ft Torque - Lb-Ft Torque Lb-Ft ________________________________________________________________ 2000 266 352 435 2500 270 365 465 3000 295 402 520 3500 314 405 545 4000 315 400 555 4500 307 390 560 5000 290 355 540 5250 269 338 480 In the example above, we have three very different engine combinations along with three very different torque curves (no surprise). The torque peaks vary for the displacement and it makes sense that each of these engine combinations will really work better with a specific transmission and rear axle ratio package. The reality is, the overall driveline gearing package should be designed to match the torque characteristics of the engine. If it isn’t you won’t have much fun driving the car. It’s clear the large displacement LS engine listed above isn’t lacking when it comes to torque. Note the figures: It produces almost 170 more torque (Lbs-Ft) at 2,000 RPM than the little 302 produces and just off idle; the largest displacement engine produces more torque than the ZZ4 does at its peak. The ZZ4 is no slug either, but you’ll note the torque curve isn’t nearly as “fat” as the hypothetical LS. The reality is the LS combination can easily get by with less gear (transmission and/or rear axle) than the little 302. There’s a downside, however: If you use too much gear (a high numerical gear set) the result will usually end up as fried tires with the bigger engine combination. This is something you can't take lightly when choosing gear ratios. Decades ago, one of this writer’s magazine assignments was to assemble a dozen winning tips from a dozen winning racers. Bill "Grumpy" Jenkins was one of the first racers interviewed. I held Jenkins in great esteem at the time – still do -- and even though he’s no longer with us, his depth of knowledge is still worth sharing. Although his exact words escape me, Bill noted that far too many drag racers over-geared their combinations (buzzing them as they went through the lights). He felt many racers failed to take engine torque and the actual power band into consideration. Jenkins went on to point out that in many cases, a little bit less rear gear could actually make a given car quicker, faster, more consistent and, ultimately, more reliable. Although Jenkins’ knowledge was geared toward drag racing, it still applies equally well to street driven hot rods and street-strip cars. Bottom line? Taming a rear axle ratio might actually make your car a better (and quicker) high performance machine. So how do you figure it all out? There are plenty of online calculators out there that crunch the numbers for you, but in our next issue, we’ll lay out a few basic calculations you can use at home. They’re not complicated and they can put you right into the performance zone for your specific gear ratio package. Watch for it.

How to Select Gear Ratio Combos for Better Performance

Click Here to Begin Slideshow

A long time ago, there wasn’t much to think about when it came to gear ratios. Bigger was better. Out back, you had to pack (at least) a set of 4:11’s to be somebody. The reality was, you didn’t have much choice either; for example, if you were a GM guy all you could get inside a close ratio Muncie was a 2.20:1 first gear set. GM Powerglides offered a bit of choice as they came in either a 1.76:1 or a 1.82:1 low gear format. Choices weren’t all that different in the Ford or Mopar camps either, although they had better (deeper) first gear ratios in their transmissions. It’s true you could mix and match certain transmissions with different engines, but that usually meant a mix of adapters, release bearings and other not-so-standard hardware.

That was then. Today things are very different. From a mechanical perspective, ratio choices for both the rear axle and the gearbox are extensive. Equally important is the fact that modern tuning, particularly with electronically fuel injected engine combinations, takes the engine torque curve, the transmission gear ratios, torque converter slippage (and indirectly, the torque converter stall speed), rear axle ratio and tire roll out (diameter) dimensions into consideration as a total package. That’s not the end of it either; overdrive transmissions are commonplace. Tires are available in a zillion different combinations. When all of the factors are tallied, it’s easy to see the combinations can have a decisive effect upon how your car runs.

As you can well imagine, there’s a veritable truckload of recipes. The mix can include the engine torque curve, the transmission gear ratios (along with the torque converter stall speed), the rear axle ratio and the overall diameter of the rear tires. Over the course of this short series, we'll take several hypothetical and real, common combinations and show the effects of the variables. We’ll show you how these variables intertwine and how you can crunch the numbers to work out the right ratios for your modified car.

First, look closely at the rolling stock fitted to a modern high performance car. It will seldom be 26.5 inches in height. If you spin back the clock, tires with diameters two, three and even four inches bigger were the norm. Nostalgia hot rods tend to be great examples – they often roll on rear rubber that’s 30-inches tall or greater in height. Today, a street-strip car might wear set of P275-60 rear tires. They’re typically 28 inches tall. Changes in tire height from 26 to 28 to 30 inches aren’t minor changes in the overall combination. They can have a considerable effect when figuring rear axle ratio. We’ll dig into this deeper in the second part of our series.

But before we get there, one of the first points to consider in the overall combination is the RPM of the engine at approximately 60 miles per hour. Just how fast is that engine under your hood turning? Not long ago, cars came with very short axle ratios (high 2-series ring and pinions were common). Coupled with 1:1 high gear (a typical pre-overdrive automatic or manual trans), the engine could be turning well below 2,000 RPM. Factor in an overdrive, as found on most later production vehicles and plenty of street rods, and the engine will be running just off idle. As you imagine, when you whack the throttle pedal at 60 MPH in an application such as that (2-series ring and pinion, overdrive automatic), the engine response will prove lackluster – particularly if the engine has been modified - we understand that an automatic will drop down a gear when this happens, or with a stick, you can drop down a gear or two, but that's not the point. Worse yet, the engine speed could easily be well beneath the torque curve, which often ends up translating as worse-than-expected fuel economy numbers.

On the flipside of the coin is the combination that, for example, turns 3,500 RPM or more at 60 miles per hour. Performance is, to say the least, lively with a combination such as this. Sure you might live with it for a once-a-month hop to the local drag strip, but using such a car with regularity can get old real fast, and given today’s gasoline prices, it can get real expensive too.

When it comes to selecting the right gear ratio for your hot rod or street machine, the real place to begin is the engine combination. For the sake of comparison, we decided to include a very lightly modified rebuilt Ford 302, a GM Performance Parts ZZ4 crate engine and a slightly hopped up 376 cubic inch LS small block. While these combinations might not exactly match the hot rod you have in the garage, the information proves thought-provoking. By the way, these aren’t real engines, simply hypothetical combinations. The torque figures for each look like this:


Torque Chart

302 ZZ4 LS

RPM Torque - Lb-Ft Torque - Lb-Ft Torque Lb-Ft

________________________________________________________________


2000 266 352 435

2500 270 365 465

3000 295 402 520

3500 314 405 545

4000 315 400 555

4500 307 390 560

5000 290 355 540

5250 269 338 480



In the example above, we have three very different engine combinations along with three very different torque curves (no surprise). The torque peaks vary for the displacement and it makes sense that each of these engine combinations will really work better with a specific transmission and rear axle ratio package. The reality is, the overall driveline gearing package should be designed to match the torque characteristics of the engine. If it isn’t you won’t have much fun driving the car.

It’s clear the large displacement LS engine listed above isn’t lacking when it comes to torque. Note the figures: It produces almost 170 more torque (Lbs-Ft) at 2,000 RPM than the little 302 produces and just off idle; the largest displacement engine produces more torque than the ZZ4 does at its peak. The ZZ4 is no slug either, but you’ll note the torque curve isn’t nearly as “fat” as the hypothetical LS. The reality is the LS combination can easily get by with less gear (transmission and/or rear axle) than the little 302. There’s a downside, however: If you use too much gear (a high numerical gear set) the result will usually end up as fried tires with the bigger engine combination. This is something you can't take lightly when choosing gear ratios.

Decades ago, one of this writer’s magazine assignments was to assemble a dozen winning tips from a dozen winning racers. Bill "Grumpy" Jenkins was one of the first racers interviewed. I held Jenkins in great esteem at the time – still do -- and even though he’s no longer with us, his depth of knowledge is still worth sharing. Although his exact words escape me, Bill noted that far too many drag racers over-geared their combinations (buzzing them as they went through the lights). He felt many racers failed to take engine torque and the actual power band into consideration. Jenkins went on to point out that in many cases, a little bit less rear gear could actually make a given car quicker, faster, more consistent and, ultimately, more reliable. Although Jenkins’ knowledge was geared toward drag racing, it still applies equally well to street driven hot rods and street-strip cars. Bottom line? Taming a rear axle ratio might actually make your car a better (and quicker) high performance machine.

So how do you figure it all out? There are plenty of online calculators out there that crunch the numbers for you, but in our next issue, we’ll lay out a few basic calculations you can use at home. They’re not complicated and they can put you right into the performance zone for your specific gear ratio package. Watch for it.

How to Select Gear Ratio Combos for Better Performance 1

The characteristics of the engine mounted in your engine compartment definitely have an effect upon gear ratio selection.

How to Select Gear Ratio Combos for Better Performance 2

A honking blown first generation Hemi or a monster cubic inch shotgun Ford is going to have a torque curve far different than something like a relatively mild LS crate motor.

How to Select Gear Ratio Combos for Better Performance 3

Keep the features of your engine in mind when considering different gear ratios.

How to Select Gear Ratio Combos for Better Performance 4

Another major factor in the selection of gears is the back rubber. Big tires such as the Hoosiers on this ‘32 are extremely tall.

How to Select Gear Ratio Combos for Better Performance 5

While the drag radials on the Buick in this photo aren’t as big as the Hoosiers, they’re still plenty big.

How to Select Gear Ratio Combos for Better Performance 6

Compare these large tires to the super short rolling stock found on the ’40 coupe in this photo. It’s not hard to find a diameter range of six, seven or more inches in street tires.

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