Tuesday May 21

Get in Gear

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So, you’ve been cruising around in your pony for a while now, wasting Camaros and Corvettes and earning titles at the stop light Grand Prix. A true performance junkie, you’ve spent all your money on engine, suspension, and power adding mods, with maybe a few bucks here and there to upgrade your stereo. But you’re getting tired of looking at those 16” Pony wheels every day, or maybe your pre-1990 Stang was born with 15” 10-hole wheels or the famous “cabbage shredders”, and now it’s time to add a bit more wheel and tire so your Stang can both look good, and stop making attempts to face oncoming traffic every time it starts drizzling. However, your car is devoid of newfangled electronics and you wisely know that your speedometer will read incorrectly once you increase tire diameter. What’s the biggest you can go without having to install a gear reducer? Well, ask no more, for here’s a handy-dandy guide to help you decide the right wheel and tire size for you. (Or you could just find an online calculator to plug your numbers into and save yourself loads of time).

First, you have to determine the RPM of your tires; this example is based on a tire size of 245/50R16:

Convert Section Width to Inches:
Section Width / 25.4 = Section Width in inches Example: 245/25.4 = 9.65 in.
(245 is the with of your tire, and the 25.4 is a constant)

Calculating Section Height:
Section With in inches x Aspect Ratio = Section Height
Example: 9.65 x 50% (or 0.5) = 4.83 in. (Take the 9.65 from the previous calculation and multiply by the aspect ratio percentage)

Calculating Tire Diameter:
(Section Height x 2) + Rim Diameter = Overall Diameter
Example: (4.83 x 2) + 16 = 25.7 in.

Estimating Tire RPM:
20,800 / Tire Diameter = Tire RPM
Example: 20,800 / 25.7 = 810 RPM
(20,800 is a constant, which you divide by the overall diameter from the previous calculation)

So we get 810 for your tire RPM. Now let’s say you want to upgrade to some 235/45R17’s

Convert Section Width to Inches:
Section Width / 25.4 = Section Width in inches
Example: 235 / 25.4 = 9.25 in.

Calculating Section Height:
Section With in inches x Aspect Ratio = Section Height
Example: 9.25 x 45% (.45) = 4.16 in.

Calculating Tire Diameter:
(Section Height x 2) + Rim Diameter = Overall Diameter
Example: (4.16 x 2) + 17 = 25.32

Estimating Tire RPM:
20,800 / Tire Diameter = Tire RPM
Example: 20,800 / 25.32 = 821 RPM

Now, in order to determine the differential between your old tires and your new tires, then you have to divide your old tire RPM by your new tire RPM and multiply by your indicated speed, so the formula is:
Old RPM / New RPM x Indicated Speed = Actual Speed:
Example: 810 / 821 x 55(mph) = 54.26
810 / 821 x 65(mph) = 64.12
Your upgrade to 17’s makes a barely noticeable difference in your actual speed vs. speedo speed. At 55 mph, the difference is only 0.74, and at 65, the difference is only 0.88. Nothing to concern yourself about in the least. How about we try a more extreme example.

Now let’s say you want to get your girlfriend to leave her husband for you, so you get some big, bloated 22” spinning wheels, and your new tire size is 305/45R22”

Convert Section Width to Inches:
Section Width / = Section Width in inches
Example: 305/25.4 = 12.0 in.

Calculating Section Height:
Section With in inches x Aspect Ratio = Section Height
Example: 12.0 x 45% (.45) = 5.4 in.

Calculating Tire Diameter:
(Section Height x 2) + Rim Diameter = Overall Diameter
Example: (5.4 x 2) + 22 = 32.8 in.

Estimating Tire RPM:
20,800 / Tire Diameter = Tire RPM
Example: 20,800 / 32.8 = 634.14

Old RPM / New RPM x Indicated Speed = Actual Speed:
Example: 810 / 634.14 x 55(mph) = 70.3(!)
810 / 634.14 x 75(mph) = 95.8(!!)
This means that after upgrading from 16’s to 22’s, then when your speedo reads 55, then you’re actually going 70, and when it reads 75, you’re actually going 95.8 mph! Try to explain that one to the Judge.

Now, lets see if we can make things a bit more complicated. Let’s throw your rear gear ratio in the mix to see if your gears work within the optimum powerband of your engine. Let’s say you already have 3.55’s in your car, and your peak torque is 350 at 3,600 RPM and your cruising speed is 70 mph. However, you plan to make some mods that will raise your engine’s peak torque to 410 lb.-ft. at 4100 RPM. Are 3.55’s still the right gear for your car? Let’s find out by using this formula, using our hypothetical 245/50R16 size tire from the first example:

(Max torque RPM x Tire Diameter) divided by (MPH x 336) = Optimum gear ratio.
Example: (4100 x 25.7) / (70 x 336) = 105370 / 23520 = 4.48

So your nearest choices for the new engine would be 4.48, so you would want to buy some 4.30’s or 4.56’s. Now, let’s verify our work by placing our new gear choice (let’s say you went with the 4.56’s) in a formula to determine your correct RPM.

(MPH x Rear Gear Ratio x 336) divided by Tire Diameter = Optimum RPM
Example: (70 x 4.56 x 336) / 25.7 = 107251 / 25.7 = 4173

So the 4.56’s would put us only 73 RPM away from our peak torque figure in our new engine. Fantastic.
But Dave, I think you broke my brain. What do you mean to tell me that there’s more? Yes, young padawan, no gear ratio calculation would be complete without including your transmission ratio also. Now you need to know how many more RPM you’re going to be turning in overdrive with your spiffy new 4.56’s vs. your old 3.55’s. The previous example assumes peak torque in 4th gear, which is 1.00. Let’s see what your new cruising gear will be in overdrive (most transmissions have an overdrive ratio of .67):

(MPH x Transmission Gear x Rear Gear x 336) / Tire Diameter = RPM
Example: (70 x .67 x 4.56 x 336) / 25.7 = 71858.3 / 25.7 = 2,796 RPM
(70 x .67 x 3.55 x 336) / 25.7 = 55942.32 / 25.7 = 2,177 RPM
Your 4.56’s are going to turn 619 RPM more than your 3.55’s at a cruising speed of 70 mph. Using these formulas, you can determine what RPM you will have in any gear in any ratio with any tire diameter. This formula also helps in determining your trap speed with a certain gearset, and it can also help you change your rev range in case your Flowmasters tend to drone at a certain RPM. I know you probably didn’t read through all this, but if you did, not only do you have my gratitude, but I’m also including a chart of popular factory and aftermarket gear ratios for your perusal. If you have a non-Ford transmission or one with nontraditional gear ratios, then you will have to refer to the literature that came with your transmission.(Ha! Like your junkyard tranny has any documentation. You'll be lucky if the part numbers haven't been worn off already. Seriously, though, most tranny ratios can be found somewhere on the 'net).

Manual Transmissions:
79 - 83 1/2
SROD 4-Spd.
 		  83 1/2 - 84
T5 5-spd.
  85-89
T5 5-Spd.
 		  89-95
T5 5-spd.
 		  96-01
T45 5-Spd.
 		 
1st 3.07  1st2.95  1st
3.35  1st 3.35  1st3.37
2nd1.72 2nd1.94 2nd1.93 2nd 1.99 2nd1.99
3rd1.00 3rd1.34 3rd 1.29 3rd 1.33 3rd1.33
4th.70 4th1.00 4th1.00 4th 1.00 4th1.00
   5th.72 5th.675 5th .675 5th.675

Automatic Transmissions:
79-82
C4/C5 3-Spd. 		  84-93
AOD 4-Spd.
  94-95
AODE 4-Spd.
  96-01
4R70W 4-Spd
 
1st2.40  1st 2.40  1st 2.40  1st 2.84
2nd1.47 2nd1.47 2nd 1.47 2nd1.55
3rd1.00 3rd1.00 3rd1.00 3rd1.00
   4th.667 4th.667 4th.667

Aftermarket Transmissions:
Tremec/TTC
TR-3550 5-Spd
  Tremec/TTC
T5 5-Spd
  Tremec/TTC
T45 5-Spd
  Tremec/TTC
T56 6-Spd.
 
1st3.27  1st3.35  1st3.37  1st2.66
2nd1.98 2nd1.93 2nd1.99 2nd1.78
3rd1.34 3rd1.29 3rd1.33 3rd1.30
4th1.00 4th1.00 4th1.00 4th1.00
5th.68 5th.720 5th.675 5th.74
         6th.50