Thinking about changing the size of your** tires**?

You’re not alone—people run larger **wheels and tires** for aesthetic purposes. We’ve also seen guys reduce the overall size of their tires for various reasons. However, there are some serious consequences to any changes to tire diameter.

Tire diameter, along with rear-axle gearing, plays a role in the final drive ratio, which in turn affects acceleration and overall performance. For example, a Camaro that has decent acceleration with a set of 26-inch tall tires and 3.55 **rear axle gears **becomes a dog with 29-inch tall tires and the same gearing. That’s why it’s important to compare final drive ratios with different tire heights before making any changes to tire diameter. By doing so, you can determine the axle gearing you’ll need to use with your new tire diameter in order to maintain (or enhance) performance. You’ll need three figures to do this: your vehicle’s current rear axle gearing, the diameter of your existing tires, and the diameter of the tires you want to use.

## Determining Current Tire Diameter

The easiest way to determine tire diameter is to call your tire dealer and ask for it—easy, but not always an option. The second way is to use the following formulas and figure things out yourself. Please note that these formulas are for metric-rated passenger and light truck tires. Slicks, large off-road, and some larger, Pro Street style tires are already rated by diameter, so you don’t have to do any calculating for them.

We will be using a P235/60R-15 tire as our example: 235 is the section width, 60 the aspect ratio, and 15 the required wheel rim diameter. Here is the basic formula:

**Tire Diameter = 2 x (section width/25.4) x (aspect ratio/100) + rim diameter**

When you plug in the values from our sample tire, the formula looks like this:

**Tire Diameter = 2 x (235/25.4) x (60/100) + 15**

Now, let’s do the calculations:

**Tire Diameter = 2 x (9.25 x .60) + 15**** Tire Diameter = 2 x 5.55 + 15**** Tire Diameter = 26.1 inches (round down to 26)**

## Calculating Effective Drive Ratio

Once you determine the diameter of your old tires and potential replacements, you can compare the effect of new tires on the final drive ratio. For example, if you have 26-inch tall tires with a 3.55 gear, and you want to go to 28-inch tall tires. To calculate the effective drive ratio of the new tires, multiply the diameters together, then multiply that figure by the gear ratio as follows:

26/28 = .92857142857

.92857142857 x 3.55 = 3.2964 or 3.30

As you can see, the larger tires reduce your final drive ratio to 3.30—a measurable difference. You can also do this to determine the effect of smaller tires on your final drive ratio.

## Calculating Equivalent Drive Ratio

Now that you know that 28-inch tall tires reduce your final drive ratio, you can calculate the gearing need to match the performance of your original 26-inch tire/3.55 gear combination. First, divide the new tire diameter by the original tire diameter, then multiply that figure by the original (3.55) gear ratio:

28/26 = 1.0769230769

1.0769230769 x 3.55 = 3.82

With 28-inch tires, you would need a 3.82 rear-axle gear to match vehicle performance with 26-inch tires and 3.55 gears. Since a 3.82 gear is not offered for most rear axles, you will need to choose a gear between 3.73 and 3.90 to get close to the optimum.

Good article, but what I really want to know is the RPM at 65 MPH and what the speed will be when I am at redline.

Scott, these calculators from Yukon Gear may be of help to you:

https://www.yukongear.com/Calculators.aspx

(RPM x W ) / (168 x R) = MPH

W= The length from the center of the axle to the ground. (Not diameter. Because of tire footprint the two radii are different).

R = is your overall gear ratio. (i.e., your trans gear x the rear gear).

Example: 26″ dia tire (is not a 13″ radius but a 12.5″ radius due to the footprint or tire patch).

1:1 trans 4th gear ratio x 4.11 rear gear = 4.11 overall.

(6,000rpm x 12.5″) / (168 x 4.11) = 108.6 MPH

Using 9th grade algebra you can solve for any term in the above equation. Write it on the back of your driver’s license. You will be using it for the rest of your life! LOL.

PS: Some people, and most of the speed calculators use 336 for the constant which is the overall diameter of the tire. But that is inaccurate because it does not take into consideration the tire footprint. Even tire growth at speed has a footprint. If you use 336 then the W value is the tire diameter and not the short radius.

1967 Olds 442?

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So with my 2.73 ratio and 295-50-15 inch and a 700r trans I’m dead in the water in a drag race lol!

Your final drive ratio in overdrive with the convertor locked is 3.00 with those tires. If you have enough torque and can get it to hook you should be just fine in a drag race. I run a 72 buick GS with a built 455 and a final drive ratio of 3.08 and have a best time of 12.53 at 106.02. Not awful for a 4000 lb car. It is all about the combination of parts working together.

i am building a 1 off truck it has a 383 stroke with 333 hp and 445 ft bl toque it is a 4×4 here the strange part i am going to run 40×15.5×20 39.67 tall reves/ per mile 523 no rear and 275x65x20 34.05 tall reves/ per mile 610 on front.this is a 1946 body on a 1994 2500 chevy 4×4 chasse it started out as a diesel truck. this is going to be a highway truck not for off road .like your opion on what gear ratio to use and how to match front and rear to run at same rate

4.56 or 4.88

I have 31# tires , I can’t change gear ratio. Indeed to lower rpms by 6-700. How can I do this. Thanks Lorne

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Larger tyres. They will have fewer RPM’s

i used this for a RC car i’m racing, final ratio was 5.8:1 , i changed 2 of 6 compound gears to get 4.7:1 & added taller tyres to get to 2.82:1 for 132 feet racing.

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Hi, I’m running a 30 inch tall drag radial on a jeep CJ more for aesthetic reasons I have a 4.10 gear. I feel it lays down in 4th gear. Motor redlines @ 6500 rpm. Quarter mile best was 13.1 @ 102 mph. Do you think the effective is off ? I was thinking of a 28”. Not sure how much the et slip will change. Also the width is presently 12.4 where as the 28 could be around 10 – 10.5. Thanks Mike

I’m building a modified VW using a 2.3l V5 engine and 4Motion drivetrain.

The stock wheels are 215/55r16 (25″) and I am fitting 235/70r16(29″)

I have gear ratios but this box setup has 4 final drive ratios so i am struggling to figure out how it might perform.

Cog 1 Cog 2 Ratio Overall

63 15 4.200 N/A

63 19 3.316 N/A

41 12 3.417 14.350

40 19 2.105 8.842

40 28 1.429 6.000

37 34 1.088 4.571

34 31 1.097 3.637

31 34 0.912 3.023

Can you help?

This is all wrong. Your acceleration is increased by larger diameter tires so long as the mass/weight is the same. The final drive ratio gets lower, but *not* at the cost of acceleration. The longer thread in a larger tire moves the car forward a greater distance with less work from the engine. For example a 235 75 R15 tire would out accelerate a 215 75 R15 tire, because the weight is only nominally different, but the diameter increases by about 2 inches.

Funny cars had/have giant rear drive tires for a reason – acceleration. The tiny front tires reduced drag and weight and didn’t need to be big because they were not pushing the vehicle.

If you put giant steel rims on your car when you increase tire size the weight increases and this will offset acceleration with increased mass. Which is why aluminum wheels were invented to offset wheel weight and “unsprung weight” which causes a horrible ride.

I see many sites get this final drive ratio change and the benefits all mixed up. The effective lowering of final drive is only for engine speed, not acceleration. In fact, the lowering of the final drive with larger diameter tires means a higher top speed for the vehicle – again so long as mass, aerodynamic issues are not introduced by some giant wide and heavy rims or something.

Whaaaaaaaaat?

I have to admit that the last comment regarding the increase in acceleration is accurate and true with regards to the increase in tire size. In my opinion a 31″ tire with a 4:10 rear end and an overdrive is a great combination for a truck.

When buying Goodrich TA’s in 1976 my dealer alerted me to loaded tyre radius. I wanted P235/75-15’s for rear of 34 Chevy coupe and he pointed out that they were intended for Rolls Royce and other big cars like Cadillac, etc., of the time. To achieve the correct pressures for a good contact patch across the tread on my lighter car, I would have to lower the recommended pressures. Much later Using “Rodder ‘rithmatic” from 1963 Hot Rod Magazine, I generated a spreadsheet with tyre radius, final drive ratio, car speed, selected gear and engine revs which produced useful data to select gear ratio and tyre size for a given car speed, engine revs and gearbox gear. The outcome for me was the selection for my 68 Camaro of 3.73:1 final drive, and 26”D tyre, for a cruising speed of 110KPH in 4th gear at just above 2000rpm, dropping to 1600rpm in 5th (.75:1) and forget about 6th (.5:1). I did this over 20 years ago for a car to spend most of its time on the road with an occasional thrash on track.

28″ / 26″ = 1.077 (a 7.7% increase in diameter)

3.55 (old ratio) x 1.077% = 3.82 (new ratio)

The comment by Tech is nuts. The distance from the center of the axle to the ground is a lever arm that drives the car forward. If you increase the length of the arm (i.e. tire diameter) you REDUCE the mechanical advantage! Remember, a lever arm can work two ways. It can increase or decrease mechanical advantage depending on where the force is applied. Big tires are for TRACTION, not mechanical advantage that is why gear ratios change to get the mechanical advantage back.