WE GET A LARGE VOLUME OF TECH QUESTIONS HERE AT ORA. A high proportion of them involve some variation of the question, "I just swapped to larger tires. What gear ratio should I run?" We have visited this topic at different times and in different ways before, but since the influx of letters shows no sign of slowing down, we think it is a good idea to address it again.
Gearing and Grunt
The effects of a larger tire on driveability and fuel economy can be mild or massive. With the gear ratio remaining constant, increasing tire diameter makes the ratio higher (a.k.a. "taller") which requires more engine effort to get the vehicle moving and keep it that way. There's a "just right" ratio for every vehicle. Often, but not always, the OE picked it in combination with the OE sized tire.
If you have a multi-speed bike, you can feel the effects on your own legs by starting off in first. Easy, huh? Now shift into 9th gear. Ugghh! You probably had to stand on the pedals to get moving and ended up walking the bike up even the mildest of hills. Now you know how the engine felt when you swapped those OE 215/75R-15s for 15.50x36- 15s. In town fuel economy went down the toilet, overdrive is useless below 85 mph, garbage trucks outrun you at stoplights and your 4x4 can't climb a fist-sized rock.
Before we go farther, let's talk about some of the variables. A lighter vehicle, or one with more power for a given weight, can successfully use higher gear ratios on the street, but featherweight and muscle won't always get you by on the trail. A rig with a good powerto- weight ratio may have the grunt to pull itself through a lot of hard stuff with tall gears, but 'wheelers also need to be able to go slow enough to avoid beating their rig into scrap and tall ratios can't give you that. The big engine/tall gear scenario works best with mud runners and worst with rockcrawlers.
The level one choice for any tire swap is to use the "Equivalent Ratio" formula in the sidebars. For the math averse, there are also some general "rule-othumb" recommendations there too. The equivalent ratio works best for street performance and puts your rig back in the same general gearing ballpark as stock. It allows you to compensate for the loss of performance that comes from a tire swap, but that's not the end of the story. The other effects are increased rolling resistance, more rotating mass and reduced aerodynamic efficiency. The performance loss from those effects is not fully recoverable, but by dropping a skosh lower on the ratio, you can gain some of it back.
Usually, the equivalent ratio formula will give you an odd number that doesn't match with any available gear ratios. Say the numbers offer a 4.38:1 ratio for a Dana 44. No such animal, but you could go back to 4.30 or even 4.10:1, but going a bit lower to 4.56 will help to recover some of the lost performance. With a particularly burley engine, you might go in the taller direction.
If your rig is mostly a trail runner and doesn't see much street time, you are free to pick the best ratios for the trail. What are they? You tell us. Certainly the ratios can be lower, but they vary according to the individual vehicle and the type of trails driven. Since we're talking mostly about rigs that are street driven here, we'll defer that topic for another time.
Gearing and Fuel Economy
Having ratios that are too tall often costs as much in fuel economy as ratios that are too low. The too-tall ratios will hurt you mostly in town, where more throttle is needed to get the vehicle moving... the engine grunting and straining. The too-low ratio will hurt you mostly at highway speeds where the engine is spinning at high rpms.
The equivalent ratio formula will usually keep you in the good rpm range for fuel economy, unless a rig was excessively low or high geared at the factory. Examples of these could include some of the '70s, '80s and '90s rigs built with 2.76 or 3.07:1 gears. If your machine is one of these, use a more reasonable 3.55:1 ratio as a starting point and you'll often get better performance/mileage results.
At the other end are those older trucks with sixes and really low gears, 4.56, 4.88, etc. Many of these started off with low power engines and used the low cogs in lieu of engine grunt. Now many of them have a more powerful engine swapped in. Again use about 3.55 or even 3.73:1 as a starting point and you'll get a more usable result. You may find the gears you have are perfect for bigger tires with a bigger engine. The formula may even indicate taller gears!
Torque Loads
There are also durability aspects to a larger diameter tire and wheel combo. It puts more strain on axle components. Part of that is the increase in the weight of the rotating mass and much is from the increase in the "lever arm" length of the tire radius. Draw an imaginary line from the center of the hub to the contact point on the ground. That is your radius and the length of that line is the torque arm. A longer arm increases the load on the axle by multiplying the torque more.
To help you understand this effect, 500 pounds off one end of a one foot arm on a rotating shaft equals 500 pounds-feet of torque on the shaft. Increase the arm to two feet and that 500 pounds is multiplied twice to 1,000 pounds feet. The same thing happens, more or less, with tire radius. The axle is the "fuse" between engine torque, multiplied by trans and tcase gearing, and tire grip (a.k.a. traction torque). Fortunately, the tire is not glued to the ground. Or is it?
Modern tires are pretty good at delivering grip. Airing down or increased weight from weight transfer on slopes will increase grip. While that's not the same thing as a tire glued to the ground, if you combine a taller tire with one that's more grippy and put in a really good traction situation, those sticky new tires may generate enough grip to snap an axle part or two.
There is a formula for getting some rough numbers on figuring axle strength and tire grip in order to determine a safe axle strength range, but we'd need extra pages to work through it. It gets really complicated when you consider the differences in beef between axles of a given general size and individual weak links that can be cured with aftermarket parts. Again, the "rules-o-thumb" below will help give you a general idea of maximum tire sizes for stock axles. Yours could go higher or lower.
The mounted and installed radius of a tire is useful information, radius being the distance from the center of the hub to the ground. Beyond the advertised classification of "35," "37" or whatever, and the manufacturer's mounted diameter, when the tire is on your rig and inflated to your specs... THAT'S the true radius. Use that number, times two, to get your real-world working diameter.
