Tire sizes and gears.

[2ndGenToyotaFan]

There are a lot of questions about this stuff, so I'll try and clear some things up. First off, stock tire size on most of your trucks is 225/75 R 15.....

Well what does that mean? The 225 is the tread width in millimeters (About 8.86 inches), and the 75 is a ratio, that's right, it's not a measurement of anything, it's a #&%*%*$&$ ratio! Who's dumbass idea that was I don't know. And the 15 is the rim height in inches.

Back on track, 75 is really 75%, so that's 0.75 times the tread width (225 mm) equals the sidewall height, 168.75 mm. Now there are 2 sidewalls (that's 337.5 mm's total, or 13.28 inches) on the way from the ground to the top of the tire, plus the rim. So 15 inches of rim plus 13.28 of sidewalls is 28.28 inches tall.

Now to the gear part, stock gears for our "28 inch tires" (As you see they are technically not 28...) is 4.10, which means your drive shaft turns 4.1 times every time the tire makes one full turn.

The diameter, or rolling distance of our "28 inch" tire is 88.84 inches (Diameter times pi (3.14159) = circumference) Divide that by 4.1 and we travel 21.67 inches for every driveshaft rotation, and that's the number I'll be referring to, because it's directly related to your speedometer (that's the thing you're staring at...)

So now the fun stuff. I'm going to make a good picture for this.... hold on....

[2ndGenToyotaFan]

Ok, so here is the chart I made. As you can see the 28 inch tires with 4.10 gears have a travel of 21.67 inches per drive shaft rotation. You can cross reference tire size and gear ratio to see what your travel would be and then compare that travel to the stock figure. If it's more than stock you can be sure of less power on the road, if it's less than you'll have more power, but lower top speed.

If you want a figure of just how much difference there will be in a percent just divide the new travel by the stock 21.67. Say you want to get some 31 inch tires and stick with stock gears, you would divide 23.75 by 21.67 and get 1.095 so your speedometer and odometer will be over by just under 10% So 50 means 55, 100 is now 110 (That was a joke! )

***PLease note that tire size has little to do with the actual size of the tire once mounted on a rim, and that as a tire wears it gets shorter, so take that into consideration when reading this.***

So by the numbers the best gear tire set-ups close to stock for a truck that came with "28 inch" tires and 4.10 are:

Size/Gears

28-4.10
29-between 4.10 and 4.30
30-4.30
31-4.56
32-4.56
33-4.88
34-4.88
35-between 4.88 and 5.29
36-5.29
37-5.29
38-a little closer to 5.71 than 5.29
39-5.71
40-5.71

****If you have an auto tranny, or your truck came with a larger tire package, you will want to refigure your travel length by multiplying your tire height by 3.14159, and dividing that by your differential gear that came stock. Then you can compair your number to the chart.****

You will start to want lower than stock gearing with bigger heavier tires, but these combos are all within 3% of stock, so no exuses for speeding tickets! That's a good one!

Any more questions?

[Tools R Us]

Good post, a couple of points to add;

Doing the math is a good start for tire size. But if possible stretch a tape on the tires, preferably mounted on the rims. Most manufactures play loose with the numbers, with most tires being smaller than advertised.

Depending on engine, transmission, model year, stock tire size, etc there were several ratios from the factory. Your stock gearing needs to be determined before looking at an upgrade, if you have a 4 cyl auto, it may have factory 4.88's and 5.29 or 5.71 would be your only lower choices. Unfortunately the factory 4.88 housing is different and only those gears will fit, so to change ratio another set of diffs will be needed. Links to diff ID pages;

http://toyota.off-road.com/toyota/ar....jsp?id=274535

http://home.4x4wire.com/erik/diffs/

[beatdown83fj60]

Yeah, I'd say that pretty much every tire I've ever ran wasn't the advertised size. I think the closest was probably my BFG AT's, and my 33x13.5 LTB's but neither of them were exactly the size written on the sidewall. Also don't forget the difference between putting that exact tire on a 15x8 or a 15x10 which afffects the final tire patch/true ride height. Im gonna lay a tape across my MTR's this weekend and see how close they got. Honestly from memory the 35" MTRs don't look a whole lot taller than my 33" LTBs were.

[tntoyota]

the best stuff I have, and cant down load is in the downey cat and Lc engineering cat

Awsom stuff you can see the %ages in the downey PER gear then compare to the %ages on there tire size chart EZ!


ALL Toyota diffs, gear ratio and r&p codes on one page!


LC has a RPM in 4th gear AND 5th gear/ tire size/ gear ratio, chart 30" to 44" tires and all gears available again EZ!

you can see the differences, then easaly choose up or down you can see too much to little, too EZ!

[mith]

Originally Posted by 2ndGenToyotaFan

Any more questions?

As a matter of fact, yes.

What would be the downside of running lower gears? For example, Your chart has 4.88 as the gears closest to stock setting for 33 inch tires. If someone were to run 5.29 on their 33s, what sort of problems might they expect. I mean, aside from the speedometer reading lower than their actual speed. It wouldn't be wearing any of the drivetrain at an increased rate than the "stock" setup, right?

[2ndGenToyotaFan]

No problem with going with a lower (numericly higher) gear, but your speedometer would show faster than actual speed if you are "Overgeared". You would get more use out of 5th gear, but at top speed your RPM's would be a bit higher than stock.

Thanks guys, I meant to put in a disclaimer on a few points.

[Tools R Us]

Originally Posted by mith

As a matter of fact, yes.

What would be the downside of running lower gears? For example, Your chart has 4.88 as the gears closest to stock setting for 33 inch tires. If someone were to run 5.29 on their 33s, what sort of problems might they expect. I mean, aside from the speedometer reading lower than their actual speed. It wouldn't be wearing any of the drivetrain at an increased rate than the "stock" setup, right?

Anytime a lift and/or bigger tires are installed it will increase the loads, angles on drive train components, slightly increasing component wear. Bigger tires are heavier and have more rolling drag, a lift makes the truck push more air, added up it takes more power to roll down the road, so going slightly lower than "stock speedo correct" will net better driveability.

The other issue is where you drive. In Ohio's hills close to stock would probably work OK. If you live or travel to areas with mountain roads somewhat lower gears are beneficial. Think of lower gears as a longer lever arm, allowing the motor to turn the tires, move the truck with less effort.

[krustyroo]

Awesome info.

Krusty

[douglashuft]

If you allready have serious meats on your truck just delete this, if you still got a stocker and care about my subjective thoughts on gearing it might be slightly of interest. Who has not had there truck/tire size evolve? As the original owner of a 22re 5spd truck I started with 225 r15 tires and 4.10 gears untill I upgraded to 30" followed by 31" tires. I felt a "decrease in power" every time I upsized. Next I installed factory v6 4.88 gears(with 31" tires) and the difference was quite noticable. I could now race folks from the stop light (and win). It did feel a bit buzzy on the highway however. I now have 33" tires with a mild lift and the truck feels like a slug again. Despite a gear ratio that is still lower than stock the bigger tires are just gonna take a lot more work. The funnest combo on the street was the 31" tires with 4.88's. I sure like the 33's on rougher trails though. the only reason I have not gone larger is the limitations of IFS.

[bkg]

Originally Posted by Tools R Us

Good post, a couple of points to add;

Doing the math is a good start for tire size. But if possible stretch a tape on the tires, preferably mounted on the rims. Most manufactures play loose with the numbers, with most tires being smaller than advertised.

Depending on engine, transmission, model year, stock tire size, etc there were several ratios from the factory. Your stock gearing needs to be determined before looking at an upgrade, if you have a 4 cyl auto, it may have factory 4.88's and 5.29 or 5.71 would be your only lower choices. Unfortunately the factory 4.88 housing is different and only those gears will fit, so to change ratio another set of diffs will be needed. Links to diff ID pages;

http://toyota.off-road.com/toyota/ar....jsp?id=274535

http://home.4x4wire.com/erik/diffs/

4-cyl autos came with 4.30 gears. The only auto's that came with 4.88's from the factory were the V6 SR5 models w/ the 31 tire package.

Originally Posted by mith

As a matter of fact, yes.

What would be the downside of running lower gears? For example, Your chart has 4.88 as the gears closest to stock setting for 33 inch tires. If someone were to run 5.29 on their 33s, what sort of problems might they expect. I mean, aside from the speedometer reading lower than their actual speed. It wouldn't be wearing any of the drivetrain at an increased rate than the "stock" setup, right?

Run em. 4.88's w/ 32's is about as perfect a ratio as you'll find for a 22re/5-speed vehicle.

Originally Posted by Tools R Us

Anytime a lift and/or bigger tires are installed it will increase the loads, angles on drive train components, slightly increasing component wear. Bigger tires are heavier and have more rolling drag, a lift makes the truck push more air, added up it takes more power to roll down the road, so going slightly lower than "stock speedo correct" will net better driveability.

The other issue is where you drive. In Ohio's hills close to stock would probably work OK. If you live or travel to areas with mountain roads somewhat lower gears are beneficial. Think of lower gears as a longer lever arm, allowing the motor to turn the tires, move the truck with less effort.

Agreed. Great points that are often overlooked.

[4RUNNY]

Awesome write up. 33's and 4.10 with a 3.0 just don't mix on mountain highways, but in low gear the set up can still perform moderately.