true tire height vs gear ratio..

[K5Greek]

well the other day i was bored and measured my tires.. i found that my 36's are indeed 34's.. i have no problem with this but does the gearing have the same effect? basically should i be regearing for a 34" tire or a 36" tire?

this question really has no effect on my outcome, as i was planning on going with 4.56's anyway but its just something i wanted to know

 

[SkysTheLimit]

All the calculators use the true diameter so I go with that. More accurate math is a good thing.

 

[broncoman6524]

the thing is...are you tires a worn out 36? therefore making them a 34, or are hety a new 36 that is a 34.

i personally would go with gears to put you back to stock at a 38. taht way when you get new tires, and if/when yuo go bigger your not screwed and unhappy.

 

[tRustyK5]

Loaded height is one thing, tire diameter is quite another. Measure the circumference and divide by pi to get the true tire diameter that's relevant to gearing and rpm's.

Think of it this way...if you air down to 4 psi your 36's could be as short as 31 or 32 inches but you'd still roll the same distance per revolution.

Rene

 

[K5Greek]

the thing is...are you tires a worn out 36? therefore making them a 34, or are hety a new 36 that is a 34.

i personally would go with gears to put you back to stock at a 38. taht way when you get new tires, and if/when yuo go bigger your not screwed and unhappy.

nope not worn down, they have at most 3k on them.. they are rated for 30 psi max cold and i have them at 25.

rene your probably right though cause when its standing still i could see how it could lose and inch or two because of that little buldge at the bottom of the tire

 

[Greg72]

Loaded height is one thing, tire diameter is quite another. Measure the circumference and divide by pi to get the true tire diameter that's relevant to gearing and rpm's.

Think of it this way...if you air down to 4 psi your 36's could be as short as 31 or 32 inches but you'd still roll the same distance per revolution.

Rene

I always thought that circumference was a more logical way also.

You could mark the tire with chalk near the edge of the sidewall......drive the truck forward until the mark rotates 10 complete times, then measure the total distance traveled. Divide by 10 and then do the calc to convert circumference to diameter.

10 revolutions instead of 1 would help to negate any measurement variations and give a nice precise number to do the gear calcs from.

 

[user]

Factor in the number of crankshaft rotations per mile

Question is what it is you're trying to optimize. A higher gear ratio number gives you a lower gear, which to me is all torque and no action, a lower top speed and less fuel efficiency, but better getaway from the stop light and towing-trailer-trips up Pike's Peak.

When you factor the tire size into that, you need to consider the leverage that the wheel/tire combination has. The radius of the circle formed by the outer circumference of the tire is like a lever with the fulcrum at the center of the hub. The longer that radius is, the less effort it will take to move it. Think of how big the rear wheels are on an old Ford tractor; or how the sprockets are arranged on the rear wheel of a ten-speed bike - the largest sprocket gives the most torque but least effectiveness at higher speeds (in terms of distance travelled).

So choose the larger tire if you want to get started faster but want a slower top speed, and a smaller tire if you don't need to burn rubber. With the gear ratio you're talking about, you won't have any trouble getting either size tire to get up and go at the stop lights. If you are going to be hauling big weights in huge trailers up the sides of the Rockies, then you definitely ought to get the larger diameter tires and be happy with a top speed of around 45 mph (unless, of course, you intend to buy a new engine every year - the crankshaft has to rotate a lot more to do the same work the higher the differential gear number and the larger the tire diameter).

 

[6.2Blazer]

Loaded height is one thing, tire diameter is quite another. Measure the circumference and divide by pi to get the true tire diameter that's relevant to gearing and rpm's.

Think of it this way...if you air down to 4 psi your 36's could be as short as 31 or 32 inches but you'd still roll the same distance per revolution.

Rene

Sorry incorrect. The rolling radius is the measurement you need to get if you really want to accurately determine this. If you are using one of the calculators that needs diameter, just take the rolling radius x 2 and use that number. This is measured by going from the center of the hub straight down to the ground. This will take into account the "bulge" the tire has caused by the weight on it (and the air pressure).

For example you have 36" tires. If the tires were a true 36 and completely solid (no bulge), than the rolling radius would 36 / 2 = 18". Now in real life the rolling radius measurement will be lower, maybe 17" or so. If you air down for off-road maybe it's only 16", in which case it would behave the same as a 32" tire according to the gear calculators, at least in relation to rpm versus speed (or distance per revolution).

If you don't believe me, you can test it yourself. Put a mark on the tire and with street pressure roll the truck (has to have the vehicle weight on the tire) one complete revolution and measure how far it went. Then air that same tire down to say 5 psi, roll it one revolution, and then measure how far it went...I guarantee it won't be the same.