i was talking about this with my friend the other day and i was plugging numbers into the calculators. i put in 3.55 gears at 55mph and put in different tire heights, like 32, 33, 35, 40. the rpm's were going down as the tire height was going up. how come this was happening, i thought the bigger tires you have the more rpm's your turning?

http://www.differentials.com/calc.html

 

It is because the larger the tire the less RPM you will have because there is more surface area. Think about it if you take a lawn mower tire and roll it 20 feet how many full revolutions will it do, say maybe 20 if it is a small tire. Now take a big 35" tire and roll it the same distance, it may make 2 or 3 full revolutions. Hope that helps.

 

this might seem stupid, but are you saying the engines rpm are based off the tires rpm? i thought you would have higher rpm because the engine has to do more to turn the bigger tiret

 

Nope, a larger tire has pretty much the same effect as a taller gear.

I think 97Ram2500 put it well.

Imagine if you could have a tire that was 1 mile tall. The circumference would be a little over 3 miles. You would only have to turn the tires one time to get accross town. The engine would only have to turn enough times to turn the axle once. With a "normal" tire, you'd have to turn the axle a few hundred times to go the same distance.

 

ok right, but now i thought the bigger the tire or the taller the gear, the worse the mpg. i thought going from 3.55 gears to 4.10 gears would lower the rpm and therefore give you better mileage, by decreasing rpms? same with tires, i thought going from 265's to 285's would give you worse mpg because the engine has to do more to turn it, ie more rpms.

but the calculator is saying the bigger the tire, the less rpm. i always thought the lower the rpms the better the mileage. but that would be conflicting with the idea of having a bigger tire=less rpms.

i think i am confusing something.

 

having lower rpms yes will get you better gas mileage.. if you were on a flat straight hwy have a 35 inch tire and 3.55's will give you BETTER mpg over a 33 or so because your rpms will be less.. but if your going around town having a bigger tire will hurt your mpg because it'll take more work to turn the tires coming to stops.. turns.. ect.

going from 3.55 to 4.10 will RAISE your rpms.. not lower.. going up in gears (3.55 to 4.10) is adding more teeth to the gear in the rear end.. in other words a bigger gear.. thus the axle spins the same speed but the gear is turning slower.. if you look at it that way your engine is spinning at the same rpms but the tires are turning slower. so if you did 55 at 2,000 rpms with 3.55's.. with 4.10's doing 2000 rpms you will only be doing aroung 50.. get it???

 

"Taller" gear = lower number. 3.55 is taller than 4.10. Going from 3.55 to 4.10 will raise the rpm.

Taller gear or taller tire will give lower rpm at a given speed. Theoretically, that would mean better mpg at speed on the highway. BIG tires can lower mpg because they are HEAVY.

Sometimes a taller gear may not give better mpg, but you are right, they generally do. That's one reason we have O/D transmissions.

The engine has to work harder to turn a big tire. Even if the rpm are lower, the load can be increased. If you had a crazy shorter gear like say, 1.00 you might get really bad mpg because the engine will be struggling to get the truck moving. At high speed the engine might be running at a fast idle, but your foot would be to the floor because the load would be high.

 

ah confusing stuff, i think i'm starting to understand. thanks for all the info

 

You may find this helpful:
http://www.carcraft.com/techarticles...rks/index.html

 

thats why people get higher (numearicly) gears when they switch to bigger tires. say you have 29" tires, and a 3.55 gear. You add 33" tires, your gonna throw your rpm "sweet spot" lower in the rpm. take those 33"'rs and add 4.10 gears your back to arround your same stats as the stock setup. ( i dont remember if 29->33=3.55->4.10, but you get the idea)