2nd Gen Non-Engine/Transmission Crazy/dumb question on gear ratio's

[Bug Out]

I was wondering about gear ratio's and was looking at Fritz web site, tire sizes etc, auto's with 3. 54/4. 10 etc.





A CTD with a auto and 3. 54 gears at 59 MPH is at 1600 RPM.



A CTD with 4. 10 gears at 1600 RPms is at 51 MPH, a 8 MPH loss.



Now, figuring that a 4. 10 takes 8 MPH from a CTD at 59, a 3. 73 rear ratio would take 4MPH from a 3. 54?..... ?( Approx>>>not exactly)



So would the same differance be applied

by changing tire sizes( from the chart on Fritz)

from a 265/75/16 to a 225/75/16 would lose approx 4 MPH???



The MPH Vs. RPM change the same, but one is only changing tire dia?



I want to make sure this would be in essence, the same as a gear change but not, and only a tire dia change.



Is this right or ?????



Thanks in advance.

 

[parcher]

Easier to think of revolutions per mile. Samller tire = more revs per mile same as higher numerical gear ratio. Taller tires= fewer revs per mile same as lower numerical gear ratio.

 

[klenger]

I didn't attempt to follow your math, but keep in mind that tire diameter is not a direct ratio of the first number of tire size. The 265 or whatever number in the tire size is the width of the tire, and the 75 or whatever number is the ratio of tire width to height from rim to tread (or vise versa, can't remember which way). The rim diameter is a constant 16" and needs to be added. The easiest way to find the tire diameter is to go to a web based calculator.



Here's one I found:



http://www.rx7turboturbo.com/robrobinette/tire_diameter.htm



In the case of the tire sizes you mentioned, the 225 size has a diameter of 29. 29" and the 265 is 31. 65". The 265 is 1. 08 (8%) bigger in diameter than the 225.



If you just calculate the ratio of the tire sizes, the 265 woud be 1. 18 (18%) larger, which is incorrect.

 

[jponder]

Yes you would be changing your effective gear ratio, But your going the wrong way MAN!

 

[2-ND TIME]

tire sizes

Are you wanting to get better milage or more power to your tires?Is the terrain in your area level or mountious these are the reasons to change tires sizes. Do you run empty most of the time ?The ones I put on are 5. 78% larger and don't make a big difference even with the hills. Always has been gutless!!!

 

[Bug Out]

The reason I asked.....

As you can see , I have a 97 1 ton that does all of my 5th wheel pulling. It has 4. 10's and does a great job etc.



I was toying with the idea of converting my 2000 QC 4x4 as my 5th puller. It just doesn't have what it takes to pull 15,000# without a struggle or as effortless as the 4. 10 geared 1ton.



So, I figured i could gear the 2000 to at least 3. 73 with a tire change,( height means nothing to me. I'd only lose an 1"+-. )



Get a BD trans or DTT ,EZ box, put the disc brake conversion on the rear and away I go.



However, I PROBABLY SHOULD just get rid of both and by a new 3500 6 speed, but then I won't have anything to get greasy with, change, fix, blow money etc etc etc . . LOL



Its kinda like the oil change, men Vs. Women.



She goes to Jiffy lube, spends 25$ and is done in 20 minutes,.



He goes to Kragens, case of oil, oil pan et etc. .



7-11 case of beer, and 4 hours later, $60 he's done. .

 

[2-ND TIME]

reply

Be glad that you have one that works. BD plug in power is good and an exhaust brake is better.