final gear ratio
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Senior User
Joined: Jan 2005
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final gear ratio
Here is something i was thinking about when i couldn't sleep
so let me know if my logic doesnt work.
If the stock tires size on a 250 4x4 is 32in and you have 4.30 gears than you run 62mph at 2000rpms what if you change tire sizes?
32in 4.30gears = 62mph at 2000rpms
32in 3.73gears = 61mph at 1800rpms
so...
36in 4.30gears = 63mph at 1800rpms
theoretically if you have the taller gear ratios and put taller tires on the truck you effectively have the equivilant of the small tires and shorter ratio gears
would the larger tires' wieght/higher truck hieght
cancel the milege increase of dropping the rpms? or would the fact that the very powerful engines in a superduty overcome the increased rotating mass and act like you just lowered the rpms by 200? All this is assuming you were driving highway, already at speed, no headwind, no load or trailor. Of course driving in town(stop and go) is worse with larger tires.
And yes I realize there is a break even point because hey-with 52in tires your not gonna go 101mph at 2000rpm and still get 14mpg as the calculators would suggest (or drive a stock truck at 5000rpms/150mph for that matter)
Just a thought
Brian
so let me know if my logic doesnt work.
If the stock tires size on a 250 4x4 is 32in and you have 4.30 gears than you run 62mph at 2000rpms what if you change tire sizes?
32in 4.30gears = 62mph at 2000rpms
32in 3.73gears = 61mph at 1800rpms
so...
36in 4.30gears = 63mph at 1800rpms
theoretically if you have the taller gear ratios and put taller tires on the truck you effectively have the equivilant of the small tires and shorter ratio gears
would the larger tires' wieght/higher truck hieght
cancel the milege increase of dropping the rpms? or would the fact that the very powerful engines in a superduty overcome the increased rotating mass and act like you just lowered the rpms by 200? All this is assuming you were driving highway, already at speed, no headwind, no load or trailor. Of course driving in town(stop and go) is worse with larger tires.
And yes I realize there is a break even point because hey-with 52in tires your not gonna go 101mph at 2000rpm and still get 14mpg as the calculators would suggest (or drive a stock truck at 5000rpms/150mph for that matter)
Just a thought
Brian
New User
Joined: Aug 2007
Posts: 9
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From: Binghamton, NY
I have a question, besides the the speedometer being affected how dose having the 6 speed manual tranny in a SD with 3.73 rear end figure in tire size? This question is in regards to towing since with a manual you shift through the gears you need.
Post Fiend
Joined: Feb 2007
Posts: 7,191
Likes: 4
From: Blairsville,Ga
Originally Posted by dash9w
I have a question, besides the the speedometer being affected how dose having the 6 speed manual tranny in a SD with 3.73 rear end figure in tire size? This question is in regards to towing since with a manual you shift through the gears you need.
an engine will only pull so much the speedo is just a measuring device
Oh my gawd...that is entirely too much math! My brain hurts!!! 
