explain gears please

[jacked_86chevyk10]

i do not understand the gearing in trucks with oversized tires... i have a 86 k10 with 35's and soon to be 38's..i have the orginal gears, whatever they may be for a 1/2 ton (and i know 1/2 dont go with 38's that well but..) truck,i would guess somewhere in the low 3's and i get around 7 mpg, if i went to lets say 4.56 or somewhere around there would my mileage improve? i realize it would make it easier to get the big meats a-rollin...is that true? if i want better gas mileage with big tires do i go with a lower gear? help please...thanks

 

[trollporter]

What engine, tranny, and transfer?

 

[pauly383]

Your engine is not in its powerband for those gears and tires you have now. The lower gears will improve the mileage , but not as drastically as you think. You will still lose MPG from oversize heavy tires and lifted trucks just from trying to cut the air with a big brick , our truck aren't the most aerodynamic things out there. Do the gears and feel the difference in performance, then drive easy and tune up once a year and filters often, that will be the best for mileage /forums/images/graemlins/k5.gif

 

[jacked_86chevyk10]

sorry bout that, 350, 700r4, 208

 

[jjlaughner]

Think about it this way...

if you had a 31" tire that makes 1 rotation how far does it go? well 31x3.14(PIE)= 97.34" on the road
Now switch to a 35" tire, 35x3.14= 109.9"
now with 38's - 38x3.14= 119.32"

SO, if you are running a 3.73 gear and a 31" tire at 65MPH in overdrive your running 1839.5RPM, if you move up to a 35" tire going 65MPH (actual not whats showing on the gauge) in overdrive still you'd be running 1629.3RPM. See how the same speed, same gears and larger tires drops you out of the power band of the motor. NOW, this is assuming you can run in overdrive at 1629.3RPM which your tranny would not be happy about.

Most people have come to relize that a 35" tire and an overdrive transmission (700r4) basically requires a 4.56 gear. Then from there I would say as the tire size steps up the gear would also. SO with an overdrive tranny running a; 35" tire you need 4.56 gears, 36-37 needs 4.88's, 38 up to about 42 needs a 5.13 gear. Working backward 33" tire at about a 4.10, 32 likes a 3.73 and a 30-31 run 3.08's pretty well. By changing the gear with the tire size you are attempting to keep you vehicle in its power band.

Hope that helps, I've read many things on here and talked to several people, this is what I have taken from it while planning my tire/gear combo with 350TBI/700R4 setups. I'm getting ready to regear my DD to 4.56's with 35's and my project truck will get about a 38 or maybe a little larger tire and 5.13's! /forums/images/graemlins/waytogo.gif

NOW, for those running a non-overdrive tranny you drop everything back one, so you use a 4.10 with a 35" tire and so one. I'm not UP on the Manual stuff yet but I guess its drops back yet another gear for granny? /forums/images/graemlins/thinking.gif /forums/images/graemlins/dunno.gif

 

[R72K5]

manual transmissions slightly drop RPMS at a given speed in a certain range/gear compared to automatic transmissions, a little more of a drop than with non lockup converter compared to lockup one

all of the above affect cruising RPM

from highest to lowest at a given speed with given axle ratio and given transmission range/gear and a given tire height:

auto non lockup 3 speed non overdrive
auto lockup 3 speed non overdrive
auto overdrive non lockup
auto overdrive w/lockup and manual non overdrive
manual w/overdrive

but if you already have 1/2 ton axle ratio IE high ratio in lower 2's or higher 3's then you are already running wayyyyyyyy to low of RPM for the engine and youre just lugging it and thats supposed ot be worse of a thing to do to an engine than overrevving it, unless youre going over redline 5500 or 6000

i like to cruise my SBC engines upper 2's to mid maybe upper 3's
my 1/2 ton 67-72 2wd trucks that ive had in the past with stock original 3.73 12 bolt and close to original tire size around 29.5-31" tall i have ran all the time at 3800RPM at 65 long or short trips and it was great, had great acceleration and performance no matter if from a dead stop or at 65 or faster, it would always get up and go form any speed, you dont get that in all these 70s and 80s trucks that have the crappy 2.73 and 3.08 something axle gears in them. i hate those ratios they should have never been made they are usueless and pointless, you cant haul weight with them and they do not perform at all, its great never had any problems with the engines,


good luck