i just bought my dodge ram 1500 2wd slt with a 4.7 magnum v8 now my
problem is i bought the truck with aftermarket 20s. i was wandering if my truck should have the oem 17x8 rims or 20s, the inside of my door
says 17x8 but then i check my glove box and it says i have a 3.92 rear axle ratio and im pretty sure the 3.55 came with 17s i need some help thanks.

 

Wheel size doesn't determine what gear ratio exactly. It's the trim and package that would do that. And 17's are better for towing so I would trust the sticker on this. But regardless since you now have 20 inch wheels I believe your gear ratio has changed.

 

Actually Effective Gear Ratio is changed by the over-all diameter of the tire, not the wheel. A 17" wheel turning a 33" tire will have the same effective ratio of a 20" wheel turning a 33" tire.

Where the difference lies as far as towing and performance is in the weight difference of a 20" wheel vs. a 17" wheel and the amount of sidewall available.

Your truck could have come with 17s and 3.92 gears, it just depends on how it was ordered from the factory. Many times dealers will automatically order 3.92s if it has the tow package, other times not. Seen a good many 17/3.92 combos and just as many 20/3.55s, although the latter sucks ***** IMO...

 

gear ratio hasn't changed.. overall drive ratio MAY have changed, but it's dependent on the overall height of the tire compared to the overall height of the other tire..

most likely, you increased the overall tire/wheel height, which reduces overall drive ratio.. you should be happy to have those 3.92's, because they help out your overall drive ratio...

tracking?

edited to add: Ninja'd by Hammer...

 

You let the old man beat ya' to it again drew... LMAO...

 

and I didn't even see it coming.. go figure...

I swapped out 3.92's for 4.56's when I bumped to 35" tires from 33".. the overall gear ratio likely remained close to the same, with a hedge toward 'lower' with the 4.56's.. this makes a tranny happy, and for towing- the engine still produces enough POP through the lower geared rear end to pull everything I need to pull..

the one thing I would LOVE to see, is different ratio's in the tranny.. more like a chevy's tranny- geared lower on first and second, then a pretty good gap to third... my 46RE has a pretty even step from first to fourth.. that 'get you rolling' low ratio INSIDE the tranny would be a great assistance when pulling or hauling..

 

thanks yall so if i change back down to 17s it shouldnt be a problem right? because right now i have 285 50r 20 id really like to get some 17s with bigger tires any suggestions

 

I ain't a fan of 20s, at least not on any 4x4. Now a street 4x2 who's going for looks, yeah, them big, honkin' 20" or larger wheels look good, if you're into that look.

You'll perform better with 17s and your tire choices will be a lot more and a lot cheaper.

As for the tires, it wholly depends on if your truck is a 4x4 or 4x2 and how/where you use it...

 

i want to lift it a lil bit and be able to haul something when i need to i really want to ditch the 20s thanks yall for the help i just bought my truck 2 days ago and i love the truck !

 

wheel size measurements are height and width
standard tire measurements are just like wheel measurements, height and width
metric measurements are in aspect ratio, or AAA/BB/diameter.. the a's= width of tire in millimeters, the b's = the height of the sidewall in percentage of the width of the tire, and the diameter is in inches.. [which proves this system is effed up, why quote mm's in the same breath as inches?].. anyway:

your REAR END has a ratio of 3.92:1.. that means every time your hub makes a complete revolution, the drive shaft has turned 3.92 times..

your wheel and tire combo REDUCES your overall drive ratio. The taller the combination, the higher the ratio (numerically lower).. Taller tires reduce the overall gear ratio.. based on leverages, the engine has to work harder to turn them.. Also of note, the heavier the tire at the same time, which also works against your engine/transmission/rear-end, and REDUCES the amount of torque you're getting to the ground..

SMALLER diameter wheel/tire combo's INCREASE your overall gear ratio.. the leverage has changes, which allows the wheels to turn quicker with less effort.. It's easier on your engine/tranny/rear-end to get things in motion, as you're getting more torque to the terra..

there is always a compromise.. you're on your own to find yours..

if you intend on towing a LOT, you're better suited having LOWER overall drive ratio... you can achieve that by either running shorter overall height tires, or by changing the ring and pinions in your rear-end.. or both.. you sacrifice economy to a degree, as the engine has to run at a higher RPM to manage the same speed (the circumference of the shorter tire is less)..

if you're not looking for speed, or don't plan to tow/haul too much, you can get by with a higher overall ratio, which will reduce your gas usage once rolling at a constant speed, as the engine doesn't work as hard to achieve the same MPH.. (lower RPM's because the circumference of the tire allows the truck to roll further with each revolution of the tire)... by lowering the overall ratio, you also make the engine/tranny/rear-end work harder to achieve that speed..

now the trick part:
It's been said, and it's a theory I subscribe to (based on experiences) that lower gears actually improve economy- regardless of how counter intuitive that seems. the reason? you draw less vacuum on the engine, meaning you don't load it near as much as you do when you attempt to accelerate with higher gears..

would you rather have a one line answer and make this easy? here you go:

smaller tires reduce engine load (regardless of RPM), and increase available power by putting more torque to the terra.. these are trucks- not race machines.. lower overall ratio = better for the purpose of owning a truck.