Gear ratio question.....wheel & tire size change

[rms25thta]

Just wondering if you guys could educate me on the net effect on gear ratio with my wheel & tire change. I went from stock 16" wheels with the 265/70/16 tires to 20" wheels with 275/55/20 tires. I have 3:42 gears in the rear, what did the change in tire size do to my gear ratio? Please get back to me. Thanks in advance for the assistance.

Rich

 

[blueflamed03]

from 30.6" tall to 31.9", so when your speedo says 60, your doing about 63 MPH.

 

[rms25thta]

What effect does that do on the gear ratio?? Does that essentially lower the gear ratio below 3:42??

 

[blueflamed03]

yes, the taller you go, the more it kills the gear. My stock was 3.42, went with a 31" tall tire, swapped to 4.11 gears, much better.

 

[Jay]

Tire height acts like a gear. Switching to taller tires is like decreasing the gear ratio in your axles (like from 3.42s to 3.23s). The opposite is true for switching to shorter tires. It affects the total drive ratio , which means all gears are affected. It does not change the total power the engine puts to the ground, just at what speeds the power occurs.

Installing 4.10s in place of 3.42s makes many people think they gained a lot of power. The "feel" changed, but the power didn't, you just changed the power and speed relationship. Power is torque times rpm with the 5252 used as a conversion factor to get the units correct for curvilinear motion. For example, in a stock truck with 30.6" tires with 3.42s in 1st gear at 25mph is turning ~2900rpm where the 5.3 is making approx 320 lb-ft of torque (320x3.06x3.42 = 3,350 ft-lb turning the driven tires at 25mph). With 4.10s, at 2900rpm in 1st gear you are moving at 21mph (320x3.06x4.10 = 4,015 ft-lb to the tires at 21mph)... you are still making the same engine torque (@ same rpm... power!) through a slightly different total drive ratio, just at a lower speed, so you "feel" much more oomph when you give it throttle. Simple leverage. At the drag strip this will lower your times simply due to having the engine hover around the peak horsepower rpm for a longer period between the start and finish lines.

With 30.6" tires and 3.42s, at 75mph you should turn 1970rpm in OD. Switching to 31.9" tires drops this to 1890rpm at the same speed. The end result is +4% change in TDR. If they were available, you could install 3.56 gears to return to your original TDR.

Google search for gear calculators and knock yourself out playing with them. See how different gear ratios and tire sizes affect your gear/rpm/speed relationships.

 

[l86MadLac]

2015 awd dod ,vvt delete , trunion rockers, 222/236 .610"/.610" 113 LSA,108 ICL W/+10 fuel lobe , lt2 intake , 103 NW throttle body, cold air, long tube no cats. Stock 285 45 22 (32.1) don't know what stall or gears size. Currently 3.23 i believe. 441 to the wheels. Any input would be greatly appreciated. Thank you

 

[Marky Dissod]

wondering if you guys could educate me on the net effect on gear ratio with my wheel & tire change.
I went from 265/70R16 tires to 275/55R20 tires. I have 3:42 gears in the rear, what did the change in tire size do to my gear ratio?

You have 3.42:1 gear, not 3:42. When doing maths you never know if a typo could royally skreu things up. OK
Every piece of advice given so far is true, but there are pieces of info missing.

GM undergeared nearly every vehicle with a V8 in it since the 70s when CAFE MpG testing became a thing, to try to get slightly better CAFE MpG scores.
Tests results are unrealistic, so they don't directly apply to the real world as well as they should.
With GMT800s, anything under 3.73 needs some help off the showroom floor (in my experienced opinion).

Installing 4.10 in place of 3.42 makes many people think they gained a lot of power.

Although the engine itself may not be any stronger or more powerful, going from 3.42 (or even 3.73) to 4.10 has two undeniable measurable effects:

*Tow rating improves - in the Owner's Manual under 'Weight of The Trailer', it turns out that:
a 4.8L with 3.73 is tow rated same as a 5.3L with 3.42 - 6800lb
a 4.8L with 4.10 is tow rated same as a 5.3L with 3.73 - 7600lb
so an argument could definitely be made that gears are worth strength, since Power = Strength / Time
There are of course diminishing returns, no sense trying to tow anything over 2x the weight of a Tahoe, just 'cause it has 4.56 - got Suburbans for that much or more.

*If one were to measure, let's say, 0-some speed, or time to distance like a 1/4 mile or 1/2 mile, even with the same engine making the same power,
it'd be impossible to deny that better gear would result in quicker time to speed and time to distance. Since Power = Strength / Time,
even if both vehicles have the same engine, the quicker one made better use of it by putting more power to the ground thru the tires.
Again, diminishing returns; typically the only Tahoes / Suburbans (1500) with 4.56 or more gear have HUGE tires with aftermarket suspensions to suit,
specifically to make up for the much taller tires - which some people forget are also MUCH HEAVIER, even without heavier wheels.

Taken together:
taller tires have a weakening effect on axle gearing
any additional weight towed / hauled would be benefited by more axle gear
GM already hobbled most GMT800 suvs with barely enough gear as is

Long story short: most Tahoes and Suburbans (1500) would be helped by 4.10 gear.