well i know that the horse power at the wheel is allwase less that the one on the fly wheel i am wodering if lower gears can '' add '' or realease those seeping horses.
its almost logical but how can we calculate this. if you can feel it when you drive there is a difference some where. i think its more in the torque than in the hp.
dose anyone ever done a dyno run after that.

and i read a lot of thread and did not found exactly what i am looking fore.
i have a 2 door auto with 33 and they are gonna last 4-5 years with my low millage. after that i go with 35 and hope to stay there. i never go more than 100kmph
so 60 mph never and a little high way. i need a calculator that is gona tell me how my rpm will be with 4.56 4.88 5.13 all i found is for manual trany 5 speed and the 6 speed here.

beater post 2 question in one post

 

Gears don't really add horsepower or torque. They can however, CHANGE where the peak horsepower and torque OCCUR, which can have a similar affect. Think of lower gears as adding an extension to a wrench. They simply give the drivetrain more leverage to do it's job.

 

i know that. lot of experiance with mountain bike. but still i wonder how a dyno run would read that.

 

The torque and horsepower peaks would just be in a different spot. Lower gears=lower peak in the rpm band. That's all I got...unless you're lookin for specifics.

 

One quick calculation is to mutiply .12 by tire diameter (.12 X 38" = 4.56), personally I don't carre for it because I think it's too far off with Jeeps and there lesser horsepower compared to trucks, etc.

Drivetrain parasitic loss from the engine to the tires whereby (just tire diameter) was changed requires a simple remedy of differenctial re-gearing to fix, provided you haven't gone overboard with tire size that weakens other areas, like axle shafts, etc, etc. For example someone putting 38" tires on a JK X model with a front D30 axle, rolling around town would be fine but offroading it will fail eventually if not right off the bat.

Automatics on average lose +or- 17% and manuals lose about +or- 13%. Re--gearing will gain horsepower at the wheels but will shift the effective torque curve. If you re-gear too low you will run a higher rpm and effectively negatively alter your gas mileage including more wear and tear on the engine being run at a higher rpm. The ideal re-gearing should put your vehicles hp/tq curves as close as possible back to stock throughout the rpm band, this will net the full capability of what the vehicle was designed for. There are plenty of gearing charts on the web to help you figure out which gears to go with for 33s or 35s to retain close to factory specs.

The best method we used was a real dyno to verify our estimates using a pretty extensive horsepower-dyno software which took into account transmission gearing (auto or manual), differential gearing, flywheel horsepower benchmarks, etc and it would give us the hp/tq curves throughout various rpms which we used to determine A) shift points or B) 1/4 trap speeds and 60' times off the line. Drag-racing software aside there are now several free online calculators which have some pretty simple input data for the offroading community, I've included a few below.

One formula you can try is: New tire height/stock tire height x stock gearing = new gearing

I'm running 35s on my Rubi so:
35/32 = 1.09375 x 4.10 = 4.48 then find a common set of gears like (4:56s)

So with my current Rubi on 35s (real-time) and wanting to run 37s in the future I'd run:
37/32=1.15625 x 4.10 = 4.74 or common gear (4.88s) some like 5:13 gears based on our little pansy 3.8L's.

formulas aside the real world aka "communities" like this can provide even better information on what has been working best on average which would net you the best results.

TIRE SIZE CHANGE CALCULATOR
http://www.4lo.com/calc/geartire.htm

GEAR RATIO, TIRE SIZE, CRAWL RATIO, ETC. CALCULATORS
http://www.4lo.com/calc/gearratio.htm

Gear Ratio & Tire Size Chart
http://www.4lo.com/calc/geartable.htm

 

wow that is a realy nice explanation. thanks a lot and i will look to those link.

thanks again, have a good day.

 

horsepower is a calculated number based on torque so if your max torque was the same but at a different rpm it would change your hp reading.

HP=(torque x rpm)/5252

 

OK... HP/tq numbers are done through a series of calculations in the Dyno software that is actually designed to figure engine hp minus(-) parasitic loss. There fore when calculating engine HP/TQ a 5.13 gear will actually show lower numbers than say a 3.07 gear. This is becuase a 5.13 gear has more parasitic loss than anything closer to a 1.1 ratio. This is why when you dyno your vehicle with a manual transmission, you want to dyno in 4th gear which is usually a 1.1 gear in the tranny. Try a dyno run in 1st gear, it will show less power, each and every time.

BUT.... dynos will actually show a power to the ground figure that is rarely used but is available. This on the other hand will show a power increase when using a steeper gear and the numbers corrolate pretty closely with the percentage increase in gear change, for instance a 5.13 gear will show pretty close to a 20% increase of torque to the ground over a 4.10 gear.

Hope this helps

Steve Logston
Miami Muscle and Performance Tuning
GM LT/LS/Duramax Specialist