Speed-o-meter

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Natty

New member
1044384

Howdy Ya'll! Don't know how many people out there are familiar with the dilemas faced within the last week, but in summary, here it is: 4.5" lift last month, 33x12.50's last week,blew up spider gears in rear diff. driving like an idiot cutting cookies on the beach. Replaced both diffs. with 4:11 gears, and now I know the speedo is not telling me the truth. Who knows of a product that will correct this situation? Like the rest of us, the least expensive option would not upset me. Thanks, Matt.
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1044439

if u take it to a tire show they can calbrate your speedometer. your suppost to do that everytime u get new tires unless they are the same size u had on there before.i am pretty sure they can calabrate ur speedometer to work with ur gears and tires[addsig]
 
1044445

Replace the speedo gear in the transfer case tailhousing with the correct tooth count gear for your tire size and gear ratio.



Or, borrow a GPS that has a speedometer like a Magellan GPS 320, then drive and determine your correct speed. Adjust your driving habits accordingly.[addsig]
 

1044458

Some simple math is all it takes to figure it out....and its free and will keep you from seeing red and blue while cruising down the highway not knowing really how fast you are going.



Some gear formuals



Gear Ratio

(RPM x Tire Diameter)/(Diff ratio x Trans Ratio x Transf Case Ratio x 336) = MPH



Crawl Ratio

Trans Ratio x Transf Case Ratio x Axle Rato = Crawl Ratio



New gear ratio

(new tire diameter/old tire diameter) x Old axle ratio = new axle ratio



Speedometer

(new tire diameter/ old tire diameter) * speedometer MPH = Actual MPH





Hope that helps (some of these were borrowed from 4lo.com)



for your deal you will probably need the first one since you changed more stuff than just your tires. Say you are going down the road (your in 2 HI which is 1:1) and your rpm is 3000 and you are 4th (manual) (thats a 1:1 ratio btw) (if you have a auto put it in D (not overdrive)) and this is what you do



Gear Ratio

(RPM x Tire Diameter)/(Diff ratio x Trans Ratio x Transf Case Ratio x 336) = MPH



(3000 x 33)/(4.11 x 1 x 1 x 336) = 71.689 or about 72mph



at 2500 you are going about 60mph

at 2000 about 48mph



try different rpms and then go out and test it

if you want to do lower speeds just adjust the trans ratio for that gear (you will need to find out what your trans gear ratios are for each gear) anything over 40 you will probably be in 4th or final for auto anyway.



its tricky but you can kind of figure out the different speeds at different rpms and then you will know about how off your speedo is. Take note that it will not be the same for all speeds, the faster you go the more off it is. For me I put 31's and kept the same gear ratio in my diffs. I use the last formula to find my speedo fix. My old tires were 28 inch.



Speedometer

(new tire diameter/ old tire diameter) * speedometer MPH = Actual MPH



at 60mph...

(31/28) X 60 = ~66 mph

at 70 I am really going about 78 mph

at 100 (like my jeep can go that fast with a 4 banger) Im going about 110

but at low speeds like 30 Im only going like 33



with 33's you will be quite a bit more off (10mph over at 60) with your stock diffs just to give you an idea.



Hope that helps... Math is fun

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1044470

Thanks for all the advice ya'll. In about 20yrs. I hopefully will have acquired enough knowledge to eventually help ya'll out. Thanks again - Matt.
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1044487

forgot to mention price.. new speedo gear only costs about $7-$15 dollars.[addsig]
 
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