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Discussion Starter #1
My ram came with 265's and 3:55 gears. I've since changed to 35's and 4:10 gears. Do you know if this these have compensated for each other and now the speedometer is running close to true? Does anyone have the same set up and know how much there speedo is off + or -???
 

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I came up with - 4% on the speedo, can anyone else confirm that?
 

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That is about right. Doing 57 when it says 60.
 

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ok,how did ya figure that one out? is there actualy a formula for something like this? gezzzz, i feel like a caveman,back when i didnt care if the speedo was right or not...lol
 

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okay. I have 35s with the 3.55. I drove by one of those "this is your speed" machines and while I had the cruise set at 50 it ready 57.
 

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Discussion Starter #7
Here's a thought, after I've driven 60,000 miles with my 4:10 ratio, my odometer will read 2400 miles more than it's been driven!! I'm not complaining.... compared to the performance gain, it's a very small price to pay. :D:
 

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Acutually, with larger tires the speedo will read slower than your actual speed. The theory behind it is, the larger diameter the wheel, the more distance per revolution you will travel. now, with gears (other than stock, let's say 4.56:1) you have to take into consideration that the carrier is travelling at the same speed for the speed sensor to pick up the actual speed, and this is what the computer reads. BUT, with larger tires, you still travel a greater distance due to the fact that the wheel is greater in diameter. Make sense? If not, just go to a cop and ask him if he will clock you as a courtesy, usually they will not have a problem with this, as you are trying not to break the law, or just spend $350 on a Truespeed sensor from Superlift.
 

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Discussion Starter #10
OK, I understand the part about the tire turning less revolutions, say in a 1 mile stretch, but- with the lower gears- isn't your driveshaft turning alot more times, thus contridicting your tire size to an extent? Or am I just thinking about this whole thing too hard..... :roll:
 

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if you had 3.55's and 265's and now you run 35's with 4.10's your speedo is 10% high. meaning, if it is reading 50, then you are actully going 45. 10% * 50 = 5, 50-5= 45.

you (i forget who) are correct, that the taller thge tire the slower the speedo reads, but the higher the gear ratio the faster it reads. in this case the 4.10's are a bigger jump than the 265 to 35 so the diffrence goes to the gears. i have confirmed this, on my truck, similer setup, that this is correct. i would suggest you testing it out tho. i think the 4.10's with the 265 are @ 20%, never really checked that one.
 

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Discussion Starter #12
Thanks for your input Big green monster. That makes scence. By the way, I just ordered the Mopar performance computer to replace my Jet chip. It will be here Tuesday. How impressed with the performance gains were you with your Mopar computer? :):
 

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Yes, I concur. Whenever you go to a lower gear (higher numerically) you will increase the revolutions that the driveshaft actually turns to make one revolution. I.E.-- 4.56 driveshaft turns to one complete turn of the ring gear (axle). With small tires, you will have awesome low end torque and takeoff, but virtually no top end. With larger tires, you start to even out the low end torque and top end speed, remember about the larger the tire diameter, the more distance travelled? For a 35" tire, 4.56's are almost ideal (basically taking the ratio back to stock tires and gears), but a little steeper gear is needed to equal out, say something like 4.78:1, or something like that. I actually figured it out once, but I don't remember offhand. With 35's, 4.88:1 is just about the best to get the speedo to read correctly. But that is not an option for 1500's, only for the 3/4 ton and above.
 

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What I did was to calculate the RPMs for a given speed (60mph in this case) using the 31.6" tire and 3.55 gears. Then using that RPM to calculate actual speed with 4.10s and 35" tires (57mph in this case)

Yes the driveshaft speed is reading correctly in the speed sensor and your spedo is showing the correct speed for the driveshaft RPM, but the lower gears turn the tires slower but the taller tire travels farther per turn. It is a careful balance between the two.
 

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hope this helps:

http://www.superlift.com/gear.htm
 

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kevin,
no problem! :D:

i run 35's w/ 3.92's and my speedometer is off by 5 mph. (65mph is actually 70mph)

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'98 Dodge Ram 1500 QC SLT 4X4 5.2, 5" SkyJacker Stage II Kit, 35/12.50/15 GoodYear MT/R's on 15X8 Weld Sidewinders, Flowmaster Cat-Back Exhaust System, Volant Cool Air Intake System w/ Ram Air Scoop, Custom Audio System.
 

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http://www.bncoffroad.com/tire_gear_cal.html
im willing to bet this will help
 

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LuvMyRam, Call your local dealer, ask for parts. Tell the guy, you just installed 4:10's and larger tires, and that you want the correct "speedometer pinion gear" for what you have now. You'll need to the metric numbers for the tire size you have. He (the parts guy) should be able to look up on a chart and identify what pinion gear to use to correct the difference.
I just went throught this about 2 weeks back. I installed 4:10's (w/lockers & 32" BFG's) and called my local dealer. I was surprized, the guy was actually helpfull!. I ended up with a 38 tooth pinion gear. (I think it was the largest he had) My old pinion gear was a 31 tooth.
Running 35" tires, the dealer may not have a speedo pinion gear large enough to compensate though. Then I'd try a speedometer shop.
Or, you can go out and buy a GPS and mount it in you truck. Turn it on when ever you drive, and it'll display your true speed and direction, and provide a line to follow back should you get lost!
Good luck!
 

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Discussion Starter #20
Thanks man, that's good to know. From the other feed I've read, I'm guessing I'm off only about 3 MPH right now. I might have someone follow me on the highway with a car and check it that way too. As long as I'm only off 3MPH or so, I'm not too worried about it. At least I'm getting it narrowed down to approx. how far it's off, then at least I'll know about how fast I'm going if I meet a cop .0
 
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