35's, 4.88's and diff in speedometer
 

i want to run 35in tire with 4.88 gear but im wonder how far off the speedometer will be off from actual speed i know that the cable is ran off the tranny and that the gear will cominsate a little for the tire change but how much and a good way to correct this miscallualation yes i know i cant spell being a redneck and a marine thats to be expected

The gear swap won't actually change you speedometer, it will just put you back in the correct power band for the larger tires.

To figure out what your new speed will be:

new tire size/old tire size=X
35/30?=1.166667
I don't know your current tire diamter so I just picked a number.

X * 70(mph)=Y
Use your X from above and multiply it by 70. This is the speed you are actually going.
1.166667*70=81.666690 MPH

Hope this helps.

thank you that answered all my questions

Doing a gear swap will mess with your speedo. If you have a 97 or 98 you are gonna need a new speedo gear, if it's later than you will need a tuner.

I had 305/70/17's on my truck and when I swapped in 4.56's the speedo was way off. I followed a friend of mine in his car adn when he was going 58-60 my speedo read 75-77.

[QUOTE=Rock Krusher]Doing a gear swap will mess with your speedo. If you have a 97 or 98 you are gonna need a new speedo gear, if it's later than you will need a tuner.

i got a 97 where do i find these speedo gears stats well they cominstate for the gear and tire size or just the gears?

i found gears on summit for 20bucks but how many teeth will they need found 23 20 and 19

Just depends what the size of the original tires was... We need that one little piece of information first.

-Joe

30in tire was stock with 3.55 gear now im going ethier 4.56 or 4.88 with 35's seen the jet speed recalibrator just thinking about going that way

Going from a 30" to a 35" tire, that means 30/35 = .857 or roughly 14% slow reading on the speedo. To recoup that, you'd want 13% shorter gearing. So, 3.55/.857 = 4.14. 4.10 would likely be the next closest ratio that will put you pretty close to stock (you'd be off by about 1% at 70 mph).

Now, with 35s, 4.14:1 puts you back at stock. If you jump up to 4.88s, now your engine and drivetrain will be turning 15% faster (4.14/4.88 = .848) at any given speed.

Hope that helps!

so 4.10 gears and 35's would almost put me back to shock speedometer cali???? if that is cause i will consider those.......

Assuming it was accurate to begin with, yes, it'll put you back to within a percent or two of stock. There will still be more rotational inertia there, so ETs on a drag strip would show a slightly slower time, but yes, it'll get you pretty close to where you started.

Rock Krusher,

You said above that for a 99 and up you need a tuner to correct your speedo if you have larger than stock tires?

I have a 2002 4.6L 4x4, i want to correct my speedo back will one of the aftermarket performance tuners do this? I've been thinmking of getting one anyway for performance but if it correcs my speedo thats a bonus!

Thanks,

thank you for your help you saved me $150 for the calibrator 1to2% isnt to bad


and derekmc1 from what i learn about reprograming you can not calibrate a tire larger that rotates less than 599 times a mile the computer will not take accept it. they can get it closer but basiclly the 35in is the biggest you can go however if you change gear ratios you must go another route like the jet or superlift speedometer calibrator does both but cost $150-$200

hope that helps

Yes, almost every programmer that I have seen allows you to adjust for tire sizes.