hi awhile ago i changed my gears from 3.55s to 4.56s and from 265/75r16s to 37x12.5r17 so few years ago when i did that i did not know u could just change the speedo gear so i bought this stupid electric modulator to change the signal to the speedo and its never worked well the speedo changes depending on temp and voltage so i want to change the speedo gear and i can not seem to find a chart with the same numbers on it i find some with 35 teeth gears fit my application then i find other charts that say 32 teeth is for me so was just wondering where i can look i already bought a 35 tooth one and the speedo seems to read way to slow the factory gear is a 30 tooth and i got a spare 36 tooth laying around any advice would be great

 

need to find your effective gear ratio.if my math is good iam sure someone will correct me your combo is ratio of 3.82.then the formula is current pinion teeth count times effective gear ratio divide by old gears.

 

My Indy came with 3.55grs stock & 275/60/17 tires on it & had 32 tooth speedo gear.
I swapped to 35 tooth speedo when i added 3.92grs.

Did your truck come with 265tires factory?
Makes sense having the 30 tooth gear for that 31.5" tall tire so the speedo would be accurate.
37" tires with 4.56 I thought worked out to 3.90 final.(used DML's tire calc)
35 tooth should be very close to accurate

http://www.dakota-truck.net/TIRECALC/tirecalc.html

 

yes indy my truck came with 265s and 3.55 gears and a 30 tooth speedo gear so u thinking a 35 tooth is preatty close then maybe its been way off and im just not use to it being like that ya know i been driving it like that for couple years

 

I checked with a cop running a radar trap after I swapped the speedo gear to 35 tooth to verify it was accurate & it was bang on.

 

alright guess ill leave it in by the way truck looks fantastic i havent been on here in awhile was looking at ur site i had a idea for myself last night thought i would see what u thought why dont ya find a srt10 rear axle for urs dana 60 with disc brakes wanted to see how close it would be to our trucks what ya think

 

you can test it by getting on the highway and running exactly 60 mph and time yourself through the mile markers. if speedo is correct it will be exactly 60 seconds.

 

ok will have to do that once its driveable again

 

I can't find the one that I had used but see if this will help.
http://www.go.jeep-xj.info/HowtoSpeedoGears.htm

 

Good link, that should take the guess work out
I didnt swap my speedo gear right away after swapping gears.
Didnt figure out there was an issue till I got to the track where I run 28 tires
Noticed speedo & traps speeds way off & the 35 tooth got it much closer (though still off a bit on 28s)
On stock tires my trap speed was almost bang on to what the speedo showed