Which speedo gear is used with 3.55 axles in our trucks? I have a t-case from a truck with 4.10s but my truck has 3.55s and I want to make the speedometer accurate again. Also, I have 33s on it now but I'm probably going back to a stock tire size soon, so I want the speedo to be accurate with stock tires if it isn't too far out of whack with 33s. I can just change the driven gear, I don't have to change the ring gear on the tail shaft, right? Thanks guys!

 

Geeeeez, You have so many trucks in your sig I don't know which your refering to. If you have the digital odomiter there should be no gear to change as your speed sensor for the speedometer is in the rear differental. Simply find the revolution of your tire size per mile at 60 miles per hour (usually found on tire brand website) and program it into your odometer.

 

All of the trucks in the signature would have a cable driven speedometer. The answer to your question should be YES you shouldn't have to change the ring gear on the tailshaft. After you get the truck all set up like you finally want it, final tire size etc. If you know a stretch of road that is a given mile, or better yet a stretch of road with several given miles in a row that doesn't have much traffic. Have the truck up to where your speedometer is showing exactly 60 and hold it as close to 60 as possible through the mile. Time it as close as possible to the nearest second. Use a factor of 3600. For example, if it took you 60 seconds you would be dead on. If it took you 72 seconds, you would have been going 50 MPH. If it took you 40 seconds you would have been going 90 MPH and you better hope the cop believes you when you tell him what you were doing. Anyway, use a factor of 3600 and count the cogs on the gear that is in there now and try to mathematically figure out how many cogs you should have on your replacement gear. I hope I made myself perfectly clear, if not post back and I'll try again.

 

No that's clear 88n94, thanks. I was just hoping somebody would know which the correct gear for a stock truck with 3.55 gears is (I am talking about my 90 F350). As it is now the speedo reads way slower than the truck actually goes, keeping up with traffic on the Interstate is reading 50!

 

If you got cable drive you have to do the calculations, but first consider this. A good GPS can be very useful in a comparison of actual ground speed vs. the speedometer indication. If you don't have one maybe you know somebody with whom you could borrow one. It is a matter of tossing it in the vehicle and drive. This has got to be about the easiest test possible. The GPS is neat because it gives speed readout at all speeds for constant comparison. Some speedos are acurate at one speed but not another. If you look very close at your speedo numbers on the dash. You might see two little white dots in a few different locations. The speedo needle is ran up between these dots and is used for calibration. I know this is used on the electronic odometers. Not sure if it is on the mechanical cable drive or not.

The gear drive may not be close or may not be all that accurate even after you do your calculation and gear swap.

Haven't read mags like Road and Track in years. But they always did a calibration test of the speedometer at different speeds. Some are lower, some are dead on and some are higher indicating than actual speed. It is the advantage of the vehicle manufacturer to have a speedo read high and have the vehicle actually drive a little slow. When you race against a stopwatch (0 - 60- or 0 - 100mph) based on a false speedo you will appear to get there faster than reality. When you cruise around, it appears you travel further because the same false indication gives you false miles driven. Then when you do fuel mileage calculation based on the false miles driven you get WOW! Fantastic Gas Mileage!

Since the GPS tells distance traveled , you can compare that readout to the odometer and see the variation there as well. Interesting stuff that most people don't consider until they get a speeding ticket...

OK, I just woke up and reread your post. I can't see that the transfer case (from a truck with 4:10 axles) is going to effect your speed indication as the cable comes off the transmission and high range should be a 1 to 1 drive ratio. If you swap back to oem tire size try the gps comparison and see what the difference is. NOW, if you pop those 4:10 gears in? Thats a hole nuther deal...

 

if you tell me what teeth are in the tailshaft now, I will tell you what you have to get to, AND I will give you a link to the guy who has the gear in stock.

the 3.55 drive/driven ratio is cake - I have 2 of them in stock, but the 410 required a 23 tooth driven gear which wore funny so in some case the tailshaft went to a different drive gear like a 6 tooth. stock drive for a 3.55 was 7 - see the issue?

take it apart, count some teeth and get back to me

 

It's got the pink 19 tooth gear in it now. Maybe I should buy a few different ones and "dial it in"?

 

count your drive gear - is it a 7 tooth? (its not the easiest to count, you need to spin the yoke exactly 1 turn and count the 'ends' of gear teeth)

and just so I get this right:

your truck has 355 rears, for some reason you are replacing the xfer case with another one that used to drive 410 gears

count the drive gear in the previously driving a 410 xfer case - is it also a 7 tooth? if not some changes to make.

the 33" tires are about the same as dropping almost to the 308 axle, so if all else is the same, id try a 17 tooth and go from there.