rpgdeity : Speedometer Correction

Speedometer Correction
Post by rpgdeity on May 3, 2006, 1:29am

I have the stock NP200 transfer case with the stock speedometer. I am currently running the 11r16 XL's. Is there a way I can correct my speedometer for the larger tire size? Is there a gear or something inside the transfer case I can switch out? If that isn't possible, anybody ever gone to an aftermarket speedo that allows you to correct for tire size? I'd really like to keep my stock speedo, but it is also rather important that I get correct speed and mileage readings. Thanks.

Re: Speedometer Correction
Post by porkchop on May 3, 2006, 2:22am

Mine seems to be 5 mph slower. You can take it to a speedo shop and they should be able to build a box for you to ajust for the tire size. I am going to move mine from the 200 to the tranny (sm465) to allow more room for the box.

Re: Speedometer Correction
Post by compexp on May 3, 2006, 8:53am

you can put the gear ratio box on either end of the cable PC, which would allow you to run it behind the dash.

Re: Speedometer Correction
Post by brute4c on May 3, 2006, 8:55am

There are correction devices around for around $100 if I recall the stuff right....might be about the same price to get the newer speedo and correct with that...the correction devices are gear driven affairs that are kinda trial and error to setup.

As to whether you can change the tcase gear or not, no difinitive info to give you...some have said they believe it to be the same as an NP205 gear but to date I recall no positive statement that anyone has ever done such a swap and had it work. If anyone wants to check it out, the stock one is:

GEAR, DRIVEN 17 TEETH
FSN 2520-741-1861
Reference Number 11640679


For bigger tires, one needs to speed up the rate at which the cable turns at a given RPM to reflect the new higher speed at the same RPM...this would require a drive gear with LESS THAN 17 teeth...how many less, I dont know...

Re: Speedometer Correction
Post by porkchop on May 3, 2006, 9:41am


Quote: <TABLE class=bordercolor cellSpacing=1 width="90%"><TBODY><TR><TD width="100%"><TABLE class=quote cellSpacing=0 cellPadding=2 width="100%"><TBODY><TR><TD width="100%">you can put the gear ratio box on either end of the cable PC, which would allow you to run it behind the dash.</TD></TR></TBODY></TABLE></TD></TR></TBODY></TABLE>

True, but if I run it off the tranny it will also be easier to remove the cable if needed. Ever try to take one off with the ebrake in place? What a pain!

Jon, you can just change the the small gear shaft in the t-case to get a different ratio. This is the one below the cable mount. I changed the one in an old D20 I had. Trying to find one for a NP200 my be tricking.

Re: Speedometer Correction
Post by brute4c on May 3, 2006, 9:44am

I found this frmula for figuring what gear one needs to change to when making a driveline change:

New Teeth = (Current Teeth x Speedo Reading)/Actual Speed.

If we take the stock tire a 34 inches and the new tire at 38 inches and we use a constant RPM to check speed, say 3000 RPM,we find that at 3000 RPM:

34 inch tire diameter yields a speedo reading of 52.13
38 inch tire diameter yields a speedo reading of 58.26

Pluggin these into the formula with the known 17 tooth count on the stock gear, we get:

15.21

or a 15 tooth gear.

BUT WAIT!!

My truck has been checked to read 60 when the actual speed is 58 miles per hour...this with 9.00-16 tires measuring 34.5 inches in height.
If we take the difference in the stock numbers and plug them into the original formula, we find that I should have a 17.59 tooth gear now...taking that, which would make me accurate with the stock setup, and then figuring in the new tire size difference, we find that a:

15.73 tooth gear

comes out of the formula...this suggests that a 15 tooth gear would read more than 2 MPH high at 60, and that a 16 tooth gear would read roughly 1 mph low at 60...so...

A 15 tooth gear at 60 would read 63 or so...

A 16 tooth gear at 60 would read 59 or so...


Theoretically...if that formula is correct....

Re: Speedometer Correction
Post by barrman on May 3, 2006, 12:00pm

Here are some sites that let you figure this stuff out as far as the numbers, not the NP200.

http://www.lightlineofla.com/gearcalc.html

Hmm, the other 3 web sites that do this aren't working today. Maybe later.