What's new
CK5 Network
Become a member of the largest online community for Chevy K5 Blazer, Suburban and C/K series truck owners!

# Speedometer recalibration

#### SpeedlabDan

##### Keepin it trashy never classy
GMOTM Winner
is anyone here currently or previously runnin an np205tcase with 4.88 gears and 35" tires by chance?
I have a right angle "Hubbard" cable adapter on there now that is 1:1.214 but as far as I could tell without doing all the math and mike marker measurements it seems to still be about 10 to 15mph fast.
It doesn't have to be perfect but it would be cool to know if anyone has a ratio somewhere close.
I know Texas electric has them but we have to do the math ourselves to order the correct ratio.

#### rampage

##### 3/4 ton status
10 to 15 mph at what speed? Too fast or too slow? It will always be off by a percentage. Find what percent it’s off by and what your current speedo gear ratio is, then either multiply or divide (depending on if the speedo is too fast or slow) that ratio by the percentage to get the ratio you need.

And just an FYI: someone with a username like yours shouldn’t have any trouble figuring this out.

#### SpeedlabDan

##### Keepin it trashy never classy
GMOTM Winner
10 to 15 mph at what speed? Too fast or too slow? It will always be off by a percentage. Find what percent it’s off by and what your current speedo gear ratio is, then either multiply or divide (depending on if the speedo is too fast or slow) that ratio by the percentage to get the ratio you need.

And just an FYI: someone with a username like yours shouldn’t have any trouble figuring this out.
Never claimed I knew how fast I was goin! Lol I'm trying to avoid all the simple math if someone has this common arrangement. I've yet to gps my speed it's just a guess that I may be going around 55 or so when it reads 80

#### wheels87k5

##### 24 Karat
GMOTM Winner
You will need some help for sure because that's more than 10-15 over.

Have someone ride with you, with GPS, and write down the different speeds of actual compared to what the speedometer says. Get several speed comparisons so you can get better accuracy on the percentages. Then go from there. Ask away, we'll get it figured out.

#### SpeedlabDan

##### Keepin it trashy never classy
GMOTM Winner
You will need some help for sure because that's more than 10-15 over.

Have someone ride with you, with GPS, and write down the different speeds of actual compared to what the speedometer says. Get several speed comparisons so you can get better accuracy on the percentages. Then go from there. Ask away, we'll get it figured out.
Could be 65 at 80 lol whatever if I felt like doin math on the fly I would leave it like that to give my brain a workout every time I look at the dash but I'm clearly lazy. I know someone on here has exactly the same setup I just want answers without all the leg work

#### Abusfullofnuns

##### 1/2 ton status
Speedbox is an app that I use. I used it all the way from NM to TN when I picked up the burb. Works really good.

#### SpeedlabDan

##### Keepin it trashy never classy
GMOTM Winner
Speedbox is an app that I use. I used it all the way from NM to TN when I picked up the burb. Works really good.
That's interesting I'll have to check that out

#### bigblock72

##### Elmer's glue taste tester
GMOTM Winner
Could be 65 at 80 lol whatever if I felt like doin math on the fly I would leave it like that to give my brain a workout every time I look at the dash but I'm clearly lazy. I know someone on here has exactly the same setup I just want answers without all the leg work

Start with pulling the cable adapter out, I'll bet it's within 10% without any correction. The math will be a lot easier to figure out when you take out the correction box.

#### SpeedlabDan

##### Keepin it trashy never classy
GMOTM Winner
Start with pulling the cable adapter out, I'll bet it's within 10% without any correction. The math will be a lot easier to figure out when you take out the correction box.
I concidered that it would be a good start once I find out how much faster I'm actually going

#### campfire

GMOTM Winner
Start with pulling the cable adapter out, I'll bet it's within 10% without any correction. The math will be a lot easier to figure out when you take out the correction box.

I disagree about the math being easier. Either way, he will be determining a speed correction ratio and multiplying that times the number of teeth on the driven gear to determine what new driven gear he should purchase.

An example from my life:

My Blazer currently reads 60MPH when it's going 55MPH. Its driven gear has 41 teeth (yellow in color). Taking the speed ratio times the tooth count give me (55/60 * 41 = 37.58). So a 38-tooth gear (blue) would give the closest reading. The math is easy once you figure out what correction ratio you need and what tooth count you currently have.

If you want to remove the gearbox you will need to correct for that in the math (or remove it and refigure your correction factor, it'll yield the same number). So (55/60*1.214 * 41 = 45.63). For my NP208 there is no 46-tooth gear option, so I would need to keep the box if this were my rig. Not sure what ratios are available for the NP205 but a quick check of Google & Ebay should tell you what's out there.

#### campfire

GMOTM Winner
And remember, the speedometer may have a bias one way or the other (e.g., reading 2MPH high), but that is not correctable. Only the ratio is adjustable, so your problem will scale with speed. If it's 20MPH off at 60, it's 10MPH off at 30.

#### brans87

##### 1/2 ton status
- In Memoriam -
Use your phone for speedo, there are apps. Mine was off when did 4.56 gears but when put 35's on I am dead on till about 62 which is fine with me.

Oh saying rang true when I asked about speedo gear keep it simple stupid.

I needed to do nothing but put my ties on and mine was pretty straight on my 87 K5.

#### SpeedlabDan

##### Keepin it trashy never classy
GMOTM Winner
I disagree about the math being easier. Either way, he will be determining a speed correction ratio and multiplying that times the number of teeth on the driven gear to determine what new driven gear he should purchase.

An example from my life:

My Blazer currently reads 60MPH when it's going 55MPH. Its driven gear has 41 teeth (yellow in color). Taking the speed ratio times the tooth count give me (55/60 * 41 = 37.58). So a 38-tooth gear (blue) would give the closest reading. The math is easy once you figure out what correction ratio you need and what tooth count you currently have.

If you want to remove the gearbox you will need to correct for that in the math (or remove it and refigure your correction factor, it'll yield the same number). So (55/60*1.214 * 41 = 45.63). For my NP208 there is no 46-tooth gear option, so I would need to keep the box if this were my rig. Not sure what ratios are available for the NP205 but a quick check of Google & Ebay should tell you what's out there.
Now I think the colors you are referring to are for the transmission speedo drive? The gears in the np205 drive are not interchangeable so adapters are used like I have but with different ratios. I need the correct ratio to order a new adapter though.

#### campfire

GMOTM Winner
Now I think the colors you are referring to are for the transmission speedo drive? The gears in the np205 drive are not interchangeable so adapters are used like I have but with different ratios. I need the correct ratio to order a new adapter though.

I don't own a 205, but reading around the web made it sound like there are multiple driving and driven gear options in addition to using the adapter (like it would be if you had 2WD or a NP208).

#### SpeedlabDan

##### Keepin it trashy never classy
GMOTM Winner
I do however have the app for my phone now I just gotta get my intake back on so I can road test

#### SpeedlabDan

##### Keepin it trashy never classy
GMOTM Winner
I don't own a 205, but reading around the web made it sound like there are multiple driving and driven gear options in addition to using the adapter (like it would be if you had 2WD or a NP208).

I'll have to look into this but from the research I've done the setup I have is not interchangeable and that makes sense bc when I rebuilt the tcase last either everything (worm gear) were set inside the case so total disassembly would be required to pull the worm gear

#### Blue85

##### Yooper
GMOTM Winner
What year is your case? Does it have a plastic drive gear (7 teeth) in there or metal (5 teeth)?

H 18
J
19
K
20
L
21
M
22

You can find used driven gears on eBay. I want to say NP205 and NP203 use the same gears, but I don't have a source to reference or know which years would interchange.

#### SpeedlabDan

##### Keepin it trashy never classy
GMOTM Winner
What year is your case? Does it have a plastic drive gear (7 teeth) in there or metal (5 teeth)?

H 18
J
19
K
20
L
21
M
22

You can find used driven gears on eBay. I want to say NP205 and NP203 use the same gears, but I don't have a source to reference or know which years would interchange.
Metal if I recall but it's a 1970 np205 non synchro

#### Fordum

##### 1 ton status
Couple of things.
First, the 205 in my Ford definitely has swap-able gears. I changed them one time back in '89, but I don't remember what was available. Mine has been a little high with my 12.50/33s, but I just lived with it for most of the time I had it.
My car, the speedo is fairly close, but it basically sucks. The numbers only go in whole numbers, and they are close together. I put a GPS in it, and never even look at the speedo anymore.

So, one day, I noticed an Autometer speedometer that used GPS to drive the needle. Way more than I was going to pay, but it started me thinking.
Bought a heads up GPS box for the truck, solved the problem. Just tossed it up on the dash, and done.
However, if I had it to do over again, I would get one that is dual display. The heads up part is great at night, and the little film that came with it helps in the daytime. but direct reading is better.

Here is one like I am talking about.
https://www.amazon.com/VJOYCAR-C60-...id=1512103963&sr=8-5&keywords=gps+speedometer

Have not tried that one, but it looks about like mine. Except that this one has the direct reading function that mine does not.

If you were to want a more traditional unit, they sell some like this.

https://www.amazon.com/Black-Speedo...id=1512103963&sr=8-8&keywords=gps+speedometer

Couple of things to watch out for. Make sure its a GPS unit. They sell a lot of them that plug into the OBD2 port and read the speed off the transmission. Which kinda defeats the purpose.
Also, some of them run off USB power and don't include an adapter. If the one you want doesn't, you need to grab one.
I would wire up a special socket that uses switched power so you don't have to unplug it when you get out of the car.

Given the relative accuracy, ease of installation, and low cost, I don't anticipate ever changing the speedo gears or trying to calibrate one again.
If you need accurate mileage for maintenance purposes, you might want to get one with an odometer function.

#### diesel4me

##### 1 ton status
- In Memoriam -
My '56 Chevy pickup never had a decent speedometer--I tried swapping 2 or more salvage yard dashes in it and only one had the speedo work somewhat normal,but nowhere near accurate..the original dash and one replacement would bury the needle after you hit 30-35 mph or so,and it was "lurchy" at lower speeds..

I just "estimated" my speed and was probably lucky I never got a ticket--after I got a tachometer and hooked it up,I was able to get a better estimate of the speed by using the rpm/tire size/rear axle ratio chart to determine what speed I'd be going at a certain rpm..
I also had a friend "pace" me on the highway at 55 mph,and I took note of the rpms--it was around 3300 rpm with L-78-15 tires and 4:56 rear end gears,so I tried to not exceed that rpm..(any faster it sounded like it would come apart after several miles too--55 mph was the speed limit here then..)