Getting my speedo to be more accurate

[Rich69shortfleet]

When I changed the truck to 3.73 gears (from 3.07 gears) I did the calculations and changed the speedo driven gear to get the speedo back into calibration. However, a few weeks ago I was traveling with a friend. Our speedometer readings didn't match. I told him I was going to drive 65 and so he did too and he pulled away. So, apparently my speedometer is reading fast, or his is off too!

Anyway, on the way home at 100 miles from home I noted the odometer reading. This 100 miles was covered on the Interstate with never leaving the Interstate. When I reached home, the odometer was reading 7 more miles than it should have given the 100 miles covered. So, my speedometer and odometer are reading 7% fast.

If I recall correctly, on the driven gears one tooth is worth about 5% increase or decrease, depending on which direction you go. So, to slow down my speedometer to more closely match the speed I'm going, I'd want to find a driven gear one tooth larger than what I have now. I still need to pull the bullet and see which gear I have since I honestly don't remember, but I was hoping others here can help check my logic on what I need to do, ie: get a driven gear one tooth larger.

TIA

[rockyrivermark]

Easiest thing to do is buy this set and try a few. Get a mph app on your phone and test. Only takes minutes to change them out.
One of these gears should put you real close.
RRM

[Rich69shortfleet]

Yeah, I did all that and ended up with the set up I currently have. Which is reading a touch fast. Maybe my speedometer head is running a bit lazy.

I checked and I have a 21 tooth gear so a 22 tooth gear should get me a lot closer to actual speed.

[rockyrivermark]

Ah. Got ya. Possibly a different drive gear?
I can't remember website but do a search online
There is a online calculator.
Your plug in gear ratio and tire dimensions and it spits out proper comination of drive gear and driven gear.
Hope it helps.
Mark

[jocko]

Recommend once you make another change (or perhaps before you make a change..) that you use a GPS for speed and distance crosscheck rather than your buddy's vehicle or hi-way markers - this will be much more accurate. And, like you said, who knows if your buddy's vehicle is correct either. 7% off on the distance, even if calculated by highway markers, is a chunk though, so at least you know which direction you need to move.

[BRL]

Originally Posted by jocko

Recommend once you make another change (or perhaps before you make a change..) that you use a GPS for speed...

OK I took out the GPS today after reading this post, and after almost running off the road trying to start the dang thing up while moving......the GPS indicated that at a speedometer reading of 45 MPH, I was actually doing about 50.

My question is after I pull out the bullet, should I be looking to increase or decrease the number of teeth?

I figure about one tooth either way before rechecking, and I have a few extra entire bullets with different numbered teeth laying around.

ETA: I just pulled out the bullet, (nice to not have to do something ridiculous like drop the trans. on these old trucks) and the center one is it.

Notice how clean and pink the fluid left on the teeth is!

One on left is 23 teeth but not sure if that bullet is even right for a turbo 350. (notice the center post on the gear is shorter too)

My current one (center) is 22 teeth, and the one on the right is 21 teeth.

Hopefully that will give someone the info. they need to suggest a direction.

[Fbks_Cruiser]

So if you want to increase your speedometer relative to the gps. You need to go fewer teeth on the driven gear. In your case the one on the right. We just had a thread on this for my rig. In my case the speedo is reading too fast.

[BRL]

Originally Posted by Fbks_Cruiser

...In your case the one on the right...

Sounds great, I'll toss it in right now.

Much Thanks!

[68is4me]

Originally Posted by Fbks_Cruiser

So if you want to increase your speedometer relative to the gps. You need to go fewer teeth on the driven gear. In your case the one on the right. We just had a thread on this for my rig. In my case the speedo is reading too fast.

Wait what?!?. His speedo is running faster then the gps, so you need to slow it down. Go 1 tooth bigger to slow down the speedo.

[Rich69shortfleet]

Originally Posted by 68is4me

Wait what?!?. His speedo is running faster then the gps, so you need to slow it down. Go 1 tooth bigger to slow down the speedo.

Exactly.

[BRL]

Originally Posted by BRL

...the GPS indicated that at a speedometer reading of 45 MPH, I was actually doing about 50...

Sorry if my quoted statement sounded confusing, but I do believe Fbks_Cruiser has it right.

Speedometer reading 45 MPH.

GPS reading 50 MPH.

So the speedometer on mine needs to be sped up to match (in my case) my GPS reading.

I'll take my truck for a spin today and see if the two speed readings more closely match.

[Rich69shortfleet]

Originally Posted by BRL

Sorry if my quoted statement sounded confusing, but I do believe Fbks_Cruiser has it right.

Speedometer reading 45 MPH.

GPS reading 50 MPH.

So the speedometer on mine and the OP's needs to be sped up to match (in my case) my GPS reading.

I'll take my truck for a spin today and see if the two speed readings more closely match.

You are right, the wording in the earlier post was kind of confusing. I had to reread it a few times over.

[BRL]

Yea, I probably could have worded it better...and I was even sober when I wrote it too.

Anyway, that IS the correct gear!

Just got back from my road test, and the speedometer and GPS match perfectly now!

Sad part is I knew the speedometer was off for years, had the extra gears for years, and did not take the ten minutes to try and fix it in all that time.

Thanks for finally getting me motivated OP, and good luck in getting yours set up as well.

[68is4me]

Ok, I get it now. I was confused thought it was the op and all off a sudden it went from reading to high then to, to low. I didn't see somebody else chime in with a different issue. Glad you got it worked out.

[Rich69shortfleet]

Originally Posted by BRL

Thanks for finally getting me motivated OP, and good luck in getting yours set up as well.

Well, you can thank me by sending me that gray 22 tooth gear.....

That's the one I'm going to need. I'll post back when I get one and get it in. They aren't expensive so I'm trying to decide what else to buy to make the shipping charges at least make sense!

[BRL]

Originally Posted by Rich69shortfleet

Well, you can thank me by sending me that gray 22 tooth gear..... ...

Not a problem, PM me with an address and you can have it,

[VWNate1]

THANK YOU for the wonderful roadside picture ! .

[BRL]

Yea, that's a few miles down the street from home.

The route to that spot looks pretty much the same but the road is more curvy.

This place is beautiful up here!

Sure makes road tests a snap!

[VWNate1]

My late Father lived in Bellingham, Wa. on the edge of Lake Whatcom, it was very nice indeed and he spent years trying to get me to move up there, no thanx, I hate the rain .

So many beautiful back roads in the PNW, Oregon and so on .

I grew up in the Country and hope to live out my remaining days in town .

[Mikes68C10]

Originally Posted by rockyrivermark

Ah. Got ya. Possibly a different drive gear?
I can't remember website but do a search online
There is a online calculator.
Your plug in gear ratio and tire dimensions and it spits out proper comination of drive gear and driven gear.
Hope it helps.
Mark

Re; Online Calculator. This site is loaded with calculators for automobiles, enjoy!

http://www.wallaceracing.com/Calculators.htm

http://www.wallaceracing.com/speedo-calculator.php

[Rich69shortfleet]

Originally Posted by BRL

Not a problem, PM me with an address and you can have it,

PM sent. I was just joking, but thanks!

[OregonNed]

Great information here. Mine has been reading 10 MPH higher than actual GPS MPH reading. Its been this way since purchase. My set is on the way.

[Rich69shortfleet]

Well, a little more follow up. My testing procedures in the past have been to time myself between mile markers on the interstate. I usually did 10 mile stretches to get it to work out better. Looks like I got it pretty correct in the end.

Yesterday I downloaded what is supposed to be one of the most accurate GPS tracking phone apps. Using that today I found that when the GPS says 60mph the speedo says 61mph. That's pretty close and good enough to me. Now I have to tell my friend that his much more modern truck is reading incorrectly. He won't believe me.....

Sending BRL a follow up PM....

[OregonNed]

The Gray 22 tooth gear was the ticket for me!

[BRL]

Glad you got it right, it is nice not to have to guess anymore eh?.

Can't tell you how many times I drove down the highway @ 45 past a county sheriff coming around a blind corner in the opposite direction, thinking I was maybe going 5 mph over, and it turns out I was doing at least 10 over.