Speedometer Pinion Gear

-
KJoeZ61

KJoeZ61

Well-Known Member
Joined
Apr 4, 2017
Messages
422
Reaction score
147
Location
Northampton Pennsylvania
My 71 Scamps speedometer is way off. It was born with a 904 trans and 2.76 rear and now has a 3.23. I'm running 225/60R 14 tires which are 24" in diameter. According to the chart I need a 35 tooth gear, does that sound correct? I was out today and the speedometer said 40 but GPS on my phone was saying about 30 MPH.

Mancini
Mancini Racing - 35 - Tooth Speedometer Gear
 
I would imagine it's the original, but I haven't puled it out yet. Anyway you look at it can't be correct since I changed the third member and tires.
 
how far off is the tire height from stock? Usually it ends up being very close to rear gear X 10 to get tooth count on stock height tires.
 
Gear teeth you have now to what you need is a ratio of true speed to displayed speed.

Regardless of tire size
 
35 is what i see as well from TCI Speedometer Gear Calculator

They do mention this though:
The housing must be placed in the correct orientation for the particular driven gear being used. The three positions broken down by tooth count are; 25-31, 32-38, 39-45
 
ChuckDock said:
Not exactly sure what you are saying, but tire diameter is definitely part of the equation I'd say
Click to expand...
If GPS says you are going 60 but your speedometer says your going 50, your speedometer is 16.67 percent slow. If your current gear is 45 teeth you need a gear smaller by 16.67 percent which is 37.5 teeth so your options are 37 or 38 teeth, no tire size needed.
 
Dana67Dart said:
If GPS says you are going 60 but your speedometer says your going 50, your speedometer is 16.67 percent slow. If your current gear is 45 teeth you need a gear smaller by 16.67 percent which is 37.5 teeth so your options are 37 or 38 teeth, no tire size needed.
Click to expand...
I'm thinking that will confuse the issue more since the poster has the opposite issue and different speeds.
His speedo says 40mph, the gps says 30mph.
Plus he doesn't know the tooth count on the current gear.

So, since he does know his tire height, and his rear end ratio, why not just use the calculator and get the right part the first time. These are a little pricey at around 44 bucks each.
 
Yes they're pricey; that's exactly why you crawl under there and remove your gear and count the teeth. Even little things like tire pressure and tread wear can change the reading, so to compare the present tooth count to the present mph reading difference would be a double check on which new gear to buy. It's not going to be perfect, but knowing what gear you're starting with will get you closer to perfection than all the calculations in the world without it.
 
ChuckDock said:
I'm thinking that will confuse the issue more since the poster has the opposite issue and different speeds.
His speedo says 40mph, the gps says 30mph
Click to expand...

My example is just that, an example.

The equation is really simple...
New teeth = Speedometer / GPS x Current teeth.
Doesn't matter speedo faster or slower.


I have a spreadheet I the made that does all the thinking for you. Just enter the current teeth, GPS speed and speedo speed and it will give you two gears and the amount each is over or under so you can get the closest.
 
KJoeZ61 said:
I didn’t think this was going to be so complicated.
Click to expand...
Lol,
Well there you go KJoeZ, all the info you will ever need to buy the right part!
When you pull out the old gear count the teeth and let us know what it is.
Its not complicated at all, just a couple methods to try and it will be interesting to see the results.
 
Sorry it took so long but it has rained every night here in eastern Pennsylvania. The existing gear has 29 teeth and is black. Again the tires are practically new and are properly inflated and measure 24" 225/60R14 and it has a 3:23 rear in it.

It's definitely going to need a seal kit as well. I noticed Brewer's Performance sells the gear and seal kit for $50.00, but Ive never dealt with them.
 
I meant that you could determine the tooth count by looking at just the color on the end of the stem without taking it out. Does a 36 tooth red gear sound right?
 
KJoeZ61 said:
Sorry it took so long but it has rained every night here in eastern Pennsylvania. The existing gear has 29 teeth and is black. Again the tires are practically new and are properly inflated and measure 24" 225/60R14 and it has a 3:23 rear in it
Click to expand...
you need a 34 tooth
speedo-gear-jpg.jpg
 
The difference between 40 and 30 = 10. 10 is 1/4 of 40. 1/4 of 29 is 7.2. 29 + 7.2 = 36.2. Maybe you're not reading your GPS accurately. Use mile markers.
 
KJoeZ61 said:
34 or 36? 35?
Click to expand...
Well..that is the question for sure. Probably no one will tell you which to pick specifically since we all seem to come up with different numbers. Once you choose, let us know which one, and how close it ends up being.
 
If the actual (GPS) speed is "30", but the speedometer reads "40", then the speedometer needs to be slowed down. Needs a bigger gear. 30mph÷40mph=0.75. And 29 teeth÷0.75=38.7 teeth. It looks like you need a 39 tooth gear, based on the observed error.
However, with a 225/60-14 (24.6") tire and a 3.23 gear, you should need a 34 tooth gear. Something is fishy here.
 
You must log in or register to reply here.
-

Back
Top