Page 1 of 2

Speedometer gear w/ radials?

Posted: Sat Mar 09, 2019 12:58 pm
by RedBird64
Like most, I have gone to 215/75 R15 radials. My speedometer is off by quite a margin.
Is there a speedometer gear tooth count that folks are using to correct the speed reading?

Thanks,
Scott

Re: Speedometer gear w/ radials?

Posted: Sat Mar 09, 2019 1:29 pm
by sseebart
I spent a good deal of time looking for this very thing many years ago when I had my transmission rebuilt. The shop owner hinted that such a thing might exist, but neither one of us was able to locate anything. I had the same experience with my F-100.

~Steve

Re: Speedometer gear w/ radials?

Posted: Sat Mar 09, 2019 1:40 pm
by RedBird64
Nothing is ever easy is it!
The BN has all the gears avail.

Maybe to go at this from another angle..
Does anyone know what gear our cars came with? Going by my lousy memory, I think the gear color I have indicates it has 18 teeth. I suspect 1 tooth difference is all I need but I was hoping to get some anectdotal experience.
I know I can jack the car up and check it but I was hoping to only do that once when I put a new gear in.

I'm assuming more teeth lowers the indication..


Scott

Re: Speedometer gear w/ radials?

Posted: Sat Mar 09, 2019 8:37 pm
by stubbie
All you need is a maths professor of which I am not. :crazyeyes:

Re: Speedometer gear w/ radials?

Posted: Sat Mar 09, 2019 8:52 pm
by stubbie
If 3.00-1 cruise o matic gear should be brown 15 teeth. 3.25-1 gear should be green 17 teeth 8 x 15 tyre

Re: Speedometer gear w/ radials?

Posted: Sat Mar 09, 2019 10:41 pm
by RedBird64
stubbie wrote:
Sat Mar 09, 2019 8:52 pm
If 3.00-1 cruise o matic gear should be brown 15 teeth. 3.25-1 gear should be green 17 teeth 8 x 15 tyre
There are 15" tires and there are 15" tires. Bias-ply to radial makes a big difference as does going from the OEM 78 aspect ratio to the radials 75. Plus, who knows how far off the speedo was to start with.

Re: Speedometer gear w/ radials?

Posted: Sun Mar 10, 2019 12:01 am
by stubbie
Your still going to have the brown or green plastic gear. Unless someone changed it somewhere along the line.
Does this help
https://www.automobile-catalog.com/tire ... matic.html
and this
https://www.tyresizecalculator.com/tyre ... calculator

Re: Speedometer gear w/ radials?

Posted: Sun Mar 10, 2019 11:55 am
by sseebart
RedBird64 wrote:
Sat Mar 09, 2019 10:41 pm
There are 15" tires and there are 15" tires. Bias-ply to radial makes a big difference as does going from the OEM 78 aspect ratio to the radials 75. Plus, who knows how far off the speedo was to start with.
Actually, that's something you can figure out pretty easily these days. There are a ton of free GPS speedometer apps available. Download one to your phone, set it somewhere you can see it and note the difference between it and the speedometer as you drive.

~Steve

Re: Speedometer gear w/ radials?

Posted: Sun Mar 10, 2019 12:27 pm
by RedBird64
sseebart wrote:
Sun Mar 10, 2019 11:55 am
RedBird64 wrote:
Sat Mar 09, 2019 10:41 pm
There are 15" tires and there are 15" tires. Bias-ply to radial makes a big difference as does going from the OEM 78 aspect ratio to the radials 75. Plus, who knows how far off the speedo was to start with.
Actually, that's something you can figure out pretty easily these days. There are a ton of free GPS speedometer apps available. Download one to your phone, set it somewhere you can see it and note the difference between it and the speedometer as you drive.

~Steve
Yeah, I have a hand held GPS I took for a drive and that's how I know I'm off.
The challenge is figuring out what 1 tooth equals in MPH.
IIRC, I'm off about 4 MPH at 60. My math tells me that's about 1 tooth. I need to get the car back together for some more testing though.

Re: Speedometer gear w/ radials?

Posted: Sun Mar 10, 2019 11:10 pm
by stubbie
I'd say almost 1 tooth but as said before I'm not a maths person.
64mph indicated divided by 60mph actual = 1.0666
15 teeth x 1.0666 = 15.999 teeth required
Did I do that right.

Re: Speedometer gear w/ radials?

Posted: Thu Mar 14, 2019 1:47 pm
by jtschug
First, calculate exactly how far off your speed is.

I use mile markers on the highway and a stopwatch. Maintain a constant indicated speed, i.e. 60MPH, and go 3-5 measured miles. Then calculate your actual speed. And see how far you are off.

As an example, let's say you went exactly 5 miles in 6 minutes. 6 minutes/ 60minutes/hr = 0.1 hours
So your speed was 5 miles / 0.1 hours. 5 divided by 0.1 = 50 MPH

The indicated speed was 60 MPH, so your error is (60-50)/50 = 10/50 = 20% fast

If your indicated speed is faster than your actual speed, like this example, you want to slow the gear down by adding teeth. In this case you want to add 20% more teeth so that your indicated speed matches your actual speed. If you pull the gear out of the transmission and count 18 teeth, then to add 20% you need 22 teeth. (the actual answer is 21.6 so you must round to the nearest whole number)

Summit has C6 gears from 16 to 23 teeth here:

https://www.summitracing.com/search/par ... ing&page=2

If your car has a C6 it should be pretty easy to find the gear you want, if it is an MX trans ('64-65 or earlier) it might be a bit more difficult, but I think it should be possible to find them.

Re: Speedometer gear w/ radials?

Posted: Thu Mar 14, 2019 1:53 pm
by jtschug
If you are having trouble with the math, just post how far the car went and the time it took and what the constant speedo speed was and I can calculate it for you.

Re: Speedometer gear w/ radials?

Posted: Thu Mar 14, 2019 11:42 pm
by RedBird64
I have to drive it again with the GPS (no more marker posts around here and GPS is far more accurate) and see for sure but I think the speedo was about 4 MPH fast at 60. From everything I've read, that will be 1 tooth (there are lots of threads regarding this on some other forums). Assuming 15 teeth presently, 60 actual MPH equals 3.7 MPH per tooth - or at least that's how I figured it. So I need a 16 tooth gear and that should get me about 60-61 indicated at 60 true.
I sent an email to the BN about asking if they knew anything about this subject but never got an answer. I figured there must be some gear they sell more of.
Before I can drive it I have to get the new surge tank, water pump and rebuilt radiator installed. Then I have to fix the #%&$ stripped oil pan.. :drinking:

Scott

Re: Speedometer gear w/ radials?

Posted: Mon Mar 18, 2019 6:24 am
by jtschug
Yes, you need one more tooth, but you should check how many teeth the gear has now. I don't think you can be sure what you have is a 16 tooth gear. These cars have had a lot of changes over 50+ years

Re: Speedometer gear w/ radials?

Posted: Mon Mar 18, 2019 12:06 pm
by RedBird64
jtschug wrote:
Mon Mar 18, 2019 6:24 am
Yes, you need one more tooth, but you should check how many teeth the gear has now. I don't think you can be sure what you have is a 16 tooth gear. These cars have had a lot of changes over 50+ years
I know I should have checked but I was ordering another part from the BN so I took a chance and ordered the 16 tooth. I'm counting on our long history with the car to assume it's still OEM.
My Dad bought the car with about 60K on it. The first owner, a wealthy older gentleman probably never touched it. After that it was only the shop that rebuilt it 30+ years ago for me that has ever touched it beyond myself.

Hopefully my gamble pays off!

Scott