61 speedo gear for 4:11 rear end

[John S.]

Need some help. Speedo is off in the 61. At 50 mph on the speedo actual is about 43-44 mph. Car stats are:

4:11 rear gear
4 speed original trans
22t white/clear/greenish driven plastic gear
P205/75 r15 Coker Classic Wide white wall tires

By my calculations I should get the 25t orange plastic gear. (Very hard to find I am learning) I believe this would still mate up right and it would put me right in where I need to be.

Next best option would be a 24t gear which I can find anywhere. I believe this would be the correct gear if the tire o.d. was back at standard. I know this will not get me all the way there but it will get me close. Any suggestions?

[Joe L.]

Re: 61 speedo gear for 4:11 rear end

Need some help. Speedo is off in the 61. At 50 mph on the speedo actual is about 43-44 mph. Car stats are:

4:11 rear gear
4 speed original trans
22t white/clear/greenish driven plastic gear
P205/75 r15 Coker Classic Wide white wall tires

By my calculations I should get the 25t orange plastic gear. (Very hard to find I am learning) I believe this would still mate up right and it would put me right in where I need to be.

Next best option would be a 24t gear which I can find anywhere. I believe this would be the correct gear if the tire o.d. was back at standard. I know this will not get me all the way there but it will get me close. Any suggestions?

John------

The original driven gear for your application was a 24 tooth currently available from GM under GM #3860347. Assuming your current gear is a GREEN 22 tooth gear (with an OD of 0.875") , the 24 tooth gear is completely compatible with your installed DRIVE gear.

However, your tire diameter is about 1.2" less than the original tires. So, the original gear for your application might not read correctly. The 25 tooth gear, GM #3860348, is discontinued. However, aftermarket equivalents for this gear are relatively easy to find. Speedometer shops as well as online speedometer parts suppliers should easily be able to get you one for a very modest price. There are some on eBay right now for as low as 10 bucks. (Note: don't be foolish and buy an NOS example which is on currently eBay for about 150 bucks; anyone that would pay that kind of money for a gear that can easily be found in the aftermarket is just plain NUTS).

[Mike E.]

Re: 61 speedo gear for 4:11 rear end

John,
what is the circumference of your tire (as opposed to that of the original 6.70x15)? My parts books, group 4.337, indicate that the greenish gear with 22 teeth is for a 3.70 or 3.73 rear end, while 24-tooth yellow is correct for 4.11. I think 25 teeth would register too high.

[John S.]

Re: 61 speedo gear for 4:11 rear end

Mike,
I agree that the 24t gear would probably be correct for the 4:11. Remaining difference would be the tire dia being small. The 25t gear would make up the difference I believe..

[Joe L.]

Re: 61 speedo gear for 4:11 rear end

Mike,
I agree that the 24t gear would probably be correct for the 4:11. Remaining difference would be the tire dia being small. The 25t gear would make up the difference I believe..

John------


Well, by my calculation you need a gear of 24.4 teeth. Of course, no such gear could be made. The 24 tooth gear is closer than a 25 tooth, though. To get your speedometer "dead-on" you would need to use a speedometer adapter. A competent speedometer shop could fix you up with one but I think the cost would not justify the benefit. Factory speedometers of the era were certainly not "dead-on". A 1-2% error was common.

Police vehicle speedometers were "certified" to a greater degree of accuracy and usually used a speedometer adapter to achieve this.

[Duke W.]

Re: 61 speedo gear for 4:11 rear end

The OE 6.70-15 bias ply tires are speced at 760 revs/mile. I don't know if Coker publishes a spec, but most 205/75R-15 tires are speced at 766 or very close, so the difference is less than one percent.

Also consider that as a tire wears revs/mile increases - about 2 percent between new and the wear bars.

Because pneumatic tires deflect under load, you cannot use inflated OD to accurately calculate tire revs per mile. Manufacturers measure it on a test drum, usually with the tire inflated to maximum cold air pressure with maximum load applied at a speed between 45 and 60 MPH. Always use the manufacturers spec, if available. If not use 97 percent of the inflated circumference to estimate revs per mile.

A 24T driven gear will increase cable speed by 24/22 = 1.091, and 1.091(43.5) = 47.4... still about 5 percent low, but accuracy will improve as the tires wear. Maybe those Coker's have lower revs per mile than typical tires of that size.

Duke

[John S.]

Re: 61 speedo gear for 4:11 rear end

Duke,
I think you got it mixed up. The speedo is reading over by about that 6mph. Speedo reads 50 mph when car is traveling 44 mph. I do get it though. I am probably just going to get the 24t gear as it is correct for the current set up. Hope to see you in Chatsworth in a couple of weeks.

John

[Duke W.]

Re: 61 speedo gear for 4:11 rear end

You need to slow the speedometer reading down, and you do with with more teeth on the driven gear. We both came up with the same answer of 24T for the driven gear. Actually 25T would be better, but no such animal may exist for the size and tooth count of the drive gear in the transmission. My Cosworth Vega has a 25T driven gear but the 7T drive gear is unique to the CV because of the 4.10 axle a 873 rev/mile OE tires. It's smaller in diameter than the internal drive gear on most GM cars of the era.

Speedometers can have gain error, offset error, or some combination of both. You should get accurate speed readings at speedometer readings from about 20 to at least 80, then plot the speedo reading versus actual from a modern car, GPS or whatever. If it's linear - about the same percent at each speed - the error is gain type. If the speedo error is constant, it's offset error.

The latter is can usually be corrected by removing the needle from the pin and positioning it properly to eliminate any offset error, then choose a proper driven gear to eliminate the gain error.

Whenever a speedometer is rebuilt it needs to be calibrated. I think the standard for that era was 1000 revs on the cable was 60 MPH.

When I installed higher rev per mile tires on the Cosworth Vega years ago, it caused the speedo to read nearly five percent high and there was no 26T driven gear, so I took it to a speedometer shop and they changed the magnetic flux on the head to reduce the gain, and now it's dead on, so that's another option if there are still any traditional speedometer repair shops around.

I'll be at the chapter meet in two weeks.

Duke

[Joe L.]

Re: 61 speedo gear for 4:11 rear end

You need to slow the speedometer reading down, and you do with with more teeth on the driven gear. We both came up with the same answer of 24T for the driven gear. Actually 25T would be better, but no such animal may exist for the size and tooth count of the drive gear in the transmission.



Duke

Duke------


A 25 tooth driven gear does exist which is compatible with his installed drive gear. It's GM #3860348 but it's discontinued. Apparently, the manufacture of this gear has ceased but there should still be plenty around in the aftermarket.

[John S.]

Re: 61 speedo gear for 4:11 rear end

I just ordered the 25t one on ebay. Will post results once I get it installed. Called around to a few local transmission shops here in the L.A. area and got the same response from all. These gears are getting harder to find and no one here is stocking them. Now on the internet I see them all over but some places are showing out of stock on the 24 and 25 tooth gears. I am thinking of buying a set of all gears to keep on hand.

[Joe L.]

Re: 61 speedo gear for 4:11 rear end

I just ordered the 25t one on ebay. Will post results once I get it installed. Called around to a few local transmission shops here in the L.A. area and got the same response from all. These gears are getting harder to find and no one here is stocking them. Now on the internet I see them all over but some places are showing out of stock on the 24 and 25 tooth gears. I am thinking of buying a set of all gears to keep on hand.

John------

As I mentioned, the 24 tooth gear is still available from GM. However, the 23 and 25 tooth gears are no longer available from GM. I think that part of the problem here is that these gears are typically used for applications with high numerical R&P gear ratios. There just isn't a lot of demand for those ratios these days, especially for street driven cars (i.e. the majority of cars and those needing a speedometer). So, when demand drops for something, the manufacturers have to decide if it makes economic sense to keep making it. In this case, they've apparently decided it doesn't.

By the way, after 1970 GM ceased using the 22 tooth (green) to 25 tooth gears and the associated drive gear. Instead they used only the 18-22 tooth (silver) set of driven gears and an associated drive gear. These accommodated rear gear ratios up to 3.70:1. For those few applications with higher numerical R&P ratios a converting speedometer adapter was used.

The fact that the 22 tooth (green) through 25 tooth driven gears have not been used since 1970 (with very few used even from that time and earlier) is probably one major factor in the reason these gears are "disappearing". I would not hold out too much hope that the 22 tooth (green) and 24 tooth gears will be around for too much longer.

[John S.]

Re: 61 speedo gear for 4:11 rear end

Update on Speedo gear. Received the 25t gear from Ebay supplier for total of $13.00. Good deal I thought. Anyways I received the item and it was exactly as described including a .880 dia. O.D. Ha! Hole in trans tail housing is .875! Put the gear in my drill motor and polished the O.D. down to about .872 with some 240 grit and a 6" scale. Slid right in. As a reference the GM 22t gear that came out measured .865 dia. Bottom line is I got it working and the speedo is now within 1 mph! Reads 50 and actual speed is 49. Not bad for a 55 year old car.