I have a transmission that came from the factory specified for a 3.23 gear ratio car. The transmission is going to be in front of a 3.42 ratio rear. Is it fairly simple to just change the gear that drives the speedometer cable to get the correct speedometer reading ?

Yes, can be done in the car. I would check how accurate the car is now, correct any error as well as take care of gear change in one step. It might require both drive and driven gear changes, possible in the car on most earlier transmissions. A extreme gear ratio sometime needs an external gear box to get in range. Bruce Roe

Need 3 numbers, rear tire size (height), rear gear ratio, and number of teeth on the speedometer DRIVE gear. 20168/ tire height = Tire Revs per mile (TRM) TRM X Ratio X Tooth count of DRIVE gear/ 1001 = DRIVEN gear tooth count. For a 1970 GS455 Stage 1 stock P245/60R-15 tires at 26.6" tall 3.42 Ratio 16 tooth DRIVE gear 20168/26.6 = 758.195 758.195 X 3.42 X 16/1001 = 41.44 or 41 tooth DRIVEN gear required. http://www.tciauto.com/tc/speedometer-gear-calculator

Transmission is currently out of the car so now would be the time to make the adjustments if possible.

The formula 20168/ tire height = Tire Revs per mile (TRM) would be true if your tire made contact as a perfectly round disc. However tires are FLAT where they contact the road, flexing (varying length) coming off contact. The real ROLLING circumference is determine by the belt inside, so the above formula will always err. There is no need for trial and error with a working setup. Just make a precise measurement of current error (GPS, calibrated mile markers, etc) and then calculate the correction needed on your current gears, you will be precisely on the first time. Bruce Roe

Bruce, I have yet to have someone explain to me how the circumference of the tire changes. The circumference of a circle is Pi X Diameter. I don't understand how that Changes just because it flattens out at the contact patch. Does it stretch, does it shrink? The tire revolutions per mile are based on the circumference. If there is an error, is it really worth talking about? I have always used the formula for calculating the DRIVEN gear necessary. Maybe I am wrong or I have a mental block about this. I just don't get it. My Speedometer is driven off the Gear Vendors. I followed the chart in the instruction manual and my speedometer is dead on according to the GPS. It was just as accurate when it was driven off the transmission and I used the formula. From the GV manual if anyone is curious.

Well, they're rubber! Air pressure, speed, tread wear.... all minor unless you're talking top fuel! I had figured 1,000 speedo cable revolutions per mile, but I think the auto makers use a slightly different figure, maybe for slippage? Bob, note that the gear holder (sleeve) is meant for a certain range of driven gear sizes. Sometimes you have to change the sleeve if the gear is too big/small.

I get that, centrifugal force might have the tire grow at speed which would increase the rolling circumference, but the fact that the tire flattens at the contact patch has nothing to do with that? Slippage, that has to be pretty small, not sure that's even worth considering. The formula isn't perfect, but it is easiest IMO. Even the factory used ratio adapters for some combinations. That is definitely the only option for some not willing to change a DRIVE gear that makes a DRIVEN gear not available.

I will try to explain in detail when I get back from mailing some owners car parts. In the mean time, some research might turn up "rolling diameter" that some tire makers release. Bruce Roe

the outside circumference of the tire isnt going to change enough to make the speedo reading change. so a 26 inch tire has 81.61 inches of tread and that stays the same even if the tire went oval or heaven forbid ...square.

1001 speedo cable turns per mile is to make the odometer correct, which has reduction gearing 13:1, 11:1, and 7:1. Multiplies to 1001:1 As I recall the target for the Metric version is 616:1 We know the factory was not super keen on accuracy, and I am not telling anyone how accurate is good enough. But I will tell how I made my car as accurate as possible. Regarding the flat side of a tire. Imagine a vehicle with tracks instead of wheels, it always has a flat section of track on the ground, the rest of the track shape does not matter. Imagine a pin was pulled and the track laid out flat on the ground. If we measure the exact distance between the holes the pin goes in at each end, we could agree that is the distance travelled when the track goes around exactly once. THEN consider this. The above said nothing about how high the pin is above the ground. In fact, if the track wore down so the pin was an inch closer to the ground, the measured length would be EXACTLY THE SAME. That is how tires work. The "PIN" is at the level of the fiber belt in the tire, not at ground level. The tread is not continuous, but flexes as it meets and leaves the ground. The outer surface of the tread is not at the effective dia, the belt is. I came to realize this messing with transmissions in the 70s. A car I modified came mostly by accident, to be about a decimal place more accurate than normal. I could (did) drive 50 miles by mile markers, and they would still be exactly in sync with my odometer. But surprisingly, when those new tires wore down, THE ACCURACY WAS UNCHANGED. The final evidence was when a different brand of tire (same size) made a measurable shift in my carefully checked readings, because its belt was not done exactly the same way. In time I measured some local roads to 1/100 mile, so I did not need to go on a toll road for each check. Using the usual formula will get you in the ball park, except any input error will blow it. How do you get the tire dia anyway? But if you measure error of what you have, the required correction is easily and exactly calculated. My PHOTOBUCKET album SPEEDOMETERS AND GAUGES shows some of this, including how to roll your car forward one tire rev and measure its effective circumference. Probably not as precise as at speed. http://s93.photobucket.com/albums/L71/bcroe/ click on an Album click on a picture to enlarge + description Bruce Roe

Thanks for the explanation Bruce. When I was a toddler, I always asked 'why?'.... I never out grew that! Nice solar array! Have I seen you post on the SolarPanelTalk forum? Thought I saw your name there. When changing gears to correct speedo error, I usually figure 1 tooth on the driven gear for 3 mph. Is that a decent ballpark for a TH300/400?

I can not give you direct teeth per mph, it would depend on the drive gear and also the axle ratio. Look in there and get the tooth count on both speedo gears to get the ratio. Measure the error on the road, figure the ratio you really need, and pick new gears to get the new ratio. Yes my solar supplies all my heat and electricity here west of Chicago. Bruce Roe .

what about cable stretch and flex? If you burn rubber a lot you will make your tire smaller too. Thats why you need STEEL TIRES!. Yes the new tire that stays hard always.