What is this thing between the trans and speedometer cable?

[RemCharger]

Bf Goodrich and the tire calculator online said my 275/60/15 tires are 28 inches in diameter. When i measured them by hand they look to be 26.5 or so. With my new speedometer gear meant for 28 inch tires, the speedometer reads too fast by about 2-4mph around town and even worse on the highway

So who do you believe?
There's 3 possibilities
Measured wrong
Read the tire size wrong
The tire shrunk in the sun

At this point if you're close, just tune your speedo by changing your gear tooth count slightly

 

[Cranky]

Yes do you want me to make sure the odometer rolls over in exactly 1 minute?

Stop watch it driving at a steady 60 mph reading and see how long it takes to go that mile then do the math to see what the % off it is. Doing that will get you pretty close. Might do 3 passes and take an average....

 

[RemCharger]

Just get a cell phone app for speedo checking

 

[Jerry Hall]

With the tire mounted on the car the measurement will be less. Width of rim will also change the measurement slightly. I don't think on our 50 plus year old cars that it will ever be exact. Metric sizes won't match the old sizes exactly either. There were calibrated speedometer's on police cars then, but were they actually correct?

 

[Black_Sheep]

I did that with a speaker wire and did the math equation to convert it to diameter and got 28.01. But when I do it by measuring just the diameter with a tape measure it looks to be 26.5

That’s because of tire squat, the loaded radius is shorter…

 

[Cranky]

Yeah, measure from across the tire and not from the ground up.....it'll be different.

 

[josh202221]

Okay I’m gonna trust that the tire is actually 28 inches like bf Goodrich says and what my circumference measurement said. I’m gonna replace my speedometer cable because it’s way too long and from a discount part store. I also don’t think it was lubricated before it was put in by the shop and that’s why the speedometer needle is shaky at low speeds. I don’t want to keep putting speedometer gears in it because it’s expensive and time consuming. I also wonder if the original speedometer is due for a rebuild. It seems like the speeds it’s off by are inconsistent because on the highway it’s off by over 10mph and around town it’s not so bad.

 

[Jerry Hall]

Okay I’m gonna trust that the tire is actually 28 inches like bf Goodrich says and what my circumference measurement said. I’m gonna replace my speedometer cable because it’s way too long and from a discount part store. I also don’t think it was lubricated before it was put in by the shop and that’s why the speedometer needle is shaky at low speeds. I don’t want to keep putting speedometer gears in it because it’s expensive and time consuming. I also wonder if the original speedometer is due for a rebuild. It seems like the speeds it’s off by are inconsistent because on the highway it’s off by over 10mph and around town it’s not so bad.

If it's shakey at slow speeds, I'm not sure one can tell how far it's off. You could be correct on the speedometer cable and if it's to long the routing could be adding to the problem.
In the past I have checked the speedometer by using a short piece of an old cable with a variable speed drill. If it's still shakey at a slow speed then a rebuild may be needed.

 

[MCODECUDA]

Got any roads with mile markers and has at least a 60 mph limit or faster?

There are apps on your phone that will give you your speed while you are driving. At least there is on my phone.

 

[khryslerkid]

There are apps on your phone that will give you your speed while you are driving. At least there is on my phone.

Yes. Maps is one. I downloaded an app that uses GPS to show your speed before Maps started showing your speed.

I used the speedometer chart to calculate what gear I needed with the tires and gear ratio and it was spot on.