Speedometer gear?

[glen courier]

Hey Guys,
Using my GPS for true speed, my Courier is indicating 75 mph while doing 65 mph true.
I have 26" tires and 3.00 rear ratio with a c6 tranny. Any idea what I need for a speedo gear?
Thanks Glen

[JimNolan]

Glen,
  My advice is to see how many teeth the driven ( cable gear ) is and go down one less tooth. Not knowing what the number of drive teeth in the transmission makes it hard to figure exactly what you need. I'd try that or go to a 15 inch tire. Jim

[RICH MUISE]

one of the big advantages of having a modified car is you can use the option of an electronic speedo...takes all of the gearing problems away. changs tire sizes or tranny or whatever...no problem...push a button...drive a mile and push it again, and everything is reset correctly...at least that's the plan.  Rich

[shopratwoody]

No fun having an old car that's electronic to me.
Mine's fast also. I'm going to try a 21 tooth gear. My trans is
proly set up for a 3:00 or so.

[JimNolan]

Shoprat,
   You know, I feel the same way. But, let them come out with a new car that's got all the new fangled gadgets on it and the body of a 57 Ford and I'd buy it. I don't mean a remake of the 56 TBird or 65 mustang. But, if I were a Dodge guy I'd have a Challenger. Those new cars don't even look like what we had back then. They couldn't even sell the TBird ( that was the most famous TBird ever and they ended up ruining it. The front end looked like a 53 corvette. If they made a 57 Fairlane 500 body and a new car running gear and interior, I'd buy it. I think the car company's still cater to the younger people. They should have learned by now the younger generation will go out and buy a Mazda or Toyoda with no thought to patronizing American Iron. I guess that's why the new American cars look like Toyoda's and Mazda's. Jim

[glen courier]

Thanks, I haven't pulled the existing gear to see what it is yet. Dropping some teeth would be a good idea, or I can drive it and do the math and hope the "give me a ticket red" won't get too expensive.

[JimNolan]

Glen,
  I told you wrong. You have to add teeth to the driven gear to slow down the speedometer. Look at it this way. If you put smaller tires on your car without changing anything the speedometer will read faster. Bigger tires will make the speedometer read slower. Well, that's what we did. We went from a 750 X14 inch Bias ply tire that was 27.65 inches tall and put on 205 X 14 radials that are 26.1 inches tall. The formula for driven gear tooth count says that when you run a small tire you run a higher tooth count on the driven gear. There's a lot of places on the internet where you can figure driven gear tooth counts. When I was using them I wanted to complicate things by trying to visulize what was happening. Quit visulizing. Just look at your figures, smaller tire = more teeth.
  My car is off by 5 mph exactly using gps with 27.1 inch tires. It's off about 9 mph using 26.1 inch tires. The difference between those two tires is about 4 mph.
If my calculations are right using the Driven Gear formula, I'll need to add a couple of teeth to whatever the tooth count is on the speedometer gear I have now.
I hope this doesn't confuse you. Jim P. S. I just found out that you need to take .8 inches off the tire dia to get a true measurement of tire revolutions per mile. Example = 26.1 inch dia. tire is actually 25.3 inches when you figure squish of tire with weight on it.

[glen courier]

Thanks Jim,I was thinking that over and realized you were going the wrong way there for my problem.
Funny how much power the written word has.

[shopratwoody]

Jim,
I would buy the same as you. They did screw up the T-Bird for sure.
The new Mustangs are the only retro cars that I think come close.
It's just in the "real" 57 I like to feel the way it was. I was 16
when the came out Can't go back, but the car can.

[Lou]

Regarding your speedometer, all auto speedometers are set from the factory to read 5 MPH faster than your true speed. The exception to this are speedometers (calibrated) included in a police package. 

[JimNolan]

Lou,
    You have got to be one of the most educated people I've seen on old Fords. Take care of yourself, we want to know more. Jim

[Lou]

Jim, If by educated you mean I have a head full of mostly useless trivia that I bore and/or piss people off with then I guess I'm a educated guy....lol

[JimNolan]

Glen,
   I couldn't send you an email that would allow me to insert photo's so if the guys will forgive me I'll post them here so you can see it. Jim

[glen courier]

Jim,
Very cool, and you flew it. That qualifys you to work on Ford brake systems.

[glen courier]

Okay, pulled the gear. It is white and 17 tooth. This back thing is getting old!