Calibrating for a taller tire

[meanchicken]

I switched to 28" tall rear tires which of course makes the speedometer read slower.

How do I go about selecting and installing the right "driven" speedo gear for the speedometer at the transmission in order to get my speedometer to read accurately again?

Stock rear gear set.

 

[77 cruiser]

This works pretty good.

TCI - TECHNICAL INFORMATION: Speedometer Gears

 

[meanchicken]

Thanks!

 

[bison]

Did you actually measure the tire or go off the side wall dimensions?

 

[meanchicken]

I still need to measure it.
Currently I'm calling it a 28" based on the manufacturer spec.

Firestone Firehawk Indy 500, 275/60/15's

 

[Turbo6Smackdown]

Have a quick question about 28" tall tires. Is the additional traction they're providing over 26" tires because of the sidewall, or the fact that they change your rear end gear because of their diameter so to speak.
If I were to change my rear end gear to keep it behaving like stock, would I still get a lot of additional traction with the taller tire?

 

[TT/Ameasap]

Have a quick question about 28" tall tires. Is the additional traction they're providing over 26" tires because of the sidewall, or the fact that they change your rear end gear because of their diameter so to speak.
If I were to change my rear end gear to keep it behaving like stock, would I still get a lot of additional traction with the taller tire?

Yes, larger diameter tires provide better traction. Taller has been proven better than wider tires for traction purposes..And of course were talking straight line performance.
Yes, a taller tire will essentially make the overall gear ratio higher.
I run 3.54 gears and 30'' tall MT DR's and it hooks from a brake torque roll without a burnout. Dead hooks at the track and carries both wheels 20ft.
RPM's are 5500-5600 through the traps.

 

[Heskahoka]

Following what 77 cruiser gave you above from the TCI website, here is the gear calculator to calculate to number of teeth for the speedometer gear:
http://www.tciauto.com/tc/speedometer-gear-calculator

In addition, I found another article that gives a more accurate way of calculation the tire diameter:
http://www.rodandcustommagazine.com/techarticles/0801rc_mechanica_speedometer_calibration/

Per the article:
"Put a chalk mark on the bottom center of the sidewall and on the pavement at that same point. Roll the car forward until the sidewall chalk mark is at bottom center again, and measure that point on the pavement from the starting point. That distance divided by Pi (3.14159265) will give you a precise real-world diameter."
Hope this helps.