Running 275/60's - Speedo advice

[TR Custom Parts]

Got rid of the 255/60 TA's I was running and now have a set of 275/60 drag radials. I am using 7" Torque Thrust 2 wheels with 3 3/4" backspacing (because that is what I had!) and they just fit. The driver's side will slightly rub when taking an exit ramp and I will probably have my welder notch the area where it is hitting. Pass side has plenty of clearance. Just wondering what to do to get proper speedo reading as I am sure going to this taller tire throws things out of whack a bit. Will this make the speedo read higher or lower than actual speed and will a simple speedo gear swap help. Is it like the older GM cars where you can just pull out the gear and swap it or is there more to it than that? Just to complicate things I am running 3.50 gears! (Ford 9")

 

[John Larkin]

I did some research on this and you need a black gear with 3.42 gears. It's basically 9% slower than the stock orange one. I can't remember the website I found this one though. Doh! I run 275/60-15's and I did some mile-marker calculations with my watch; that's where I got the real world percentage verified by some calculating. Your speedo presently will read slower than actual speed.

Hmmm, with a 3.50 gear vs. a 3.42, that's a 3% difference meaning you are 3% closer to the speedo being accurate (you aren't going as fast at your speedo reading as I am with mine).
So you're running 6% higher than the factory speedo with the orange gear presently. Looks like 60mph speedo reading @ 64mph actual road speed. Is it worth it? Dunno, but you'll be hard pressed to ever calculate your gas mileage if you don't correct it! Hehe.

 

[Black Car]

John is in theory correct.
The problem is that most speedometers (in older cars, like ours) are wrong to start with.
You will need to do as John suggests " do some mile-marker calculations with a watch" and see what you have.

If it is close I wouldn't worry about it.
I run 305x50x15 on my Silver T with 3.72 gears and am off about 3-4 mph but I also have a Cyberdyne dash.

Gary

 

[Lee_Burough]

I'm not smart on this subject, but the website below has a lot of info on the gears you'll need to get a correct indication on your speedo....

http://www.tciauto.com/tech_info/speedo_gears.htm

Good luck