I understand how changing tire circumference affects actual gear ratio. What I don't know is how to tell which tire size is correct to use to be at a "zero" reference point to go up or down from.. Hopefully some one will understand what I mean......

_________________
1966 Dart GT
4 spd. OD conversion
8 3/4 - 3.55 SG (Sold)

62 Lancer GT
B Body 8 3/4

There's no standard baseline size. You pick your own baseline size and go from there. If you're starting from a big question mark (initial stages of a buildup) then probably the easiest way to go is to peg your calculations to engine RPMs. Figure out what RPMs you want to run at any given MPH, and then work through the calculations from there.

For reference, 3.23s with P185/80R13 tires gives 2642 RPM at 60mph, while
the same 3.23s with P205/70R14 tires gives you 2574 RPM at 60mph. Easy calculator is Here.

Thanks for that calculator, that's really handy (had to scroll to the bottom to get that magic tire diameter calculator).

I've now gone from 3.23s to 3.16s with my new tires. Wooohooo big change

MJ

New tires? What'd I miss? What'd you wind up with?

Thanks for the calculator. One think I don't quite understand. On the calculator if youre tire diameter increases youre gear ratio gets higher (numerically) On my cars it always been the opposite taller tires lowered the final drive ratio(numerically). Is there something I'm not understanding here? Thanks for any help........

_________________
1966 Dart GT
4 spd. OD conversion
8 3/4 - 3.55 SG (Sold)

62 Lancer GT
B Body 8 3/4

For an example I went from 24" tires to 26.4" tires . The calculator says my gear ratio went from 3.55 to 3.90?

_________________
1966 Dart GT
4 spd. OD conversion
8 3/4 - 3.55 SG (Sold)

62 Lancer GT
B Body 8 3/4

EDIT

The calculator works correctly. You and I both made the same mistake; we misinterpreted the intent of the calculator at the top of the page. When you put in the original and new tire sizes and the axle ratio and hit "Compute", it spits out the gear ratio you'd have to change to in order to keep the same engine speed at the same vehicle speed with the new tires.

You and I were both looking to it for a "What's my new effective gear ratio?" answer, which isn't what it does...UNLESS you "trick" the calculator by swapping your new and old tire sizes, in which case you'll get the answer you're looking for.

Consider a '66 Dart that came stock with (in today's nomenclature) P185/80R13 tires, which are 24.65" diameter. You're thinking about putting on P205/70R14 tires, which are 25.3" diameter. The car has a 2.93 rear axle ratio.

You plug in the old (24.65) and new (25.3) tire diameters, and the rear axle ratio (2.93), and hit "Compute". The result you get (3.01, rounded up) tells you the rear axle ratio you'd need if you wanted to have the same engine RPMs at the same vehicle speeds with the new tires as with the old tires.

But if you want to know your new effective gear ratio, you swap the old and new tire diameters: Put in 25.3 for the old and 24.65 for the new, plug in your 2.93, hit "Compute", and you get 2.85, which is the new effective gear ratio.

mine worked, I had 185/80/13 and my new 215/60/14 gave me the 3.16 vs. 3.23

Ah you just been busy I picked up some 14x6 cragars with 215/60/14 BFG Radial TAs on them. I was going to stick them on tonight when Cami goes to bed (i'm banned from car work right now since there is SO much to do on the house) and see how it goes.

BTW in that vein, taking off the lug nuts. driver's side LH thread, passenger RH? I cant' recall which way it went.

MJ

Yep, left on the left, right on the right.

good everything loosens towards the back...that's easy to remember

Ty sir

MJ

Or you could just use this handy page:

http://www.moparts.com/Tech/tech/pages/speedo.html

you're right Dan thanks.

_________________
1966 Dart GT
4 spd. OD conversion
8 3/4 - 3.55 SG (Sold)

62 Lancer GT
B Body 8 3/4