My current Fox has 3.25:1 with P225/60R15 and runs the 19 tooth. At 3.40 the 20 tooth should be darn close but for your exact combo you may need the 21.

Good luck

 

Did your speed seem correct before the change (you were keeping up with traffic and your speedo read somewhere near the speed limit)? Have you verified your speed with gps?

Gear sets are nothing but mathmatical ratios. If you calculate the change in ratio going from a 2.8 to a 3.4 the ratio changed by 0.60 or an increase of 21%. If you increase the original speedo gear count (17) by 21% you get 20.6. If you want to try a 20 tooth gear I've got an extra you can have. PM me if you're interested.

 

I did use a gps both before and after the change to verify the correct speed against what the speedo said and like I said it did seem very close before changing to the 3.40's.
I will be sending you a PM quik thank you

 

Its this easy . . if you were happy with it before the change, you just have to adjust the teeth the same percentage that the gears changed. You made a 21% change. 17 x .21 =3.57 . . added to the original 17 . . . is a 20 or 21 tooth gear.

 

Seems legit ,I guess the caculator's I used weren't quit right:shrug:

 

No the calculators are fine, but they include tire diameter. That is what complicates the math. But my opening part of the question covers that part . . .
. . . if you were happy with it before the change. . . .

 

Just a little update I was finally able to get under the car and swap in a different speedo gear I went with the red 21 tooth gear. As noted in the "what did you do for your Mustang" thread I drove about 4 miles and it stopped working mid shift to 4th and a thought aw crap stripped the gear:skullnbones: as I recall reading about this with the 23 teeth gear .
When in fact the cable broke I noticed a "funny" spot in it and it was the wrong one any way and I have been giving a proper cable in the meantime from when that one was put in.
So I got under the car again this morning feed the other cable around and through went for a test drive and 45 looks right on the money 65 is about 62/63 close enough :shrug:

 

When I told MOdern Driveline my rear gear which of course are 3.40, he told me he's never heard of them for a Ford 8. Hoping the new speedo gear is accurate, but not sure if the one I'm using would still be accurate.

 

Just in case you need it, here's the speedometer gear calculator:

http://www.latemodelrestoration.com/products/Mustang-Speedometer-Gear-Calculator

 

Just in case you need it, here's the speedometer gear calculator:

Mustang Speedometer Gear Calculator

actually as it turn out the calculator doesn't have a 3.40 rear gear listed anyway:loco:
Nope I'm good now ,But it might help others I'll post the ones I used as well :thumbsup:
Speedometer Gear Calculator and Charts - TCI® Auto

Speedometer Gears

Ford Mustang: Mustang Tech: Calculators & Decoders: RPM Calculator

 

Forget all the calculators. Take your GPS and drive the car as is. Compare your speedo to the GPS. Calculate the % difference and change the number of gear teeth by the same percentage. If you indicated speed is too slow, you need less teeth. If your indicated speed is higher than actual, you need more teeth.

 

In retrospect this would have saved me some time ,although I'll admit math sometimes eludes me :shrug:

 

Or maybe your car IS that fast!

 

I wish

 

2nd 66: What size gear eventually worked out for you? I also have a 3.40 gear, but I’m running a C4 and a 25” tall tire.