OK, just finished rebuilding a 68 Charger with a 383 auto, installed Edlebrock heads and manifold and a Holley 750 DP and a 2500 Hughes, I had a 2:76 in the back stock, put in a 3:55, hole shot was great but now cruising sucks because I'm stuck at 55 at 2800 rpms running 60x15 tires, and, the speedometer was about 12 mph off (read faster) and was 4 miles over at 10 miles reading 14 miles, I'm going back to the 2:76 to cruise again and here is my question as silly as it sounds, is a 70x14 the same as a 60x15 and should that 2:76 gear and the 60's bring back to the right speed and correct odometer reading, thanks in advance.

 

Bill -

Go to

http://www.summitracing.com/expertad...atioCalculator

And see what you should be getting.
275-15s should be 28" in diameter.
You'll need more info on your tires, are they 235-14s, etc. Should be on the sidewall.

Pluggingin your numbers, @ 55 mph, you should be around 2343 rpm.

Gotta tell ya, I'm running 3.23s /275 60s on my Charger and have never had a problem on the highway. But yes, you will have to change your speedo gear. That's fairly easy to do.

Archer

 

Richard Ehrenberg says a 3:23 is the best all around gear and that's one thing I agree on. Go here to get your speedo gear and they also have a speedo gear calculator on that site.

http://www.transmissioncenter.net/sp...n_______va.htm

 

I also agree.
The 3.23 will give you just a touch more at the bottom, cruises the freeway nice and will still bury a 120mph speedometer.
I made the 2.76 to 3.23 swap in my first Fury 30yrs ago with a '69 Dodge 383 Magnum/Torqueflite and 3.23 Suregrip.
The car would do about 65 in low, almost 100 in 2nd and 120mph would come and go....

 

By the way... Welcome to the forum!

 

2.76 freeway gears. I used to shift 2th to drive about 95 mph in my 360 Coronet

 

Thanks fellas and thanks for the welcome, looks like I'll be spending a lot of time here, looks like I'll stick with the 2:76, I'm ok with it and not crazy about watching the tach every other second, so, if anyone is looking for a 3:55 let me know bf1260@charter.net . Also on this 68 I put a new cam and chain kit form Comp, a CRB 270h-10, and after market a/c from Classic auto air, an electronic disributor from Mopar, fromt disc brakes, headers, rack& pinion from Unisteer, hammered out electrical issues (nightmare), now ready for body and paint.

 

With a 750 double pumper and a 2500 stall and a comp 270 cam a 2.76 is just too low. I would go with the 3.55 gear it is best for your setup. At the minimum a 3.23 gear. Yes cruising rpms will be up a bit but your tranny will last longer from not building up heat from the converter slipping all the time. It's your car so definately go with the gear you want just don't be afraid of overrevving at 60 mph with a 3.55. You won't. You might want to glance at the tach when you hit 110 mph or so. You could allways go with an overdrive. Good luck have fun with it. BTW I have allways wanted a 68 I love the lines and the taillights over a '69 Cool car!

 

I would keep the 3.55's an look for some drag radials with something like a 30" height