[pete315]

Buddy has a 76 GP with a dead 2.56 rear. Will he see much of an improvement in off the line performance with a change to a 3.08 rear? He knows he will lose a little gas mileage. Will his speedometer be off then? Is it hard to change the speedometer gear in an automatic tranny? Thanks.

[Skip Fix]

My 78 TA had a speedo adapter in the cable to adapt for the 2.56s removing it and swapping tranny speedo gears worked for that. Also depends if he changes tire size also.

[abefromen]

"Buddy has a 76 GP with a dead 2.56 rear. Will he see much of an improvement in off the line performance with a change to a 3.08 rear?"

a little


"Will his speedometer be off then?"

yes


"Is it hard to change the speedometer gear in an automatic tranny?"

Speedo driven gear, no.....speedo drive gear yes it's more involved as it's on the output shaft and you have to remove the tailshaft.But their are other ways to deal with it as posted above

[The Champ]

He'll probably feel like Superman on full throttle acceleration with the change to the 3.08's...

Yeah, he might lose a little fuel mileage - but how many miles does he drive per year? These cars are supposed to bring a smile to our faces - 3.08's will do that better than 2.56's.

As Skip posted above, the gear change will affect the speedometer, but tire size can also do that. It may be that the combination of the new gears and taller tires than stock gets the speedometer right on.

[ta man]

2.56 to 3.08 is a great swap about 20 percent more torque..still good on the highway..Is it a 455?

[pete315]

400 with a turbo 350 trans

[1971WARBIRD]

you might even gain some mileage if you are lucky. if this moves the engine RPM into it's power/ most efficent range during crusing, you may gain. sometimes gears are too high which can cause the car to lug, and not be as fuel efficent.

[zeebo]

yeah..the 3.08 swap will be nice with a stock 400. there are calculators on the net to get you the driven gear you need for the trans...not difficult to do at all..

[pete315]

Will I be able to just change the driven gear? I figure I can remove the driven gear in car now, count the teeth and just put in a gear with less teeth. It doesn't have to be exact, just close. Tire size will remain the same. When and why would you have to change the driven gear and drive gear in tail section? Hopefully I can get away with just changing the driven gear because it's not that much of a jump from 2.56 to 3.08 gears.

[zeebo]

Originally Posted by Tom Vaught

Rear Gears come as a matched set.

Typically you cannot use a pinion gear from a different gear set and have any real success.

I would rather see you swap the whole rear axle assy (3.08 rears are typically cheap and they made a lot of them). I could at one time swap a complete GM rear axle in my driveway in 3 hours working by myself with hand tools and a floor jack/jack stands.

Tom Vaught

tom, i think he is talking about the gears for the speedo in the trans....

[zeebo]

well, you may be able to change just the driven gear, but it would be really helpful to know the tooth count on the drive gear too..but for example..lets say your driven gear was a 36 tooth---now you are going to be spinning the driveshaft faster, so your speedo is gonna read faster---now you will need to add teeth to your driven gear to slow the speedo needle back down to where it should be..say going to a 40 or 41 tooth. note that on a th400 there are 2 different driven gear adaptors..i think one for 34 to 39 tooth and one for 40 to 45 tooth. easy to find on the net. tci has a good gear calculator on their page, but from some experiences, if you calculate you need a 40 tooth driven gear, many have found their speedos to be more accurate going up one tooth. if you have a gps, this really rocks in helping see your true mph cruising and calibrating your speedo..no reason with some testing it cant be practically dead on...

[shineybike]

I went from a 2.56 to a 3.23 rear, a ST-300 to a TH 400 and 14 inch wheels to 15 inch wheels. On the TH 400 there is a plastic gear behind where the speedo cable mounts. My speedo of course was off. I am not at home so can't look in my records, but there is a company in Texas that I called. Gave them my tire diameter and some other info and they sent me a replacement plastic gear. When it arrived the shaft diameter was not right for the mount in my tranny, so they sent me a replacement mount. I think the total cost was about $60. Worked like a charm and easy to install yourself. Maybe someone can chime in with the name of the company.

You will love the 3.08's vs the 2.56. I don't worry about gas mileage since I don't put that many miles on, but my rpm's are not to bad at 65 mph, so 3.08 would be a little lower.