thinking of replacing rear end gears

[snampro]

I just went to a 19" rear wheel with a 345/30-19 tire and am considering changing the rear end ratio to compensate for the change in overall wheel diameter and maybe go a little farther to bump the performance. What ratio is stock? what are my options? how much does something like this cost?

 

[mad0953]

In 1997 I know the ratio is 3:07. I had a Mustang Cobra once (didn't keep it long) that I changed the rear gears to a 3:73 and it woke the 'Stang right up.

 

[KenH]

How much larger is the tire diameter?

Stock gearing is 3.07. Most common gears put in are 3:55 which work well with a mostly stock motor. 3.33's are sometimes considered optimal, but are not readily available. If you put in 3.55 and your tires are 2" bigger in diameter, I think that makes an equivelent gear ratio of about 3.33. 1" bigger would probably put you around 3.45.

Some Gen1 owners like 3.73's. For some reason they work better on a Gen 1 than they do on Gen II's and III's. Personally, I think they would be too low.

I paid $1200 for my 3:55 gears installed, but I am sure there are others that have gotten it done cheaper.

 

[snampro]

the stock 335/35-17 has a diameter of 26"

the 345/30-19 on there now has a diamter of 27.4"

thanks for the information

 

[joe117]

I put in 3.55 gears. So now 6th gear can be used more often.
I went to the sticky Koumo tires fro some added traction.

I'd say that a 3.55 might be ok for a Mustang but it's probably going to be too much gear for a Viper.

 

[Steve 00RT/10]

the 345/30-19 on there now has a diamter of 27.4"

Same tire OD I have with Michelin Pilot Sport package on the 2001. Effective gear ratio for a stock rear end thus becomes a 2.92. Plugging into a calculator I found shows that to return to OEM, you would need a 3.22 gear set. 3.33s are probably the closest you'll get---if you can get them. Pretty short supply the last few years.

There is a marked difference in acceleration between a 2.92 gear and the 3.45 set we have in the other car. ....Both cars with roughly the same power output.

Steve

 

[ViperJoe]

http://www4.forum.viperclub.org/showflat.php?Cat=0&Number=739099&an=0&page=0#Post739099

 

[FE 065]

Unitrax just emailed me back today saying they'll have the 3.33 in December. Otherwise they carry 3.07, 3.45, 3.55, 3.73 etc.

I had 3.73s in my mostly stock '94 RT/10 ten yrs ago and loved that ratio for street,and the strip once in a while.

The SOTP difference was HUGE!

 

[Daddy Long Stroke]

There is a marked difference in acceleration between a 2.92 gear and the 3.45 set we have in the other car. ....Both cars with roughly the same power output.

Steve, how much of a acceleration difference?

 

[rcdice]

Steve,

Are both your cars running 19" rears? In other words are the 3.45's really spinning 3.45 vs. stock or less because of the increased tire size?

Thanks,

 

[PhoenixGTS]

The difference between the taller tires and the stock size is right about 0.20:1 of ratio, i.e. a 3.73 gear with the taller tires is the same as a 3.55 gear with the shorter stock height tires.

And in conjunction with sticky tires, short gears are unbelievable. My car with 3.55 would kill my car back when it had stock gears from a stand still. It is So much easier to control your launch with shorter gears.

 

[rcdice]

So technically, the gears in our Vipers have been getting TALLER as rear tire sizes (OEM) have gone from 17" to 18" to 19". Yes I know that all Vipers came with 3.07 but the effective ratio has changed.

Correct?

 

[PhoenixGTS]

So technically, the gears in our Vipers have been getting TALLER as rear tire sizes (OEM) have gone from 17" to 18" to 19". Yes I know that all Vipers came with 3.07 but the effective ratio has changed.

Correct?

No. The 17" and 18" GenI-II tires are the same diameter (progressively shorter sidewall). It was not until the Gen III 19"s that there was an effective ratio reduction.

 

[Steve 00RT/10]

Steve, how much of a acceleration difference?

I have not done any actual timing yet with the 2.92 car, but the SOTP is very noticeable to me. Both cars have 5 pound Roe blowers so in 1st gear they both kind of explode with the instant torque. (neither have water/****) I noticed the difference more in 2,3,4,5,6. I am not finished tuning the new car yet, it will be spring now, but I'm very close for around a 60 degree temp. It was running very rich when we got it. I had to get all the WB stuff to start tuning as well as a VEC2 upgrade from Sean to be able to log.

With the 3.45 car,I can go from 60-100 in 5th gear in around 7.25 seconds. That's pretty quick for .74 OD. 60-90 in 3rd in 3.03 seconds. 60-100 in 4th in 4.78 seconds. 60-120 in 4th in 7.45 seconds. These times are with a hand held stopwatch and pushing about 4100 pounds. It is a see saw thing back and forth with different gears, but the 3:45 car definitely seems faster to me. I'll know more next spring.

Are both your cars running 19" rears? In other words are the 3.45's really spinning 3.45 vs. stock or less because of the increased tire size?

The 2001 car has 19"s all the way around. The 2000 car is stock rubber--335/30/18s in the back. They measure 26.1" or so OD. So yes, the 3.45 car is pushing the OEM set up.

Steve

 

[Knight Viper]

I purchased the whole 3:55 unitrax assembly and speedometer calibrator used from a fellow VCA member for a little under $1,000 and love it!!!! So if you ever add more power, just pull one out and put the other in and you're good to go.

 

[V 10 MAFIA]

3:33's are probably available at Randy's Ring and Pinion
www.ringpinion.com

Thats where I got my 3:55's a few years ago.

 

[snampro]

so it sounds like stock was 3.07, with my larger rear wheels I would need approximately a 3.33 to bring it close to original, 3.55 would be quicker (more revs for a given speed and gear)...

Pulling the pumpkin off looks easy, so I think I'll just order the new parts, remove the diff, and take it to a shop to have the gears installed. It should be hard to find a shop that will work on a Dana 44.

from that link, the only ratio for a Dana 44 is 4.10, and that's a bit much.

 

[FE 065]

6000 x 27.4
---------------(div by)
3.55 x 336

= 137.826mph @ 6000rpm in 4th gear


6000 = rpm
27.4 = tire diameter
3.55 = overall gear ratio (trans gear x ring and pinion)
336 = constant



Variations of the same formula:

To find gear ratio

rpm x td
--------
mph x 336



To find rpm

gear ratio x 336 x mph
----------------------
tire diameter



To find tire diameter

gear ratio x 336 x mph
----------------------
rpm

 

[joe117]

"I'd say that a 3.55 might be ok for a Mustang but it's probably going to be too much gear for a Viper."

What I meant was that I think the 3.73 would be to much.