Swapping rear gears

[Mark1972]

Regarding swapping the rear gears in my 8.25. I have 2.71 years and an open diff now, and while I'm not building a light to light car, I'm also not spending much time on the highway. I was thinking of looking for a 3.55 set. It seems middle of the road. For anyone that has made this change, what can you tell me about how the car felt after swapping those high gears out for a lower 3.55-3.90ish gear? Just looking for a little seat of the pants improvement. It's my 1970 Plymouth Satellite with 318 and A904.

 

[747mopar]

You'll notice a significant gain in acceleration especially with the 318 but I wouldn't go higher than 3:55s. To me 3:55s are the limit for all around use unless you live in the sticks and only drive 55mph roads, 3:55s will still have you up around 3,200-3,500 rpm at 70mph depending on tire dia.

 

[Fran Blacker]

3.55 a good all around gear. 3.73's if you do limited highway driving. If you drive 20 miles to get anywhere 3.55 max, MO. Here's a thread covers everything twice.
https://www.forbbodiesonly.com/moparforum/threads/opinions-on-rear-gears.160238/

 

[rumcustom]

3:55 gears in my 69 charger with 383/727. Same type of driving you are talking about. I’m very happy with them.

 

[Jcharger]

Like what the others said you will notice a significant difference in how the car feels. your ratio is changing by a factor of ~1.3 which is like upping the torque by 30%. I did a similar magnitude change (different starting and ending gears). 3.55 should not rev not too high, but you will need to get used to having the engine reving higher on the highway, a hit on gas consumption.

To see what it will be like at 70, put it in 2nd at 60. 1.54/1*60 = 92is almost equal to 3.55/2.71*70 = 93. The 1.54 number is the ratio for 2nd in a 904. The 3.55 and 2.71 are the rear gears ratios and the 1 is the final drive ratio.

I don't think your driveshaft will be spinning too fast, but as it spins faster you may find balance issues you didn't know were there because it was spinning slowly. Not a big deal to fix if that happens, but worth knowing about it if it does.

 

[Mark1972]

Well it sounds like 3.55 is the way to go. The car is in storage for the year now because its snowing here, lol. Last year it was t shirt weather most of October. So it gives me some time to find some gears. Thank you all for the responses.

 

[RT4ever]

If you ever drive in the states, on the interstates, most cars are going 75 to 80 mph. The 2:71 gears I have are nice at 80 mph.

 

[BlooDSMeaR]

In my opinion, I wouldn't use a gear higher then 3.23 for any thing less then a race car. My car has 4.10 and @ 55 im pulling 3000 RPM. I am in the process of building a 3.23 rear for my car, so I can drive it to more places. The post above this one is correct about putting it in to 2nd and see how you like driving it for 30 minutes on the hwy.