[jhein]

Originally Posted by scott70

I’m going to do the 2.87 / .81 OD. With my driving style , 3.31 gear and 255/60-15 tire I think it will be good. It’s about a 500 rpm drop from 4th to 5th which is good enough for me.

That is the exact combo I have. Same trans gears, rear gear and tire. As I recall, you and I both rarely if ever drive at Interstate speeds. 55 mph is really the highest speed limit I encounter. The .81 OD is perfect for 65 and even 70 mph cruising. Even 75 mph @ ~2500 rpm is still very comfortable.

[scott70]

Originally Posted by jhein

That is the exact combo I have. Same trans gears, rear gear and tire. As I recall, you and I both rarely if ever drive at Interstate speeds. 55 mph is really the highest speed limit I encounter. The .81 OD is perfect for 65 and even 70 mph cruising. Even 75 mph @ ~2500 rpm is still very comfortable.

Yes,, most highway driving around me is a 60 mph limit which means I’ll be cruising 65. If I hit the interstate ( rarely) it would be 75 and like you said that still manageable .

[Formulajones]

That's a lot of money to spend for a small gain. If you guys are really driving that slow (55 mph) why are you even looking at overdrives? For as little as you drive these cars and cruising at the old speed limits I wouldn't even be thinking about an overdrive.

[jhein]

Originally Posted by Formulajones

That's a lot of money to spend for a small gain. If you guys are really driving that slow (55 mph) why are you even looking at overdrives? For as little as you drive these cars and cruising at the old speed limits I wouldn't even be thinking about an overdrive.

Well, for a 55 mph limit I will go up to 65 (my local LEOs ignore that here and in most places I think). That's around 2700 rpm in 4th gear vs 2150 in 5th. That's a pretty big difference. Plus you also get the 2.87 1st gear, that turns my 3.31 rear into the equivalent of a 3.77 when compared to a Muncie with it's 2.52 1st gear. It works for me. Your driving habits are different and if I used my car like that I'd most likely get the .68 too.

[Formulajones]

Yeah 65 starts to get in that realm of "do I want one or not" type of deal. I'd do the overdrive with anything that speed or more. They just make the car a pleasure to drive any distance.

Honestly I find I use 5th more than I thought I would anyway, even cruising around town at 35-45 mph and still have plenty of ooomf to accelerate from there with itty bitty engines so I'm okay with it.

[scott70]

Originally Posted by jhein

Well, for a 55 mph limit I will go up to 65 (my local LEOs ignore that here and in most places I think). That's around 2700 rpm in 4th gear vs 2150 in 5th. That's a pretty big difference. Plus you also get the 2.87 1st gear, that turns my 3.31 rear into the equivalent of a 3.77 when compared to a Muncie with it's 2.52 1st gear. It works for me. Your driving habits are different and if I used my car like that I'd most likely get the .68 too.

My 2.20 first gear with the 3.31 is obviously doable but the 2.87 should be much more pleasant. So you get a 550 rpm drop going to 5th?

[jhein]

Originally Posted by scott70

My 2.20 first gear with the 3.31 is obviously doable but the 2.87 should be much more pleasant. So you get a 550 rpm drop going to 5th?

I'm rounding slightly. If I use my tach that's what it looks like, but it's hard to get a really precise reading with the resolution of the tach needle. You need a really flat, level surface and then get in a steady state with zero acceleration or deceleration and then try to get an exact reading from the tach. Maybe it's me but it's not as easy as it seems to get the resolution down to the tens of MPH. When I use an online RPM calculator I get 509. So it's in that ballpark.

I'm sure you've used them, this is the one I use:

https://purperformance.com/p-29669-rpm-calculator.html