Stupid Question - Overdrive

[MomoJeep]

(1) For a long time, I've been thinking of the 5th gear in a 5-speed manual as the "top gear". In other words, the gear to use at your highest speeds. Also, on the Novak website, they talk about "gear span". On other words, it's not how low your lowest gear is, but how big is the span from lowest to highest Then combine that with the right axle ratio). This implies you would use 5th like any other gear, but at your highest speeds.

(2) However, I have also heard of "overdrive" as the gear you shift into when you are *already* at top speed, but to drop the rpm's (and lower the torque) for fuel efficiency while cruising down the freeway.

So with that said, when I'm looking at dropping my axle ratio's, and want to figure out my top speed cruising rpm's, should I do the math for 4th gear (and treat 5th as in (2) above, a bonus gear not for higher speeds, but to lower rpm's at the same top speed. Or should I treat it as in (1) above, a gear to use like any other to reach higher speeds?

 

[Randyzzz]

(1) For a long time, I've been thinking of the 5th gear in a 5-speed manual as the "top gear". In other words, the gear to use at your highest speeds. Also, on the Novak website, they talk about "gear span". On other words, it's not how low your lowest gear is, but how big is the span from lowest to highest Then combine that with the right axle ratio). This implies you would use 5th like any other gear, but at your highest speeds.

(2) However, I have also heard of "overdrive" as the gear you shift into when you are *already* at top speed, but to drop the rpm's (and lower the torque) for fuel efficiency while cruising down the freeway.

So with that said, when I'm looking at dropping my axle ratio's, and want to figure out my top speed cruising rpm's, should I do the math for 4th gear (and treat 5th as in (2) above, a bonus gear not for higher speeds, but to lower rpm's at the same top speed. Or should I treat it as in (1) above, a gear to use like any other to reach higher speeds?

I have found that if you choose your gearing to accommodate scenario #2, then your overdrive gear is usually useless unless you are cruising at 55+ on level ground.

If you gear for scenario #1, 5th becomes much more useful.

My experience only. I’ve heard both ways.

 

[tower210]

Also depends on the torque characteristics of the motor. 4.0 seem to like humming along at 2600-2900. 4.2 much less. If you have an LS loping along at 900 is likely fine with plenty of reserve...

 

[Chamba]

My 351W (about 300 horse) with 3.73 axles, 34" BFGs and AX15 (HD version from Novak) cruises brilliantly at 75 mph turning about 1750.
It pulls like a school boy at that rpm, so I very rarely downshift unless I'm on a very long grade..... which are few-and-far-between in Florida.

 

[sdsupilot]

Look at the Grim Jeeper gear calculator. There are lots of considerations to make. I feel the biggest is overall engine power. As stated above a healthy V8 can happily pull hills at much lower RPMs than a 6. I am running 4.88 gears with a GM 4.3 and a 700r4. One of my concerns was being able to run in 3rd vs 4th for longer distances if the 6 couldn't maintain speed on hills at highway speed. In my group there is 1 TJ with 4.56, a TJ and my CJ8 with 4.88s and 35" tires. 4.88s and 35s work well but I would also consider 5.13 or 5.38.

Also, don't obsess with keeping the RPM low like a modern vehicle. We are driving bricks that probably don't have enough power.

 

[ag4ever]

Over 10 years ago I was driving my ’82 from Houston to Llano for deer season. Going through the hill country, I would be in 3rd gear on the “rev limiter” (aka, she just won’t spin any faster) to barely maintain 50 - 60 mph. 5th gear was only useful between Houston & South Austin. Past that, it was either 4th or 3rd most of the time.

But, that was a worn out 232 from a ‘79 AMC Spirit with a single barrel carb. If it had 50 hp, I would be shocked. It does have 4.10 gear and 31” tires, so gears helped tires not so much. That said, I was happy with the 4.10 gears and the highway rpm on flat land. If I lived in Colorado, I would be looking for lower gears (but in reality, just an LS swap like I am doing for flat land Texas).

For my LS swap, I plan to gear it for 1,750 +/- at 70 mph In 5th gear. But plans might change and I would be fine with up to 2,250 at 70 as well. Only reason I want lower rpm’s at cruise is less driver fatigue with less engine noise. Listening to a screaming engine is just no fun to an old guy like me. (Well, fun in short intervals.)

 

[MomoJeep]

Also depends on the torque characteristics of the motor. 4.0 seem to like humming along at 2600-2900. 4.2 much less. If you have an LS loping along at 900 is likely fine with plenty of reserve...

Yes, I have determined my 4.2 (MPI and 4.0head) likes to be around 2200 at cruising speeds. It's got torque down to around 1800 at speed, and I like to shift up around 2700rpm.

 

[MomoJeep]

Over 10 years ago I was driving my ’82 from Houston to Llano for deer season. Going through the hill country, I would be in 3rd gear on the “rev limiter” (aka, she just won’t spin any faster) to barely maintain 50 - 60 mph. 5th gear was only useful between Houston & South Austin. Past that, it was either 4th or 3rd most of the time.

But, that was a worn out 232 from a ‘79 AMC Spirit with a single barrel carb. If it had 50 hp, I would be shocked. It does have 4.10 gear and 31” tires, so gears helped tires not so much. That said, I was happy with the 4.10 gears and the highway rpm on flat land. If I lived in Colorado, I would be looking for lower gears (but in reality, just an LS swap like I am doing for flat land Texas).

For my LS swap, I plan to gear it for 1,750 +/- at 70 mph In 5th gear. But plans might change and I would be fine with up to 2,250 at 70 as well. Only reason I want lower rpm’s at cruise is less driver fatigue with less engine noise. Listening to a screaming engine is just no fun to an old guy like me. (Well, fun in short intervals.)

Totally.. Engine noise is one of my considerations as well. With my current 3.31 gears and a low-end torque 4.2, I can shift into 5th at 70mph and be around 1800 rpms and the main benefit is the **relief from the engine noise**!