Sustained Cruising RPMs: how low is too low?

[Ralph LoGrasso]

With regards to cruising RPMs, how low is too low? With the .5:1 6th gear of a T56, the possibility for some really low cruising RPMs exists, and I’m wondering when it becomes detrimental to the engine? With the gas prices as they are, extra highway mpg is always a good thing in my book.

For example:

.5:1 6th gear / 3:42 rear end ratio / 26” tall tires

75mph =1650RPM
70mph = 1540RPM
65mph = 1430RPM
60mph = 1320RPM

The same combo with 3:23 rear end ratio:

75mph =1560RPM
70mph = 1450RPM
65mph = 1350RPM
60mph = 1250RPM

With the 3:23s, how low is too low: 1300, 1200, etc? I’ve heard people say you’re lugging the car, if cruising below 1500rpms, and I tend to agree, but GM recommends shifting into 6th at 49mph, figure they’re assuming a 55mph speed limit. With a bone stock 4th generation F-body, RPMS are at 1230. Basically I’m just wondering where you guys draw the line? 500rpms above idle, 700, 1000? I realize cars with large cams will need larger gears to keep the RPMS up around 2000. However, considering cars with mild or even stock cams, in the case of LSX motors; what is sufficient?

Just for the record, I’m not considering changing gears in my 4th gen, but rather thinking of another car, where highway mpg is not necessarily a concern, but any increase is appreciated. If you’ve got the HP (500+) and the car is mostly all street driven, I don’t really see the need for 3:73s or 4:11s, since you’ve likely got more than enough power. With the wide spread of gears, why not keep the revs really low on the highway and net 32-35mpg?



Thoughts?