Need some feedback on overdrive gear ratios

[Bill D]

Engine: 331
Est hp/torque: 450/400
Rear gearing: 4.10:1
Cam power range: 2500-6800

Trying to decide between .63 and .73

Using an online calculator, at 70 mph and the .63 5th gear, the engine will spin 2267 RPM. With the .73 5th gear, the engine will spin 2627 RPM.

Any thoughts on which one would be better suited? I'm leaning towards the . 63 as it's less wear and tear on the engine for long cruises.

Thanks

[RodKnock]

Originally Posted by Bill D

I'm leaning towards the . 63 as it's less wear and tear on the engine for long cruises.

Bingo! Of course, assuming the 1st four gear ratios are equal. Also, the reality of freeway driving is that most of us will exceed 70 mph. So, the 0.73 would be too much, given your calculations.

[AL427SBF]

I had the same question re: TKO600 and the .82 vs .68 OD. The feedback I got was generally better to be in the range your cam is spec'd at. Wear & tear cannot be quantified by average RPM over time, loading the engine slightly outside of its' optimum RPM range probably increases wear & tear. Have you thought about a temporary slow down to 60 MPH from 70 MPH, with the .63 OD pretty much mandates a downshift, you have a little room to play with if you're spinning at 2627 RPM with the .73 5th. One thing that clinched it for me was top end, these cars get flighty at 140-150 MPH (unless aerodynamically modified), kind of nice to use as much motor as possible through the taller gears when the situation arises ... just my 2 cents.

[Mr Jody]

Originally Posted by Bill D

I'm leaning towards the . 63 as it's less wear and tear on the engine for long cruises.

Less wear and tear on your ears as well, although you may be planning on going the earplug route regardless of RPM.

Especially with a 4.10 rear, I would say 0.63 is the way to go. It may depend on what your main driving plans are as well. I guess I’m thinking that unless you are planning on a significant amount of track time, the 4.10 might even be a bit much, but that may already be a done deal. But if the engine isn’t happy cruising at 70 mph at 2267 RPM, I don’t think it would be much fun trying to just cruise around at lower speeds in any gear, so maybe the 4.10 is OK. Hard to tell without knowing where the peak HP and torque are occurring. I would also be curious of what the compression ratio of the engine is.

For a good simulation of the difference in these RPMs, maybe you know someone with a car that has an engine similar to yours, or that would be about as loud as yours, who could take you for a ride and cruise for a while at the two different RPMs, even if it means dropping down a gear to keep from exceeding speed limits, depending on their gearing. If you can’t find someone to accommodate the experiment, then use your daily driver. Although engine-wise, this may seem like comparing apples and oranges, it will at least give you some feel for the DIFFERENCE of these two RPMs. It may feel or sound like 2627 is screaming and something that you know you would not want. Although this engine is probably significantly different from your daily driver, you may have already noticed that at 70 mph it is somewhere well below 2267 rpm, depending on the vehicle of course.

While your cam’s power range may be 2500 – 6800, I think if you were in a situation where you wanted the power, you would be downshifting anyway to raise the RPMs farther up into this range. And if you have to downshift because your speed drops to 60 or 50 mph, going through the gears is half the fun anyway.

Here is a gear/RPM calculator I was looking at recently, just to give you another point of reference it not the one you are already using.
Tire Size and Gear Calculator | GTSparkplugs

[Anthony]

I'd get the 0.73. the difference in mileage/wear tear is negligible on the amount of driving you'll likely be doing, and the 0.73's will make the car more peppy at highway speeds.

[jhv48]

What 5 speed transmission gives you the choice between a .63 or .73 fifth gear?
That's a new one on me.

[Bill D]

Originally Posted by mr jody

i would also be curious of what the compression ratio of the engine is?

11.5:1

[Bill D]

Originally Posted by jhv48

What 5 speed transmission gives you the choice between a .63 or .73 fifth gear?
That's a new one on me.

I'm rebuilding a T5. I actually have a choice of .59, .63, .73, or .81 depending on gear sets used

[jhv48]

With that 4.10 rear end, first and second gear are going to be pretty low and you're going to have a 1000 rpm drop between 4th (1/1) and fifth (.63/1).

I just put in a super alloy T-5 with a .80 fifth and a 3.46 rear end. I used to have a TKO 600 with the .64 fifth gear. I'm turning 2500 rpm at 70 mph and my engine loves it. No more rowing back and fourth between fourth and fifth on the freeway when traffic slows to 60 mph, which it does all summer long.

[Bill D]

Originally Posted by jhv48

i'm turning 2500 rpm at 70 mph and my engine loves it. No more rowing back and fourth between fourth and fifth on the freeway when traffic slows to 60 mph, which it does all summer long.

Good point

[mdross1]

Went through the same thing could not be happier with the .63. Cruising out on the highway is where you will really appreciate that taller gear especially with a 4:10 cog.

[joyridin']

I have a 3.90 rear with a .54 overdrive. A little touchy when you drop below about 60 MPH, but it runs great at 70 MPH.

I would go with the .63 or something in that range with those gears.