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What is the optimum gear ratio and RPM for 5th gear?
I am researching transmission options and I am concerned about gear ratios. I have a 3.08 rear gear and worried the engine will bog at a .64 5th speed ratio. This will put the engine at 1830 rpm at 75mph according to my calculations. Does this seem too low? That's the only part I can't find. Searching the web for optimum rpm at 5th gear doesn't yield a lot of accurate results.
454 BBC so 2000 rpm is fine. It's more driving on city streets where if I need to slow down to 40 I'm at 1500 rpm in 4th. Also limits 5th to speeds over 60-65 mph. 3.55 gears would hit the sweet spot, I think. It would let me do city driving in 4th at 1700-2200 rpm, and 5th from 55 mph and up.
Which engine? I'm doing 3.70s with a TKO-600/.64. Engine is a 400ci SHP w/ AFR195s and a fairly mild cam. It will do pretty well down to about 1500 in 4th, maybe 1800 in 5th. I'm very careful not to lug it too much in the higher gears and give slight throttle, or downshift if real acceleration is required.
I have a local friend with the same transmission and 3.08s, but it is powered by a 496 BBC. That handles it easily. My 400 would not.
So a lot depends on your engine.
Honestly, I think a stock L-48 would handle it better than most built 350s.
I run a stock zz4 with a 3.36 rear, and a .67 5th gear. The car is happy down to 40 mph, maybe a little less on the level ground. First gear works out well too.
I would get the .8 od rather than the .64 if I had a 3.08 rear. You will be able to use 5th gear at 55mph running 1700 rpms. Everyone talks about 5th gear at cruising highway speeds and that is all good. However if you are on winding roads, mountain roads, etc. you will appreciate being able to use that 5th gear. I have the .64 with 3.55 rear and I can use 5th pretty much on level ground unless I can run 60+ mph. 1830 at 75mph is probably ok but just how much are you really going to be driving 75 compared to 40 or so that most people average?
I would get the .8 od rather than the .64 if I had a 3.08 rear. You will be able to use 5th gear at 55mph running 1700 rpms. Everyone talks about 5th gear at cruising highway speeds and that is all good. However if you are on winding roads, mountain roads, etc. you will appreciate being able to use that 5th gear. I have the .64 with 3.55 rear and I can use 5th pretty much on level ground unless I can run 60+ mph. 1830 at 75mph is probably ok but just how much are you really going to be driving 75 compared to 40 or so that most people average?
I would get the .8 od rather than the .64 if I had a 3.08 rear. You will be able to use 5th gear at 55mph running 1700 rpms. Everyone talks about 5th gear at cruising highway speeds and that is all good. However if you are on winding roads, mountain roads, etc. you will appreciate being able to use that 5th gear. I have the .64 with 3.55 rear and I can use 5th pretty much on level ground unless I can run 60+ mph. 1830 at 75mph is probably ok but just how much are you really going to be driving 75 compared to 40 or so that most people average?
Originally Posted by qwank
X3
If you stick with a 3.08 gear, I'd go with a 0.82 overdrive 5th. If you want to stick with a .64 overdrive, add some rear gear (3.55 or 3.70). You will be able to utilize your 5th gear much more this way instead of just being able to use it at 75mph +.
If you start adding much camshaft duration and overlap to a final gear ratio of 1.97:1 (3.08's and a .64 5th), you are going to have issues at lower rpm's, especially in OD.
Last edited by 71454Chevelle; 01-11-2015 at 10:20 AM.
I think you guys are right... My only worry with the .82 5th was that if I ever want to go to a lower rear gear to get 1st closer to a 10:1 (3.08 * 2.87 = 8.836,) then my 5th gear cruising would be pretty high RPM.
I'm thinking that with the torque of this engine that the 8.8 ratio at 1st will probably be OK though, so the TKO-600 Road Race 2.87, 1.89, 1.28, 1.00, .82 will probably be the best option short of replacing the dif and tranny at the same time.
Thanks a lot for the advice. I definitely could not do this without the help of this forum. Soon I will start a mini-frame-off restore thread with my progress. Frame is in powder coating now!
I'm thinking that with the torque of this engine that the 8.8 ratio at 1st will probably be OK though
I disagree. Even the 3.27:1 1st gear is not enough with 3.08's. It's barely acceptable, IMO. Overall ratio of 10:1 makes steep driveways painful, and lots of clutch slipping in traffic. I'd much prefer the 11.6:1 of 3.27 1st & 3.55 rears.
With that torque curve you will have no issues with a .64 OD even with a 3.08 rear gear and no issues in 1st either. Remember, unless you are road racing or really into highway racing, an OD is for cruising-it is not intended to be used in most cases for aggressive acceleration. 4th gear is the highest gear you should be using for accelerating moderately….Your engine will thank you as well as your wallet (gas mileage) with the .64 OD. I absolutely love the 6th OD gear (.50) in my 10 Z06 for cruising (seen 28 MPG at 70 MPH, 1,750 RPM @80 MPH) and drop to 4/5 if I need to accelerate-that is what the lower gears are for.
I had a 1988 Mustang LX 5.0 (220 HP) with a 5 speed and 2.73 gears and had zero issues. I always wonder about the amount of discussion surrounding OD gears and rear gears with a 400-500 ft/Lbs V8 be it SBC or BBC….
When I'm referring to rear gear ratios and overdrive ratios, I'm NOT talking about having the power/torque to pull the car down the road without any issues. What I'm referring to is driving down the road, in OD, at low rpm's, at a constant speed when you have a performance camshaft with some duration and overlap. A 1.97 final drive ratio is pretty tall, especially with a performance cam. IMO, I think there will be issues with surging /bucking at lower rpms.
Even with my setup, 496 cubic inches, over 650hp and 600 lb/ft torque, I have issues with low rpms (1800 and below) and that's with 4.56 gears and a .63 6th gear (2.87 final drive ratio). I have more than enough power to pull the car down the road even at much lower rpms than 1800, but it is not happy down there at a constant speed.
If it were me, I'd through some gear at it (3.55 or 3.70) and go with a .64 5th gear, but that just my opinion.
Last edited by 71454Chevelle; 01-11-2015 at 09:55 AM.
With that torque curve you will have no issues with a .64 OD even with a 3.08 rear gear and no issues in 1st either. Remember, unless you are road racing or really into highway racing, an OD is for cruising-it is not intended to be used in most cases for aggressive acceleration. 4th gear is the highest gear you should be using for accelerating moderately….Your engine will thank you as well as your wallet (gas mileage) with the .64 OD. I absolutely love the 6th OD gear (.50) in my 10 Z06 for cruising (seen 28 MPG at 70 MPH, 1,750 RPM @80 MPH) and drop to 4/5 if I need to accelerate-that is what the lower gears are for.
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I agree. For me, OD is a cruise gear. I have a pretty good cam and don't have any problem with 1st either.
When I'm referring to rear gear ratios and overdrive ratios, I'm NOT talking about having the power/torque to pull the car down the road without any issues. What I'm referring to is driving down the road, in OD, at low rpm's, at a constant speed when you have a performance camshaft with some duration and overlap. A 1.97 final drive ratio is pretty tall, especially with a performance cam. IMO, I think there will be issues with surging /bucking at lower rpms.
Even with my setup, 496 cubic inches, over 650hp and 600 lb/ft torque, I have issues with low rpms (1800 and below) and that's with 4.56 gears and a .63 6th gear (2.87 final drive ratio). I have more than enough power to pull the car down the road even at much lower rpms than 1800, but it is not happy down there at a constant speed.
If it were me, I'd through some gear at it (3.55 or 3.70) and go with a .64 5th gear, but that just my opinion.
I agree that if you have a pretty radical cam that what you are saying is true about bucking/surging etc when the engine's power is more concentrated in mid to high RPM and off the cam at low revs which is pretty probable with 650 Gross HP. I have no doubt-zero-that my 355 L-82 with AFR heads and moderate Roller cam (power is very strong between 2,000-4,000 RPM, cam operating range 1,500-5,600 RPM) would easily cruise all day long at 1,800-2,000+ RPM….no issues. Granted I have 3.70 gears but the engine easily can stay in 4th gear (1:1) at 1,200 rpm steady revs…..no bucking/surging…all depends on how you are engine is set up to run...
I would get the .8 od rather than the .64 if I had a 3.08 rear. You will be able to use 5th gear at 55mph running 1700 rpms. Everyone talks about 5th gear at cruising highway speeds and that is all good. However if you are on winding roads, mountain roads, etc. you will appreciate being able to use that 5th gear. I have the .64 with 3.55 rear and I can use 5th pretty much on level ground unless I can run 60+ mph. 1830 at 75mph is probably ok but just how much are you really going to be driving 75 compared to 40 or so that most people average?
I X4
Don't make the mistake I made... I have TQ down at 2krpm... But it is pretty much WORTHLESS for any thing other than 70mph+ freeway cruising.... go .82 .64 sucks with my 3.52 rear ..... Your 3.08
I would like to point out that your original combination uses the most Popular aftermarket Transmission designed for Muscle Cars. And the most common original gear ratio is also what You have. But all this doesn't make any difference if Your Engine or driving habits don't fall into this category. Figure all Your performance driving will be done in the first 4 gears, and the OD will be used for gas millage and emissions only as that's what they were originally intended. If You use the search You will find this subject has been covered a lot, and find that the same combination from two different people works perfect for one and not at all for another.
The TKO was not designed for muscle cars. It was designed for fuel injected pick up trucks and ford mustangs. It was installed in the mustang as the TR3550. It took the place of the old toploader OD outside the US and even has some of the same parts. It was not designed for us as hot rodders, but we make it work, because its cost effective, reliable and installations are pretty well documented; but there are some eccentricities because it was designed with fuel injection in mind.
It takes a little bit of effort, but the reality is you need to figure out where your engines "happy place" is when you cruise. If thats 2,000rpm; or 2500rpms...use it. Gear the car so that when you drop it in overdrive, you are right there. As far as first gear; if your engine makes as much torque off idle as a hyd cam 350, then you can use the old 9:1 rule for Total Gear Reduction (rear x 1st). If you have tall tires, or an engine that doesn't make power off idle; then 10:1 is more likely. If you have a lightweight flywheel, tall tires and no low-rpm torque; I'd use 12:1 for your TGR.
Note: tall tire would be anything above about 27"
01-10-2015, 10:52 AM
What is the optimum gear ratio and RPM for 5th gear?
Soon I will start a mini-frame-off restore thread with my progress. Frame is in powder coating now!
