What's your enjoyable HWY cruising RPM?

[jhv48]

I keep going back and forth between the .82 and .64 fifth gear ratios.

With my 3.73 rear end, and a moderately cammed engine, I can cruise at 75 mph on the freeway (with the .64 fifth gear) and still hear myself think. Mileage is good and the noise is kept to a minimum, somewhat. Need power, downshift and go.

HOWEVER, when flying on mountain roads, fifth gear is all but worthless on those long straightaways. Too big an rpm dropoff.

Wish there was a .73 fifth gear ratio so I could go somewhere in the middle.

[patrickt]

Originally Posted by jhv48

With my 3.73 rear end, and a moderately cammed engine, I can cruise at 75 mph on the freeway (with the .64 fifth gear) and still hear myself think. Mileage is good and the noise is kept to a minimum, somewhat. Need power, downshift and go.

HOWEVER, when flying on mountain roads, fifth gear is all but worthless on those long straightaways. Too big an rpm dropoff.

Yes, it's important for those on the fence post with the TKO's fifth gear to look at their rear end ratio, tire size, and engine's behavior in the low 2000's before making a final decision and then realize that the .64 is only for pleasant cruising (and the occasional visit to the salt flats).

[YerDugliness]

Originally Posted by patrickt

....64 is only for pleasant cruising (and the occasional visit to the salt flats).

I'm faced with a dilema regarding this issue. I'm considering the purchase of a used roadster with an engine/T5 combo from a '89 Mustang GT. According to the seller, the car has a 3.70 ratio Ford 9" posi unit. I asked about cruising RPM and found out that the car turns 4K at 70MPH--that is with a 0.68 OD gear, according to this article:

http://www.allfordmustangs.com/Detailed/349.shtml

I'll be relocating to a highly rural area, which means primarily road use with practically no in-town usage, so this is too high for me. A change will have to be made. Obviously, the transmission already has the longer OD gear, so the change will have to come in the differential.

Considering ratios in the 3.2 to 3.35 range, probably at the lower end of that range. Should I replace just the R & P, or go for a totally different pumpkin?

Thanks!

Dugly :cooldude:

[jhv48]

Originally Posted by YerDugliness

I'm faced with a dilema regarding this issue. I'm considering the purchase of a used roadster with an engine/T5 combo from a '89 Mustang GT. According to the seller, the car has a 3.70 ratio Ford 9" posi unit. I asked about cruising RPM and found out that the car turns 4K at 70MPH--that is with a 0.68 OD gear, according to this article:

http://www.allfordmustangs.com/Detailed/349.shtml

I'll be relocating to a highly rural area, which means primarily road use with practically no in-town usage, so this is too high for me. A change will have to be made. Obviously, the transmission already has the longer OD gear, so the change will have to come in the differential.

Considering ratios in the 3.2 to 3.35 range, probably at the lower end of that range. Should I replace just the R & P, or go for a totally different pumpkin?

Thanks!

Dugly :cooldude:

That's 4000 RPM in 4th right?

No way that car should turn 4000 RPM with a .68 fifth gear at 70 MPH. Mine turns 2200 RPM at 75 MPH with a .64 fifth gear and 3.73 rear end.

Something is missing in this equation.

[patrickt]

Originally Posted by jhv48

Something is missing in this equation.

My rough, back of the envelope, calculation for typical Cobra tires (295-50-15) with a 3.70 rear and a .68 gear ratio shows around 122mph at 4000rpm.

[rokndad]

Whaaaattt?? I can't hear you!! Oh, I have my earplugs in, Mickmate has heard it. 3,000 rpm at 75 mph. What gas mileage, it's a Cobra. 3:31 rear on Sports Car Specials.

[REAL 1]

Around 2700 rpm at 60 mph. I have 3:54 rear.

[1985 CCX]

3000rpm at 70mph in 4th

[YerDugliness]

Originally Posted by jhv48

That's 4000 RPM in 4th right?

No way that car should turn 4000 RPM with a .68 fifth gear at 70 MPH. Mine turns 2200 RPM at 75 MPH with a .64 fifth gear and 3.73 rear end.

Something is missing in this equation.

Heres' what the current owner said in an email (I had asked about RPMs at 70MPH):


"Doug,
>
>The RPM at highway speeds is around 4000 in 5th. The speedometer is relatively accurate."

The rear tires look pretty large, but not uncommonly large for a Cobra replica--until I can see the car in person next Wednesday, I'm dealing with photos.

I'd be looking for a RPM range more similar to that which you mentioned above. I'm hoping that he was thinking 4th gear, not 5th......but this gentleman has made numerous upgrades to his roadster and all of them have been well thought out, so that would be uncharacteristic of him.

What size tires are you running to get that cruising RPM?

Dugly

[patrickt]

This is a simple calculator, but does the trick. And it confirmed by guesstimate of around 122mph.
http://www.angelfire.com/fl/procrastination/rear.html

[Chaplin]

I have 3.31s in the rear and the tko with the .82 fifth. In 5th doing 65mph I am turning somewhere around 2300. At 75mph in 5th I am turning about 2800. The other factor that you have to keep in mind is your cam shaft, you need to match your gearing to your cam shaft and vice versa. If you have a big cam that doesn't start to make power until 3500 or so, it will not be happy chugging along at 2000 rpm. It all has to work together.

[jhv48]

Originally Posted by YerDugliness

been well thought out, so that would be uncharacteristic of him.

What size tires are you running to get that cruising RPM?

Dugly

I'm running 285X40X17 inchers on the back. 4000 rpm is still too high for 75 mph in fifth. I think he was talking about 4th gear. And with his cam, he may not be able to pull low rpms without bucking and bogging down. Better check it out and make sure it isn't too temperamental an engine for what you intend to use it for. Might be a race engine. In that case, make sure you can live with that on the street. Not real friendly in traffic.

[YerDugliness]

Originally Posted by jhv48

I think he was talking about 4th gear....Might be a race engine. In that case, make sure you can live with that on the street. Not real friendly in traffic.

What little I know about it doesn't seem to indicate it is a race engine. The motor is reported to be a stockish 1989 5.0HO long block, not EFI but a simple Edelbrock manifold and small (500CFM) Edelbrock carburetor--not exactly exotic race stuff there.

Too many of you have reported 5th gear highway RPM's in the range I would have expected, the gentleman must have been mistaken about the gear he was running in. He does live right in the middle of Dallas, TX, so perhaps he was in 4th.

That would be the my hope for the way things turn out.....I'll have to drive that thing 300 miles to get it home and eventually over 800 miles to get it to my retirement home.....not exactly a happy prospect with 4,000 RPM's exiting through sidepipes less than 3' from your ears! He does say that at road speeds he can hear the sound system, so.........perhaps the tach is out of calibration?

Thanks, guys....maybe I won't need to go to the expense of changing out the rear pumpkin after all. that would make me !!

Dugly