What's your enjoyable HWY cruising RPM?

[DocDirk]

I have a 2.75 rear end in my West Coast which allows 65MPH cruising at about 2250 RPM, and my 533 BB seems not to mind at all. Low end response isn't what I'd like it to be so I'm thinking of a 3.33 which would put me at about 2750 RPM in top gear, 1:1, and I bet it would give me a lot more low end streetdriving seat of the pants fun. Not sure if 2750 would get old after a while as my car is new, shorter test drives are all I've done so far but planning a long Route 66 trip for this summer.

What RPM do you like to cruise at?

Thanks!

Dirk

[CHANMADD]

I can tell you after owning my car for 20 yrs I went from a 3.33 to a 3.07 to a 2.88 and I still use the overdrive on the freeway, with all the power I need in the lower gears. Unless youare trying to win some drag racing championship the high rpms get old very soon on the freeway,,, that is of course assuming that you are going to use the car as a car and not a trailer queen..in which case it would'nt make any difference!

[patrickt]

Originally Posted by CHANMADD

Unless youare trying to win some drag racing championship the high rpms get old very soon on the freeway,,,

I agree, and that's exactly why I went with the .64 fifth gear with my TKO. 70mph is just a hair over 2000rpm and I can actually chat with my passenger -- the wind noise is louder than the pipes. And for a quick blast up to triple digits you just drop it to third. And if you really want to show off, you put it in third, rev it to 6000, and then pop the clutch....

[undy]

Originally Posted by patrickt

the wind noise is louder than the pipes.

Then yo pipes tain't loud 'nuf..


I like around 2200 rpm.. The 482's not in her power band but she's also not in her buck 'n studder band either. A quick downshift will put me where I want to be. I have a TKO600 with a 3.25. I too have the .64 OD but am wishing I had a .82 now. I got the .64 when I had the 3.73s and it was fine. Fast forward to the 3.25s and the .64 is a little tall for the local 55 mph freeways. It's not a long trip car (thus far) and it only sees local freeway duty, say 65 max. It's not enough of a bummer though to cause me to change it.

Oh! .. and btw, I can ALWAYS hear my pipes

[trularin]

We long trip once in a while. I am at 2700 ( or there abouts ) with a TKO600 with a .64 final.

I like the .64. I get decent millage when I am on the HWY and I can still down shift and put my foot into it if I want to.

[Bob In Ct]

I've got a 0.68 OD in front of a 3.73 rear. With the 275/60-15s I'm running I turn about 2,200 RPM at 70 MPH (I get a power stroke about every 8 inches I go down the road). This allows me to keep up with the guys on Long Island whenever we join them for a ride. If I need to go, I can drop down to 4th and be 1:1.

Bob

[Tony Aprile]

I have a .82 OD with 3.27 gears. Rear tires are 275/60-15s and turning 2,500 RPM's at 75 MPH.

I had the same combo as Bob in CT but changed to a TKO600...now with a 2.87 first gear it's a bit more usable and no 1000 rpm drop from 4th to 5th gear...

...and Bob if you don't want to bring your cobra over on the ferry...my passenger seat is usually vacant.

[patrickt]

Originally Posted by trularin

.. and I can still down shift and put my foot into it if I want to.

You should try dumping the clutch at highway speeds. Take it from me, nothing brings out the grins and 'thumbs-up" from the troopers like a quick bit of wheel spin at highway speeds.

[Slick61]

I'm running the 0.82 overdrive & a 3.73 rear end. I've got a snotty cam which is NOT happy below about 2400 rpm. I turn about 3k on the freeway at 75 mph... is just right... engine's in the sweet spot, and it sounds WONDERFUL!

[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.

[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

[REAL 1]

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

[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.