? for those with tach, 4 speed, and 3:50:1 gears

66gtk · June 7, 2007 at 9:40 AM

Anybody with the above combo care to share what their RPMs are at various highway speeds? I don't have a tach, but would like to know where I'm at with RPMs in the 60-75MPH range. I also have stock 14" wheels and tires. My engine is also stock.

Edited by - SixT5HiPo on 06/09/2007 10:40:00

SixT5HiPo · June 7, 2007 at 10:07 AM

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Anybody with the above combo care to share what their RPMs are at various highway speeds? I don't have a tach, but would like to know where I'm at with RPMs in the 60-75MPH range. I also have stock 14" wheels and tires. My engine is also stock.

You'll be running at around 3700-4000+ RPM's...

round2K_old · June 7, 2007 at 10:08 AM

I run about 3,200-3,400 at those speeds.

Edited by - round2K on 06/07/2007 10:09:15

66gtk · June 7, 2007 at 10:30 AM

So far, 2 very different results. Are you both running the same setup as mine? I'd be most curious as to the 60 MPH RPM and the reultant peak torque area/speed. I'm also very curious about the 70 MPH area as I will be driving the car about 450 miles at that speed in the summer heat this year. Don't know how much more gas I'll burn at 75. I've actually heard from some Mustang owners that their cars get better gas mileage at around 80 MPH than at 70 MPH. Seems wierd to me, but maybe there is something to it...

SixT5HiPo · June 7, 2007 at 11:08 AM

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So far, 2 very different results. Are you both running the same setup as mine? Don't know how much more gas I'll burn at 75. I've actually heard from some Mustang owners that their cars get better gas mileage at around 80 MPH than at 70 MPH. Seems wierd to me, but maybe there is something to it...

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I run about 3,200-3,400 at those speeds.

Sorry, I forgot to mention that the radial tires on my car are on the "small" side of the spec, and account for the higher RPM range as well as a higher indicated speed than I'm actually driving...I plan to get some "taller" rubber eventually, and this will change the 4th gear numbers I posted here.

These tires came with the car, and were barely used when I bought the car about 5 years ago. That's the main reason I haven't replaced them yet. You can figure my readings are off by as much as 15% because of this. <img src=images/icon_smile_blush.gif border=0 align=middle>

I think that without the benefit of overdrive, you'll get the best economy vs speed if you cruise around 3000-3500 RPM or so...

289kford · June 7, 2007 at 11:17 AM

25" tall tire 1:1 top gear 3.50 gear. 2000 rpm = 42 mph...

2500 = 53..... 3000= 64.... 3500= 74. you can search rear axle calculators on the web and find some to play with tire size etc. you will be surprised at how little tire size changes effect rpm.

Edited by - 289kford on 06/07/2007 11:19:29

Murf_old · June 7, 2007 at 11:57 AM

I have a 289, 271 HP, 3:50 gear, 6.95 X 14 tires (reproduction Goodyear) and have been driving with some regularity the same car for 22 years. If you want a good rule of tuumb to use, divide your speed by 2 and you will be very very close to the engine speed in RPM. Remember this only works with the 6.95 X 14 tires. One hardly needs to look at the tachometer with this method.

Edited by - Murf on 06/07/2007 11:58:08

66gtk · June 7, 2007 at 12:39 PM

So who wants to throw out a number on what speed (4th gear highway) a stock K code with 3.5:1 gears dets the most efficient fuel economy, while maximizing the torque/RPM curve? And when does the economy start to dramatically drop off as speed increases?

Thanks,

Troy

Murf_old · June 7, 2007 at 12:51 PM

At 65 - 70 MPH using "good" gasoline I normally get 20 MPG, normally it is very warm when I drive this car (summer). Have put in 500 plus mile days going to car shows and it is pretty much something I can count on over the past many years. Have a "C" code conv (thermactor) that does not do this well as a point of reference. The hipos run as if they are on ball bearings, and my car typically used almost a quart of oil every thousand miles, as I suspect it probably did when it was new. They run very easy at highway speed but the motor sounds very "busy" by todays standards. I Do not monitor the oil temp but water temp runs about 190 (thermostat setting) on even the hottest days. Overheated once when the thermostat fell apart and stayed open, strange as that sounds. Once the stat was replaced with one that modulated properly the temp stays constant, although over the years the radiator core has become less able to transfer heat and had to be replaced. Nothing really stopped up the core, they simply lose their abiity to transfer heat due to some type of deposit that I cannot see!

Edited by - Murf on 06/07/2007 12:53:59

66gtk · June 7, 2007 at 1:51 PM

20 MPG??!!?? That's fantastic! What psi do you keep your tires at?

Murf_old · June 7, 2007 at 2:08 PM

32 PSI COLD! Remember that I still run the OLD bias ply tires. Radials I am sure would give a 8 - 10% inprovement. These old cars really are not that much of a fuel hog IF you have mature driving habits.

Kman_old · June 8, 2007 at 5:55 AM

My kar ran at just over 2,900 RPM at 60 mph when I was running the Styled Steel wheels with 195/75/14 redline radials. Runing 215/65/15 on 15" Torque Thrusts now and it is just about the same RPM. Never actually checked my gas mileage in the 11 years I have owned the car, but for sure it is much less than 20 mpg at highway speeds, probably closer to 15.

Jack

BoS_old · June 8, 2007 at 10:04 AM

May I suggest you use the Automotive Programs

by Grippo & Bowling and all your questions will

be answered.

Bo

Beagle1_old · June 8, 2007 at 2:28 PM

This won't help your 3.50 rear end + RPM = What MPG question,

But for comparisons sake, my 3.91 gear 4spd with 235/60/15 (26.1" tall) radials on TTD 15" wheels turns 3525RPM @ ~70 MPH. I am at about 3020 RPM @ 60mph and can get about 17 MPG with the center carb on the Tri-Power if I keep out of it. But it's hard to do.

I have only owned one other vehicle that pulled so hard from 50-100 MPH without downshifting. And that was a 2.3L Turbocharged Ranger Pick Up. I am thoroughly impressed with the flat out torque and pull of this K engine from 50-80 mph and how smooth it continues to 100. Doesn't really feel like a soon to be 42 year old car.

The car was sort of a slouch with the 3.50 gear due to my taller tires I think. With my tires and a 3.50 gear it only turned 2700 RPM at 60mph and 3100 at 70 mph. Just seems happier with the 3.91 and these tires. When I go back to the stock size next month on the SSW I may have to put the 3.50 back in.

zray · June 9, 2007 at 8:51 AM

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My kar ran at just over 2,900 RPM at 60 mph when I was running the Styled Steel wheels with 195/75/14 redline radials. Runing 215/65/15 on 15" Torque Thrusts now and it is just about the same RPM. Never actually checked my gas mileage in the 11 years I have owned the car, but for sure it is much less than 20 mpg at highway speeds, probably closer to 15.

Jack

Several quick question: What year K is this on? what brand tires are you using ? Standard backspacing on the Torque Thrusts ? Did you have to roll the fenders?

thanks,

Z. Ray

'66 GT-350

Kman_old · June 11, 2007 at 5:52 AM

Z Ray - Sent you back an email on you questions, but if anyone else is interested: The car is a 65 coupe and the wheels are 15 x 7's with the standard 3 3/4" back space. The tires are 215/65/15 BFG radial T/A's. My car has had all 4 fender lips nicely rolled by a previous owner so there are no rubbing issues with this setup. If the lips are not rolled, the rears should be fine, but the fronts may have a rub issue due to the height of the tire which is the same as the Coker 195/75/14's that I previously ran. I have freinds that have run this combo on untoched fenders OK and others who did have some rubbing on the front.

Jack

wkc51_old · June 24, 2007 at 12:45 PM

66gtk,

I just came back from a 700 plus mile round trip from Northern VA to just north of Charlotte NC. My daughter was getting married and want me to take her to the church and then have her new husband drive my car to the reception, in my 66 2 plus 2. Six hours both ways.

On the way down and back, I traveled VA H29 most of the way with a 15 mile drive on I64 and a 29 mile drive on I85. I tried to drive 65 mph when I could. The RPM varied from 3000 to about 3200. My speedometer bounces a bit, and may not be accurate, and I think the Rally-Pac may not be totally accurate. However, the odometer was right on when measured against the measured mile markers on the interstate. I averaged 18.1 mpg both ways. I average about 17.5 in everyday driving, mostly on side roads, which includes a few WOT blasts.

My old Hipo had a rebuild at 135,000 miles (about 2000 miles ago) with a .030 overbore, TRW Speed Pro pistons, and stock intake and exhaust. I am also running the Comp Cam reproduction cam and a reproduction dual exhaust from VA Mustang. It burned about ¼ of a quart of oil during the 700 miles. I am running 195/75/14 Michelin X radials on 14X6 American Racing Vector wheels.

The only thing I didn’t like about the trip was lack of A/C, the AM radio (no reception), and the noise. I must be getting too old and too fat!

In the 35 years that I have owned this car, I think this is may first time I did the MPG calculation.

GT Joe_old · June 24, 2007 at 2:38 PM

For what it is worth; based on 3 1966 GT-350's and 2 K-GT's, with a 3.50 ratio with either four speed or auto trans you should be pulling 3000 rpm's.

66gtk · June 24, 2007 at 8:19 PM

Thanks for all the info- you guys are all great! Just picked up a '65 shelby clone. This one is special, and has a great motor built to Shelby specs. It has the 4 speed, but only a 3.00:1 rear. Runs great on the highway. Was doing 75 MPH at just under 3000 RPMs and getting 15 MPG on the way home with it. It's a great driving car and is really fun with the Hurst shifter. Sure beats the hell out of my factory Ford shifter on the K.

b9folk_old · June 25, 2007 at 9:49 AM

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The only thing I didn’t like about the trip was lack of A/C, the AM radio (no reception), and the noise. I must be getting too old and too fat!

Maybe a "swamp cooler" would help, if you know what that is. I recently found a place online that makes new ones, though a vintage one would be cool (no pun intended).

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