Rear end ratios?

[ToyTrainGuy]

Hello,

Thanks for the info about the availability of a modern limited-slip / Posi. units. Now, I am looking for opinions on best gear ratios between the tranny & rear. Bill Martin’s TKO-600 kit will be available in either a .64 or .82 fifth gear option along with a 2:87 first gear ratio, so I am wondering which rear end gear would be ideal for 90% street-driven car. Depending on which rear ratio I use with the LS / Posi, impacts which ratio fifth gear ratio I order.

Even though modern computer controlled fuel injected cars can drop the rpms down to 1500 on the highway, some say that our old carb-equipped motors can not operate below say 2500 rpm or so efficiently. The motor is a 274hp / 310 lbs, original 289. The Dyno spec sheet I have started at 3K rpm (nothing noted below) & it was already at 290 ft lbs. of torque. I want to retain the original LAT 13” wheels & with the 60-series tires have an overall diameter of about 22”. Using one of those RPM calculators available on the Net, I came up with the following RPMs at 70 mph for these two combos. With a stock 2:88 rear & the .82 fifth it should run at 2525 rpm at 70 mph. A set of 3:31 gears with a .64 fifth should be about 2265 rpm at 70 mph. Either one appears to me to be somewhere near a decent rpm combo.

What do most street-driven Tiger owners run, the stock 2:88s? Would 3:31 gears be too much with those little 22” tires & the 2:87 first gear of the TKO-600 tranny? Would first gear with that combo not make it through the intersection? Just curious what combos others have had luck with. Thanks….

 

[michael-king]

I think most people stil run 2.88, as if you go shorter they start to rev a bit high at 60mph + and use a fair bit of fuel.

i run 3.07 rear and stock C?R box, the rear helps with the tall first bit not a huge amount and it does hurt the car a little on the highway.

What RPM are you comfortable with at 60/65mph? Use that as a guide for the rear ratio and top gear ratio, then use something like a wide ratio box first to get you good punch off the line.

I would love to run 3.34 or 3.54 with a .64 so i got nice cruising and some quick acceleration down in the lower gears.

 

[Moondoggie]

I am running 3:07 gears with the TKO 600 Road race transmission ( .83 5th)
and I highly recommend that fifth gear ratio. The .63 is just too much of an over drive in my opinion. I am also running 16" wheels ( I don't know the diameter ) This combination is perfect for highway cruising and at 70 mph the motor is turning about 2300 rpm...no need to down shift to get the rrr's back in the power band. My 302 is putting out 375 hp & 380 ft'lbs with a fairly high lift cam...I have no idea what sort of mileage I am getting but it's at least twice what the Cobra got !!!

Moondoggie

 

[ToyTrainGuy]

Moondoggie,

Punching in the numbers to an RPM calculator, it seems the tire diameter of your car must be about 26" with the .83, 3.07, at 70mph combo. Put a tape measure on one, just curious.

Even though so many have upgraded to the larger wheels as you have, I want to keep my original LAT 13's, that with 205 / 60 tires is only 22" in diameter. Big differance from your tires.

In any case, to answer my question Tigers have no problem running down the highway at around 2300 rpm, no matter what combo of ratios gets you there?

Anyone else not like the 3:31 gear ratio with 13" wheels & a lowered 1st gear (other than the close-ratio top-loader) on the street?

Thanks..

 

[at the beach]

I have run 3.31s with 13" wheels and a T-5. I think I had 185s at the time.

The first gear was so short that I'd start out from a light and have to shift by middle of the intersection. First gear was practically useless. I have since swapped back to 2.88s which I like much better.

The only "problem" with the .625 OD in a T-5 / stock 289 are the less frequent visits to gas stations which lessens the opportunity for "rest stops. On a trip to SUNI, two Tigers with T-5s drove there with one trailing the other all the way from Los Angeles to Rapid City via Yellowstone and the Beartooth Highway. It was highway and mountain driving. The lead Tiger had a .625 OD while the other had the .80. The lead Tiger with the .625 always got better mileage, usually about 2 mpg better.

The advantage to the .80 ratio is that you'll have less of an RPM drop when shifting into 5th. I suppose this is important to some who want absolute maximum performance. However if you want maximum performance you'd be winding it out to redline and that shift would then happen at over 120 mph. I don't know how often you'll do that but personally I doubt I'll be doing that a lot with either of our street Tigers. The track is a different story. For a street Tiger the .625 works quite nicely. And the resulting MPG of mid to upper 20s is also quite nice.

bt

 

[ToyTrainGuy]

I have run 3.31s with 13" wheels and a T-5. I think I had 185s at the time.

The first gear was so short that I'd start out from a light and have to shift by middle of the intersection. First gear was practically useless. I have since swapped back to 2.88s which I like much better.

The only "problem" with the .625 OD in a T-5 / stock 289 are the less frequent visits to gas stations which lessens the opportunity for "rest stops. On a trip to SUNI, two Tigers with T-5s drove there with one trailing the other all the way from Los Angeles to Rapid City via Yellowstone and the Beartooth Highway. It was highway and mountain driving. The lead Tiger had a .625 OD while the other had the .80. The lead Tiger with the .625 always got better mileage, usually about 2 mpg better.

The advantage to the .80 ratio is that you'll have less of an RPM drop when shifting into 5th. I suppose this is important to some who want absolute maximum performance. However if you want maximum performance you'd be winding it out to redline and that shift would then happen at over 120 mph. I don't know how often you'll do that but personally I doubt I'll be doing that a lot with either of our street Tigers. The track is a different story. For a street Tiger the .625 works quite nicely. And the resulting MPG of mid to upper 20s is also quite nice.

bt

So now you are running the stock 2:88s with a .625 OD? If so, how is that working out for you?

 

[at the beach]

Yes, I'm back to running 2.88s & a 289. The first 289 was a HiPo & the current is stock but with an F4B & a Holley quad. Both are fine. My next engine will probably be a hot rodded 260.

bt

 

[ToyTrainGuy]

Thanks to all for the input!