[Tommo99]

Hi there, I'm new to the thread, have 66 lemans with a 400 and original powerglide, car was original 326 engine. Have purchased a Tremec 5 speed to put in .68 overdrive ratio and running boss 17 inch wheels, 215/50 series, I'm not a burnout guy but will thrash it up through the gears and be open road cruising at 65 mph, can some one suggest a diff ratio that will have me at 2000 rpm at 65 mph, sorryI'm **** at maths 😀

[gtospieg]

67 GTO 255/45/17(26.1" tire) 3.55 rear and .69 OD was about 2200 RPM at 75...now with 3.90 about 2400...this should give you an idea of where to start. Your tire looks to be 25.5" so your RPM would be higher given the same gear ratios in my example

[Doug]

Tommo99,
The standard tire size for a 66 Lemans was 7.35x14 which had a diameter of 25.51”. Your current tire size 215/50/17 has a diameter of 25.46”, virtually identical diameters.

The standard tire size for a 67 Lemans was 7.75x14 with a diameter of 26.1”(similar to gtospieg's 255/45/17 with a diameter of 26.04). The standard tire size for a 67 GTO was F70x14, slightly larger than a standard Lemans tire. According to Coker Tire website , the F70-14 is 26.2” tall, virtually the same diameter as the standard 67 Lemans tire (and gtospieg's tire).

In the 1967 Pontiac sales brochure there is a chart listing the car speed mph per 1,000 engine rpm with varying rear gear ratios. For every 1,000 engine rpms, the speeds are: 3.08 gear=24.5mph, 3.23 gear=23.4mph, 3.36=22.5mph, 3.55 gear=21.3mph, 3.90 gear=19.4mph

With the Tremec final drive gear at 0.68, for every 680 actual engine rpm, it is equivalent to 1,000 engine rpm. Or, in other words, 1000 rpm of actual engine speed is equivalent to 1471 rpm engine speed for calculating car mph. (1 divided by .68= 1.471)

Likewise, with the Tremec, 2000 rpm of actual engine speed is equivalent to 2,941 rpm engine speed for calculating car mph. However, since your tire’s diameter is 25.46” and the chart in the 1967 Pontiac brochure is based on the GTO which used the 26.02” diameter F70-14 tire, adjustment must be made for the tire diameter differences. (26.02 divided by 25.46= 1.0291) This amounts to a 2.91% “extra” you will need to achieve by rear gearing.

Accordingly, If you are willing to accept an engine speed of 2,000rpms at a cruising speed of 65mph, by multiplying the mph of each rear gear ratio listed above as per the Pontiac brochure, you can determine which gear satisfies your requirements.

Accordingly, it appears that the 3.23 gear is what you need.

Per the 66 Pontiac brochure your original gear was likely either a 2.56 or 2.78. A 2.93 “performance” gear and a 3.36 “special order” gear were optional. The 3.23 was not listed but it was available in the 326HO and in the GTO so it should work for you. However, there may be differences between the 2.56/ 2.78 carriers and the 3.23 carrier that may be an issue.

Here is a website that has info you may use:
http://oldcarbrochures.com/static/NA...ine-52-53.html

Good luck!

[tom s]

Which Tremic did you get?What is your first gear ratio?I run a TKO 500 with a 3.31 in the back but my first gear is a 3.27.Tom

[Doug]

Tom,

What tire size are you running?

If you are running a tach, with the engine at 2,000rpm what is your mph number? (I assume your speedometer is corrected for the 3.31 gear and 0.68 final drive ratio.)

Thanks!

[tom s]

245-60 15s,will get it off my GPS tomorrow when I drive it to a show.Tom

[tom s]

If tach is correct it is exactly 70 MPH at 2000.tom

[pokey1]

Another easy way to figure is you can go to Tremec website with a computer or get the app (free) with a smartphone, you can put in your engine rpm, tire size for the height, select the transmission you have (it shows almost all types of manual transmissions to select from) and an axle ratio. It will give you the mph for any rpm you put into it. Can change any part of the formula and get everything else to adjust with it. For 2000 rpm, with a 215/50/17 tire which is 25.46 in tall and putting in a tremec TKO-500 trans which has a .68OD it shows with a 3.31 gear ratio in 5th gear is 67 MPH. If you change gear ratio to 3.55 it will put 5th gear at 63 MPH at 2000 rpm. It actually shows mph of each gear at that rpm if you wanted to know. No math needed it does it all for you...

[tom s]

I'm 100 percent sure I was 70 MPH.Just not 100 percent sure my tach is 100 percent accurate.Tom