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914World.com _ 914World Garage _ Another Transmission Gearing Question

Posted by: 914Bryan Feb 10 2012, 02:00 PM

Howdy, I know that our fifth gear is considered an overdrive, but has anyone gone a bit lower so as to lower the rpm for extended freeway driving. 3400 is about 75 mph and I was wondering about dropping it below 3K to increase mileage...Any thoughts?

Posted by: AZ914 Feb 10 2012, 02:03 PM

When I had my trans rebuilt last summer, I had 4th and 5th swapped for lower gearing. I put an X in 4th and a flipped H in 5th. There is another thread (Gear ratio chart) currently discussed that shows the various ratios. I think the flipped 5th probably lowered my RPMs by 400-500 though I can't be certain. I'm very happy with it.

Posted by: Dr Evil Feb 10 2012, 05:17 PM

Root Works did the X and flipped H for 4th and 5th and loved it.

Posted by: shoguneagle Feb 10 2012, 05:22 PM

I have had Doc Evil build my road machine transmission with the same thought; lowering the 5th gear to somewhere below 3200 rpm at 70 mph; not in yet, but I think it should lower it somewhere below 2800rpm at 70mph.

Since these are oil cooled engines, you probably will not want to lower them too much without having an external oil cooler to take care of the lacking of cooling air and oil to the engine. I am running a 3.2 Carrera engine and the whole transmission has be build close to what the engine demands per each gear. I am also running a 911 oil tank along with the external oil cooler which gives me somewhere around 24 quarts Mobile-1 in the entire system.

I feel I can run across the Mojave Desert or down to Phoenix without too much trouble of any overheating from the lowered rpm.. Now you are seeing the reasons I built my car this way. It is anticipated to be a road cruiser with great acceleration, quickness, and speed getting great fuel mileage.

If you are running a four cylinder, the building process could be something else. You can always down shift and run slower if your car is overheating. I will let everyone know this summer when I get the transmission changed, project finalized for road, and do some test runs.

My resident expert is Doc Evil who you can find on this board.

Steve Hurt

Hope the above helps