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TR6 TR6 Differential Ratios

towbiz

Freshman Member
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I am trying to decide the most economical approach to allow my TR6 to be more adaptable to freeway cruising. With my current non OD 4 speed, 2:05X65-15 tires, and 3.7 gear ratio, 70 MPH will be at about 3250RPM. I am considering using the euro 3.45 gear set to lower this. With the torque of the motor I have no concerns about lower gear performance. I would like to be able to cruise at 75 MPH without exceeding 3,000 RPM. I welcome and appreciate any feedback, or suggestions. Thanks in advance.
 

TR3driver

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Seems workable. I'm planning to pretty much do the same for my TR3, except I already have OD and 205/55 tires.

Your numbers don't seem right to me, though. 205/65-15 tires should be about 820 turns per mile (varies somewhat with brand & wheel width), which comes to about 3540 rpm @ 70 mph with 3.7 gears & no OD.

820 tpm means that at 60 mph (1 mile per minute), the wheels are turning 820 rpm. Times 3.7 gives 3034 driveshaft rpm. 4th gear w/o OD is 1:1 so that's also 3034 engine rpm. Times 70/60 to scale up to 70 mph gives 3539.666...

With the 3.45 gears, that will only drop to 3300 rpm.

You might be happier with the OD, especially once you figure out what a 3.45 diff is going to cost.

Here's some tpm numbers from TireRack:
https://www.tirerack.com/tires/tire...ehicleSearch=false&fromCompare1=yes&tab=Specs
https://www.tirerack.com/tires/tire...ehicleSearch=false&fromCompare1=yes&tab=Specs
 
Last edited:

Gliderman8

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When faced with same problem, I replaced my non-OD with a 5-speed conversion using a Toyota Supra transmission. I can now cruise at 70mph around 2200 rpm... This conversion makes my car much more drivable and enjoyable on the highway. Of course you could always source an OD tranny which would also solve your problem.
 

smaceng

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My calculations are a little different than yours.....I find that with a 205x65-15 tire at a 3.7 ratio at 70 mph that works out to about 3417 rpm. Now at 75 mph, using the same ratio, the rpm would be about 3661 rpm. If a 3.45 ratio is substituted, the rpm would decrease to about 3413.
I don't think you can get there from here! Maybe an OD is your only solution. Or ear plugs!
Scott in CA
 

martx-5

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An overdrive (OD) transmission swap will give you the best of both worlds. Plus you can split shift 2-3 and 3-4 besides the 4th OD. You could also have the OD added to your trans. John Esposito at Quantum can do that for you. I think you can also buy outright and send yours in as a core.
https://www.quantumechanics.com/qm-htm/home1.htm

Another alternative is a Toyota Supra 5-speed conversion. 1-4 ratios are just about the same as the TR four speeds with 5th being an overdrive. That's the route I went.
 
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T

towbiz

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Thanks TR3 Driver. Yes my numbers were not correct. I was dealing from memory on the RPM and understated it.
 

TR3driver

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Gliderman8

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Wow! How'd you get those tractor tires in the wheel wells ?
Just going from memory Randall... I'm using 205/70 R15's. One thing for sure is that my rpm's are significantly lower now.
If originality is what towbiz is looking for then contacting Quantum for an OD unit might be the way to go.
 

martx-5

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Yeah, at 70 mph, the Toyota conversion in fifth is more like 2600-2700 rpms depending on which box you have.
 
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