[wbferran]

I have not yet received my tach from Rocketman, so I don't know what the rpm is on the highway. I only know it "sounds" like it's fairly high even at 60 mph.

Someone suggested i change the rear end ratio. It is stock right now, code 6 which I believe is a 3.00. Can i significantly reduce high speed rpm by changing the rear ratio, AND not negatively affect performance? 351W. What would you suggest? thanks-

billy

 

[1969XR7Vert]

Billy,

No, you can't trade cruising RPM without affecting torque off the line. However, there is absolutely nothing wrong with 3.00 for the street and 2.75/3.00 is what our car commonly came with. Unless you are running tiny tires (<< 26" diameter) your highway RPM's should be very reasonable.

Having said that, the way to get what you are asking about is with a higher (numerical) gear ratio and an overdrive transmission with the right final drive ratio. The common way to do this with an automatic is the AOD transmission and an appropriate set of rear gears to go with it. I'll leave to the guys who have done this swap to say what "appropriate" means for the gears.

Regards,

Bob

 

[wbferran]

Billy,

No, you can't trade cruising RPM without affecting torque off the line. However, there is absolutely nothing wrong with 3.00 for the street and 2.75/3.00 is what our car commonly came with. Unless you are running tiny tires (<< 26" diameter) your highway RPM's should be very reasonable.

Having said that, the way to get what you are asking about is with a higher (numerical) gear ratio and an overdrive transmission with the right final drive ratio. The common way to do this with an automatic is the AOD transmission and an appropriate set of rear gears to go with it. I'll leave to the guys who have done this swap to say what "appropriate" means for the gears.

Regards,

Bob

thanks Bob. can you define a "reasonable" rpm for the highway with the FMX and 3.0, 351W? should have my tach soon from Rocketman-

 

[1969XR7Vert]

Billy,

Well, going by my car, I turn about 3000 or so at 70 MPH with 3.25 going by memory. With 3.00 you should be under 3000 at 70 MPH. There are calculators online for this, here is one from Richmond a quick search turned up: http://www.richmondgear.com/101032.html My recollection is correct, right around 3K at 70 MPH with 3.25 gears and 26" tires. 2700 RPM for 3.00.

Regards,

Bob

 

[ahunt191]

I made a spreadsheet a while back for this. Yeah, I'm a huge nerd. These are 8 inch gear ratios, but you get the idea. For tire size, I used 812 revs per mile as specified by the manufacturer (BFG gForce sport 225/60R15).

GR - [email protected]

2.79 - 2,643.06
3 - 2,643.06
3.25 - 3,078.83
3.4 - 3,220.93
3.55 - 3,363.03
3.8 - 3,599.87
4.11 - 3,893.54

As for reducing performance, your torque to the rear wheels will change by the ratio of your new to old gear ratio. Torque is proportional to acceleration. If you drop your gear ratio by 10%, your 0 to 60 time will increase by about that much.

 

[madillow]

If you are used to driving a modern car with an overdrive transmission, the engine will definitely sound like it is running at a higher RPM, 'cause it is. Definitely don't go with a gear numerically lower than 3.00. You will be sad.