3.50 gears

Hi all,
I am soon changing my 2.75 open for a 3.50 traction loc.  It's in a '56 panel truck.  Engine is a 460 .030 over, 9.5:1, aluminum cobra jet heads with 2.19/1.76 valves. Cam duration at .500 is 224/234 with .569 lift.  What do you experienced guys think?  Will this gear change/improve city mpg, yet still be happy at 65mph with 29" tires?  Should be around 2600 rpms.  The converter is a towing 12" converter, (long story).
Thanks
James

It won't be to bad. I run 3.50's but with a 2,500 to 2,800 convertor and 33's" tires, and 70-75 was OK.

I think you would be happier with a 3.25 considering mpg.

Gonna get them in next week, I need to be ready for power tour!

I'm sure with the cam and intake valve diameter the 3.5:1 ring & pinion (3.36:1, your actual final drive, Ford 9") will offer acceleration and performance. I'm surprised off idle performance isn't sluggish with the cam and converter combination, especially with 2.75:1 ring and pinion (2.61:1). If I were going cruising, I'd leave the 2.75:1 in or install a 2.5:1. The 460 displacement seems to overlook a bit of "tall gearing" and the lower ratios don't offer much assistance unless your application changes dramatically! The slower the engine speed the better chance of economy, so to say!

JMO,
Dave.

Well, we'll find out soon. Not sure I follow all your info. 3.36 final ratio? It's a C6 (1.0 in 3rd gear). A looser converter would be better, but then wouldn't taller gears keep cause more cruising slippage? Thanks for the reply.

Hwyhogg wrote:Well, we'll find out soon. Not sure I follow all your info. 3.36 final ratio? It's a C6 (1.0 in 3rd gear).  A looser converter would be better, but then wouldn't taller gears keep cause more cruising slippage?  Thanks for the reply.

A 9" Ford axle assembly's ratio is figured at 27" tire diameter. For every 1.0" you alter the tire diameter, 0.07 is subtracted or added to the ring and pinion's ratio changing the ACTUAL FINAL DRIVE ratio.

Yes a higher ratio could contribute to a converter's stall speed, although a minimal amount. However in your example, a 12" RV converter will have very little slippage, if any.

Dave.

Gotcha. I hadn't looked at it that way, just used rpm calculators to estimate rpms in the 60-65mph range. It is sluggish and rich around town with the 2.75 gears, often I leave it in 2nd gear in town. I'll have a couple months to experiment with cruise speed mileage. I'd be happy with 11 at 65mph.

Hwyhogg wrote:Gotcha.  I hadn't looked at it that way, just used rpm calculators to estimate rpms in the 60-65mph range.  It is sluggish and rich around town with the 2.75 gears,  often I leave it in 2nd gear in town. I'll have a couple months to experiment with cruise speed mileage. I'd be happy with 11 at 65mph.

Shifting into second gear (D2) will assist with an engine that idles a bit rough when a tight (low stall) converter is part of the application. I wish you luck in your quest for 11 MPG, as you have described your application! Enjoy the Power Tour.

On another thought. You could install a 2.5:1 ring and pinion, shift into D2 and enjoy a 3.55:1 (3.41:1 actual) effect in your application.

Dave.

Lol...thanks Dave, if you're gonna dream, dream BIG, (11 mpg)

LivermoreDave wrote:

Hwyhogg wrote:Well, we'll find out soon. Not sure I follow all your info. 3.36 final ratio? It's a C6 (1.0 in 3rd gear).  A looser converter would be better, but then wouldn't taller gears keep cause more cruising slippage?  Thanks for the reply.

A 9" Ford axle assembly's ratio is figured at 27" tire diameter. For every 1.0" you alter the tire diameter, 0.07 is subtracted or added to the ring and pinion's ratio changing the ACTUAL FINAL DRIVE ratio.

Yes a higher ratio could contribute to a converter's stall speed, although a minimal amount. However in your example, a 12" RV converter will have very little slippage, if any.

Dave.

    I'm confused.. maybe even frustrated.. Why would you over-complicate something like rear gear ratios?  Ring:Pinion tooth count (ratio) is what it is, regardless of tire diameter.  For sure, the FINAL ratio is affected by tire size, but you stated the "axle assembly's ratio" is based on a finite number (27"), which makes no sense to me.. unless you're talking about the baseline setup of a stock driveline, perhaps   

Ford made the rules, I'm just offering an explanation of those rules. Why, cause I can!

Not confused nor frustrated,
Dave. ...    ...

LivermoreDave wrote:

Hwyhogg wrote:Gotcha.  I hadn't looked at it that way, just used rpm calculators to estimate rpms in the 60-65mph range.  It is sluggish and rich around town with the 2.75 gears,  often I leave it in 2nd gear in town. I'll have a couple months to experiment with cruise speed mileage. I'd be happy with 11 at 65mph.

Shifting into second gear (D2) will assist with an engine that idles a bit rough when a tight (low stall) converter is part of the application. I wish you luck in your quest for 11 MPG, as you have described your application! Enjoy the Power Tour.

On another thought. You could install a 2.5:1 ring and pinion, shift into D2 and enjoy a 3.55:1 (3.41:1 actual) effect in your application.

Dave.

That is how they used to get overdrive out of a 3 spd auto before 4 spd auto's came out. The off the line accelaration was pretty dismal, to say the least. Too Bad the old ICE I believe they where called, are so expensive, might be the ticket for the Power Tour?

It shouldnt be too bad. We ran Power Tour in '08 with a 3.00 gear, 460/C6 combo, Lunati BracketMaster II cam, Air gap intake, and 850 carb. Overall average for 4350 miles round trip was 12.5, in a very aerodynamic '57 - chebby.

I got to eat some crow ..... and make mine in the form of humble pie!

I was thinking the gear ratios for the C6 were 2.42/1.42/1.0, wrong Dave! I do believe they are 2.46/1.46/1.0. Now the example I listed as the transmission's gear selection effecting the ACTUAL FINAL DRIVE is wrong. A 2.5:1 ring and pinion installed in a Ford 9" rear axle, the transmission shifted into D2, the ACTUAL FINAL DRIVE ratio with 29" diameter tires would be 3.51:1.

What can I say, I can't be perfect everyday! ...    ...
Dave.