Calculating Final Drive Ratio in Overdrive

[345 DeSoto]

How do I calculate what my final drive ratio would be in overdrive? The differential is a 3.92, and the overdrive is .67. Do I multiply the two? If that's so, it comes out to 2.54...which doesn't sound right...

[rstrasser]

Yes you do; but I thought the overdrive ratio was more like .70 or .72
Ron

[345 DeSoto]

The overdrive is in a 200 4R automatic transmission...

[64V-K7]

1st 2nd 3rd 4th Reverse
GM
Powerglide 1.76 1.00 1.76
TH350 2.52 1.52 1.00 1.93
TH400 2.48 1.48 1.00 2.00
200r4 2.74 1.57 1.00 0.67 2.07

3.92 direct drive would be reduced to to 2.62, in 4th and OD on the 200 4R


(Nothing I do, will align those columns correctly. 200 4R has a .067 Overdrive ratio / x / 3.92 = 2.62 final ratio in OD)

[studegary]

200 4R has a .067 Overdrive ratio / x / 3.92 = 2.62 final ratio in OD)

You need to move a decimal point. It should read 0.67 overdrive ratio.

[64V-K7]

You need to move a decimal point. It should read 0.67 overdrive ratio.

I don't need to do anything. The answer is correct. and so is the chart above it....

[studegary]

I don't need to do anything. The answer is correct. and so is the chart above it....

"...a .067 Overdrive ratio / x / 3.92 = 2.62 final ratio in OD)"

This would yield a 0.26 final ratio.
0.67 X 3.92 yields a 2.62 (technically a 2.63) final ratio.
Simply multiply out what you posted and what I posted and you should understand the correction.

[SScopelli]

I have an Excel spreadsheet you can enter tire size, Diff Ratios, and Transmission ratios to calculate engine speed at a given MPH..

Items you can edit are in Gray..

This example is at 85mph with the 200r4 ratios as T1 and your 3.92 as the R1 ratio..

IM me with your email, and I can send it to you..

[wittsend]

It is amazing what an overdrive (auto 700R4 in this case) does. My Daytona has 225-60-16 tires and a 3.07 rear ratio.

65 MPH in 1 to 1 "third" gear is 2,518 RPM and that isn't accounting for convertor slippage.

65 MPH in .7 to 1 "fourth" gear (overdrive) drops to 1763 RPM's (a 755 RPM drop) and with the convertor locked there is no slippage. BTW, the effective final ratio is 2.14.

The 3.06 first gear of the 700R4 gets the car moving fine (far better than the second gear start of the BW trans), but other than the freeway with a minor incline the fourth gear overdrive is pretty useless. And this with a rather "torquey" 350 Chevy. Supposedly some of the Camaro's with a 700R4 came with 2.73 rear gears. That factors to 1.91 with the 700R4 .7 overdrive!

I'd be curious to hear about your 3.92/.68 overdrive experience.

[345 DeSoto]

WITTSEND - The locked converter is mostly for 55 mph and above, such as on the interstate. My Cherokee with 3.73 rear and Chrysler/ Merced Benz OD does 65@ 2100 RPM. I'm hoping for about the same for my 289. It'll have to wait until next Spring...the Hawk is up in NY for the Winter, and I'm in FL...

[PackardV8]

65 MPH in .7 to 1 "fourth" gear (overdrive) drops to 1763 RPM's (a 755 RPM drop) and with the convertor locked there is no slippage. BTW, the effective final ratio is 2.14. . . but other than the freeway with a minor incline the fourth gear overdrive is pretty useless. And this with a rather "torquey" 350 Chevy. Supposedly some of the Camaro's with a 700R4 came with 2.73 rear gears. That factors to 1.91 with the 700R4 .7 overdrive!

FWIW, for fifty years, while turning relatively high RPMs, the Chevrolet V8 never had any main bearing problems. When GM went to computer controlled spark, EFI and lockup torque converter, with the 6,000# pickups/Suburbans, they were able to put the bottom end into hard near-full-throttle pulls at 1500-2000 RPMs. It was better for fuel economy, but they began to see main bearing wear for the first time ever.

Our carbureted/distributor Studebaker V8s wouldn't hold the same loads at that RPM without pinging. Some experts recommend to gear a 75-80 MPH cruise at the torque peak; usually 2800 RPMs.

jack vines

[karterfred88]

Best of luck with your conversion, remember though, the engine you are using is not a modern FI engine designed and "tweeked" to work well at a specific engine RPM range and highway speed. The secret is knowing what range your engine likes-the "sweet spot" in RPM, torque curve etc. and match the final combo of gearing, tires and engine needs to it. As noted in post #9-if it will only travel on a flat road at 65 and need to downshift to go up a small incline it won't be much fun to drive.

[wittsend]

WITTSEND - The locked converter is mostly for 55 mph and above, such as on the interstate. My Cherokee with 3.73 rear and Chrysler/ Merced Benz OD does 65@ 2100 RPM. I'm hoping for about the same for my 289. It'll have to wait until next Spring...the Hawk is up in NY for the Winter, and I'm in FL...

Not sure if the 200R4 is similar to the 700R4 but the 700R4 must lock up in fourth gear. It has something to do with the way the oiling routes and there is insufficient lubrication without the lock up. The guy behind me is the local transmission guru in town. His shop builds heavy duty and racing transmissions and he has a good reputation (though I hear he isn't cheap). He told me to drive in 3rd gear (1 to 1) if 4th gear was too much. He said NOT to use 4th gear without the convertor locking. Frankly it isn't the 4th ratio so much, but when the convertor locks up it really pulls the engine down. 45 MPH is about 1,200 RPM. I'd think your 3.92 gears should be fine, but do check into the 4th gear lock up on the 200R4. I'd hate to know the trans went bad if it is similar to the 700R4.

Jack - The 350 Chevy engine is out of an '85 Corvette. Generic to every other SBC except it has forged pistons (factory) and slightly warmer cam* than the garden variety cam Chevrolet used. Designed for their small tube EFI setup it was noted for torque (cam likely plays into that) but was said to die off in the 4,500-5,000 RPM range. Mine has a Performer manifold and an Edelbrock 650. Still seems to have the same characteristic regarding torque and none of the attributes of EFI. (smiley face and sad face both applicable)

202* Intake Duration @ .050
206* Exhaust Duration @ .050

269* Intake Duration @ .006"
271* Exhaust Duration @ .006"

Cam Lift is

.269" Intake
.276" Exhaust

Gross Lift is

.404" Intake
.415" Exhaust

The LSA is 114.5*

It is installed on a 115* intake centerline and has an exhaust centerline of 114*.

Timing Specs @ .050"
IVO = 14* ATDC
IVC = 36* ABDC
EVO = 37* BBDC
EVC = 11* BTDC