Little Rear Help

[Cuse99]

I know there is a thread on here about how to calc gear ratios based on trans and rears and RPM's but I can't find it. Little help please.
I have a 220 4r trans in my '84 Elco, low power motor and a 2.73 rear. I can't get this rig into 4th gear (maybe if I hit 120 mph or something). I'm looking to upgrade the rear to a 3.42 but not sure if it will do the trick.

This car is a daily driver, not a hot rod or weekend warrior. It has the original 14" wheels for now but I'll upgrade to 15" eventually. Gas milage is a concern. If I can't get it into 4th gear on the highway its just wasting gas. I appreciate the help. Thanks in advance.

 

[64nailhead]

15” what? Need to know the tire diameter. For example, a 28” tire (275/60R15) will work great with. 3.7. A 27” (255/55R15) will work great with a 3.55 or a 3.42. My point is that it’s hard to give good above in rear gear ratio with the rear tire diameter.

I used to run a 215/65R15, 26”, on my car with a 4L80 and 3.73. It was awful. It turned 3000 rpm’s at highway speed. But it worked pretty good with the 27”, and works fabulously with 28” tires.

 

[78Delta88]

No 4th is a transmission problem. I was first thinking servo, but no. Governor, valve body or possible clutch pack.

I'm looking for the shift chart but in mean time below is good for just some basics.

How to Build a TH200-4R Transmission for Drag Racing

Art Carr shows us how he builds the TH200-4R four-speed OD transmission to handle supercharged big-block power at the drag strip.

www.motortrend.com

 

[Cuse99]

15” what? Need to know the tire diameter. For example, a 28” tire (275/60R15) will work great with. 3.7. A 27” (255/55R15) will work great with a 3.55 or a 3.42. My point is that it’s hard to give good above in rear gear ratio with the rear tire diameter.

I used to run a 215/65R15, 26”, on my car with a 4L80 and 3.73. It was awful. It turned 3000 rpm’s at highway speed. But it worked pretty good with the 27”, and works fabulously with 28” tires.

Thanks for the feedback. Lets start with what I have. 195/75R14 Hankook Kingery ST with a 25.4" outside diameter. The car has no computer and I believe the trans is running on a pressure switch base on RPM or something like that. I'm not totally sure. Again this is a daily driver and not a muscle care or hot rod.

Where can I find the calc tool to help me understand the ratios once I decide to change tires and wheels etc? In the future I plan to install 15" wheels, same size as 88 Monte SS. Thanks again.

 

[78Delta88]

Calculator on summitracing.com

https://www.summitracing.com/newsandevents/calcsandtools/gear-ratio-calculator

 

[64nailhead]

If you stick with a 25.4 tire, then a 3.23 would be my choice for highway driving at 70mph if mileage is a concern.

Assuming stockish cam.

 

[78Delta88]

I have the books but just finding a pic to load was a chore.

No fourth is going to be the 4th or "Overdrive Clutch" . From the factory it's only 2 clutches so they do burn up.

Also possibility is 4th accumulator and or the governor. If it's shifting ok in the other gears, doubtful it's the governor. If the line from the accumulator is plugged by debris could cause it, but that is fairly rare.

You mention high RPM to get it to shift when it does and the shift isn't very good. This is indicative of burnt clutches or poor application of the piston.

 

[Cuse99]

Calculator on summitracing.com

https://www.summitracing.com/newsandevents/calcsandtools/gear-ratio-calculator

This is what I was looking for to get me started. It doesn't have the overdrive calc but I called summit and gave them all my details and they performed the calc for 3 different size rears so that I could pick the best option for me. 3.73 calculated at the right spot at 75mph at 2500 rpm on the highway. Actually it will be a little less RPM and when I get larger wheels it will be a little better. I'm only adding 1" to the outside diameter of my tires once I replace the wheels. I'm good, thanks again.