bad gas mileage when using a 200R4. Help!

[66velle]

Hi All,
I have a '87 Monte Carlo 200R4 transmisstion behind a 350 ci stock 249 hp, 4 barrel, dual exhaust engine with an open 2.56 rear gear on a '66 chevelle 2 dr hdtp. My problem is my gas mileage have gotten worst since replacing the PG with the 200R4. Doesn't seems like it going into overdrive. How many times does it shift before going into OD? What mph should OD kick in? Is it 3 or 4 times? Will a lower ratio gear work (3.08, 3.55, 3.73)? My thinking is the 2.56 gears is the problem. I don't have a clue.
Thanks,
Stan

 

[Kevin Turch]

Mine shifts into OD on the 3rd shift, at 45 mph. Not sure if yours will be the same tho.

Kevin

 

[66velle]

What is your rearend ratio?

 

[Geerrhead]

with a powerglide you had a 1:1 ratio. this multiplied by your rear end ratio is 2.56.

Overdive in a 2004R is .67. .67 X 2.56=1.7152

I'm sure a gear this tall requires a lot of throttle to pull. I would suggest a rear end gear change. In the mean time, you could leave it in 3rd gear 1:1 ratio. This way you still get the benefit of the lower gears, without stressing the engine in overdrive.

A grand national has 3.42 rear gears. in OD this equates to 2.2914 .

 

[77 cruiser]

What size tires are you using? I`ve got a 200-4r with 3.73 gears with 275-60-15 & seems to work out well. 60 mph is 1900 rpm 70 = 2200. Mild 383 if driven with a light foot would prolly get 18 mpg on the hi-way.

Jim

 

[66velle]

My rear tires are 235x15-60 26" diameter. Hate to say but my mpg is about 10 mpg conpared to the original pg tranny at 14 mpg. Desperately need some suggestions..
Thanks.

 

[77 cruiser]

Try driving with your trans in Drive & not OD would be the same as the Glide then.

Jim

 

[66velle]

Leaving it in D and not in OD sound like a good idea and changing the rearend out also. I'll see what happens. Thanks.