229/th350 to 305/200r4

[The Driver]

Im going to be swapping out the worn 229/th350 combo for a 305 with flat top pistons, vortec heads, performer intake, headers and full exhaust with a 600cfm eddie and a 200r4. opinions? What type of mileage will i see? Its going in an 81 malibu coupe

 

[DRIVEN]

Should be a nice combo. What rear gears are you running?

 

[The Driver]

I believe high twos but I am not 100%

 

[DRIVEN]

Might be a little tall for the overdrive but okay with the TH350. You might be happier with 3.23/3.42/3.73s. Maybe plug your specs into this calculator to see what works best for you. http://www.angelfire.com/fl/procrastination/rear.html

Since you asked about MPG estimates; my stock 305/200-4R/3.42 combo gets 20.

 

[JohnTN]

Might be a little tall for the overdrive but okay with the TH350. You might be happier with 3.23/3.42/3.73s. Maybe plug your specs into this calculator to see what works best for you. http://www.angelfire.com/fl/procrastination/rear.html

Since you asked about MPG estimates; my stock 305/200-4R/3.42 combo gets 20.

The overdrive gear is 0.67, so to calculate the effective final drive ratio when in overdrive multiply the rear gear by 0.67. If you have a 2.56 rear gear the effective rear ratio in overdrive is only 1.71. As suggested above you need to go to at least 3.23 if you have the overdrive transmission. That would equate to a final ratio of 2.16 which is much less than with your TH350. I think 3.73 would be good.

I have a mild Olds 350/2004R in a street rod (3600 lbs) with 3.42 gears and 27 inch tall tires. I get 21 mpg at 60 mph and have averaged 18 mpg over the 21,000 miles the car has been driven. At 60 miles per hour the engine turns 1775 rpm.

John