Well, after 40 years my rear end gave up. I lost enough teeth and finally decided to switch gears. I have 8" 2:80 stock gears and was thinking of installing 3:50s for a tighter launch.

My car is a daily city driver with 10 miles of freeway driving a week ('69 coupe 302 4V/C4). I average 12 MPG right now. Will this cut my MPG in half? I don't know how to gauge the MPG loss.

I think anything over 3:50 would be excessive. Any thoughts?

Carlos

 

what trans do you have now? is the final drive 1:1 or does it have overdrive?

how tall is the rear tire?

-Gun

 

No OD, rear tire height is 24".

Thanks.

 

If you want to drive it over 70 mph for any lenght of time dont go over 3.20

AS it stands now 3.50:

60mph= 3000 rpm
70mph = 3500 rpm
80mph= 4000 rpm

With 3.20

60mph= 2700
70= 3100
80= 3600

I suggest staying with your original gears as anything above 3.00 is almost unacceptable 3.50 IS unacceptable if you ever drive it above 65mph.

Furthermore the you wont feel the diffrence between 2.80 and 3.00 but you can see the difference in the speedometer which will now be off and the tac which is going to add about anoter 200 rpm at anyspeed.

stay with the 2.80s

-Gun

 

Gun Jam, you're a pansy:P hehe, jk

But still, I drive 70-80mph fairly often in my car with 24" tires and 3.25's. I have no problems making the 4hr trip home (aside from gas prices) cruising at 3200rpm the whole way.

10 miles of freeway driving isn't a whole lot. I do think the 3.50's might be a bit excessive for a car that sees everyday freeway use, but I think 3.25's would do you well. It would noticably improve in-town driveability, and it wouldn't hurt mileage that much on the freeway. It may actually improve in-city mpg.

 

Righton. Thanks, Gun.

There are times when I DO need to cruise the freeway and 4000+ RPM is too much and too loud. I thought the taller than stock tire would allow for a higher than stock axle ratio.

Anyway, good advice.

Carlos

 

Right a taller tire would allow for a higher rear end ratio....with a net gain of ZERO. The taller tire would reduce engine rpm from factory setting at all speeds the lower axle ratio would then increase rpm at all speeds assuming they were matched then you would be back to where you started.

I had 2.75 and had to switch to 3.00 to accommodate my tru track LSD...I could not feel a difference in acceleration But the Speedometer was off about 7mph at 75 mph and engine rpm went up 200. Really it did me no good but I had to because for some reason I cant get 2.75 for the tru trac.

I did the T5 swap with .63 overdrive and now the 3.00 bog it down....enter 3.80s

-Gun

 

I jumped from 2.80's to 3.25's and it made a very noticable difference in acceleration, especially at lower rpm's.

 

Humm

I couldn't feel a noticeable difference from the 2.75 to 3.00...Im guessing I should be able to feel the 3.80s though

-Gun

 

Geez, I'd hope so:P I think a lot of the problem with my setup was that my cam was way too aggressive for the 2.80's. I don't have a whole lot oflow-end torque to play with. The 3.25's let it wind up a little more