Will 3.9 gears require a speedometer adjustment?

[Hrududu]

I have been looking to replace my busted diff in my 280Z for a while now. There are only 2 I have found around here and both seem awful expensive, but I'm running out of options. The first is the stock 3.54 gears from a 78 for $175 and the second is the 3.9 gears out of an 82 for $200. I figure for $25 I may as well go for the 3.9s. If I was to install this on my 77, would it require adjusting my speedometer or anything like that? Also, will this hurt my fuel economy on the highway all that much? I finally got up to 22mpg from the 17 I was getting and would kinda like to keep it at or above 20 if I can. Thanks.

[Arne]

If I was to install this on my 77, would it require adjusting my speedometer or anything like that? Also, will this hurt my fuel economy on the highway all that much?

Yes to the first, and probably to the second. You'll need to replace the speedo gear in the transmission to correct the speedo reading. Your highway mileage will go down, how much I can't say because I don't know how you drive. But the gearing difference is over 10%, so use that as a guide. Also remember that your revs per mile will also go up - by the same more-than-10% factor.

[beandip]

I agree with Arne . Have you looked at the tire calculator and entered your tire size and the trans you are running and the different rear end ratios ? This will show what RPM you will be turning at any speed in any gear . It won't give you MPG figures but you can input what gearing you are running now and compare . Then make a good judgement call . Gary

[hls30.com]

you have a PM!

Will