alphy

I was thinking of changing my rear diff to 4.10 to get faster off-line times and Ive heard this is the "best" mod you can do on your car. However, I dont want to completely ruin my highway milage. Will the 4.10 ratio destroy my highway gas milage? What RPMs do you guys usually pull in 5th at say, 85mph? Just curious because I really want to do this mod, but not at the expense of killing my highway gas milage or keeping the revs too high at speed.

Thanks,
alphy

CrazyAl

The change in acceleration will be exactly the same as your change in engine RPM.

Assuming you have a manual trans, your stock rear-end gears will be 3.55. By switching to 4.10s you will be gaining 16% more acceleration (torque at the rear wheels): simply 4.10 divided by 3.55. At the same time, your engine will be spinning 16% faster in all gears.

So, if you were cruising at 1800 RPM at a particular speed, after the swap you'd be doing a bit over 2000 RPM at the same speed.

This will, of course, diminish your highway mileage. However most of the people on this forum report that the difference in mileage is relatively small.

If you want to know what your 5th gear RPM will be, just take your current RPM crusing on the highway and multiply it by 1.16, then you'll have your RPM after the gear swap.

alphy

Sweet! Great info.

Thanks CrazyAl

firestang70

alphy your RPMs will be about 350-400 higher. Mine cruises at 75 at 2500rpm. Not bad at all.

spyder7724

i have an auto with 4.30's and still get over 17mpg average and well over 20mpg on the highway. 4.10's are a great choice also. i had 3.73's and removed to put in the 4.30's last week. these cars like low gears!

troystoy

I've had my 4:10's in for a few hundred miles now and haven't seen any decrease in fuel milage! I usually run 65mph and under as i'm in the country but do get on the interstates maybe 10% of the time and run at 75mph. If I remember right, 75 mph is at 2500 rpm, 80mph is at 2800rpm. About 300 rpm over stock gears in a manual trans. Don't think twice about going to 4:10's, get them! After you put them in you'll start thinking 4:30's next?

alphy

4.30's seem a little bit too crazy for a daily driver. But your prolly right, I will most likely kick myself after I install the 4.10's, but I doubt it.

Thanks for the replys everyone, its much appreciated.