Higher gears in rear axle to increase gas mileage

[98frontier]

Hello, this is my first post as I just registered today, I was curious to know what kind of mileage increase I could expect if I were to install a 3.54 rear axle in my 1998 Nissan frontier in place of the 4.33 axle I have now, my engine is a stock 2.4 Dohc with a four speed auto, my truck is 2wd and I am running 235 75r15 tires in place of the factory 215 65r15.

[98frontier]

Also, if this is posted in the wrong place please redirect me to the proper location.

[user removed]

What does the tach read at 60 MPH? It it's around 2k RPM, you might not see much improvement.

regards
Mech

[Gealii]

Originally Posted by Old Mechanic

What does the tach read at 60 MPH? It it's around 2k RPM, you might not see much improvement.

regards
Mech

wouldnt it depend more on what kind of driving you do?

going from 3.54 to 4.33 should help in the city
while 3.54 to say 2.73 would hurt city help highway

[Daox]

Welcome to the site!

I've never heard of taller gearing hurting city mileage. Care to explain Gealii?

I'd recommend taking a look at our wiki page:
Car MPG Efficiency Modifications Main - EcoModder

There are several here who have done regearing stuff. The gains seem to be from 7% to 20%. I'm sure its application specific, but those are pretty huge gains.

[mcrews]

It kinda depends on how much city/hgwy driving you do.
The more hwy the better the result.
you can look at my posts w/ Q45 and taller tires. I did 30-40k a yr so a lower rpm at 60mph was a home run for me.
First: what is rpm at 50, 55, 60 on frwy?
Second: how much (%) hwy driving do you do?

since its a truck, its probably geared to 'pull', so going to the 3.xx gear would probable work.
my 66 mustang w a straight 6 200ci had a 2.76 rear.

[mcrews]

oh by the way WELCOME!!!!

increase the psi in your tires to 40+

[aerohead]

Originally Posted by 98frontier

Hello, this is my first post as I just registered today, I was curious to know what kind of mileage increase I could expect if I were to install a 3.54 rear axle in my 1998 Nissan frontier in place of the 4.33 axle I have now, my engine is a stock 2.4 Dohc with a four speed auto, my truck is 2wd and I am running 235 75r15 tires in place of the factory 215 65r15.

*You need to know where you are,with respect to road load horsepower and the engines Brake Specific Fuel Consumption (BSFC)profile.
*For every engine rpm and combined aero,plus rolling resistance load,you'd see a particular rate of pounds of fuel per brake horsepower hour,after factoring in the driveline efficiency.
*This is best done on a chassis dynomometer,where all the loads can be simulated.
*With all the data,you'd construct an engine 'map' which showed a number of islands of constant BSFC,with one in particular yielding the best mpg.
*This would be the best your engine can do.
*And it may be that Nissan has already optimized the gearing for your automatic 4-spd,and changing the ratio arbitrarily might actually lower your performance.
*Until you figure out a way to get a map,you're shooting in the dark.

[98frontier]

Well my truck has no factory tach but I did have an aftermarket tach in it a while back, I think with factory size tires it was around 2300, with my new 235 75r15 tires it should be around 2100, and also my tires have a max of 35 on the sidewall, so I run them about 34 or so.

[98frontier]

Also I drive mostly highway, probably around 70%