Gear ratio vs. fuel consumption

[60/40GUY]

I'm working on adding a turbo 1HZ and a H55 to a FJ60. I was going to add some FJ62 axels with 4:10 gears to the truck. But now I am thinking to keep the 3.70 axels with the 1HZ. I will be running 33 inch tires as well. I plan to use the truck as an expitition type vehicle. 70-80% of the time will be on paved roads. 20% of the on road time will be spent pulling a 1500lb. pop up camper. Off road will be mostly moderate trails.

My questions are as follows:

Is anyone using a turbo 1HZ and 3:70 rearend with 33inch tires? How does it drive? How does it tow?

What type of MPG differance should I expect with the 3.70 over the 4.10? I know it is a 9.75% mathmatical difference but could I expect a 9% difference in fuel consumption?

Any input would be helpful.

Thanks

Frank

 

[60/40GUY]

Noone has any comment?

 

[toynut]

i am using 33" tires with a stock 3b with 3.70's , not fun , not the same as a turboed 6 cyl but if you think that truck originaly came with 29" tires ? , the motor is probably going to labor a little and you would be out about 12% over stock with 33's and 3.70's , you will notice it if you have any hills to climb as well as your planning on doing some towing i would go with the 4.10's about a 2% difference over stock ? your speedo should be close to original and you will be happier when off road believe me even in low range with 33's and 3.70's there is no crawling .

i am looking at swapping to 4.10's with a detroit rear and open front as soon as i can .

 

[HZJ60 Guy]

I have a 1HZ in my HJ60. Because I have 37" tires Im going for 4.56 gears. The point is to keep the power/torque curve as close to stock as possible. Here's the math I used to keep my highway RPM around 2100rpm at 60mph.

MPH/Gears/5th gear/Constant/tire size

60 x 4.56 x .845 x 336 / 36" tire = 2157 RPM

Your example:

60 x 3.70 x .845 x 336 / 32 = 1969 RPM
60 x 4.10 x .845 x 336 / 32 = 2183 RPM

Look at those numbers. You could go either way. At this point Id go 4.10. The lower gear will give you a little more grunt and it'll crawl better too without hurting your mileage. P.S., those 33" dont meaure 33"tall.

You have to decide what you want that engine to spin at YOUR highway speed. I like 60mph because it's a mile a minute and it keeps the engine at a respectable rpm. Diesels are VERY rpm touchy. In my 2001 TDI Jetta I get 55mpg at 60mph. At 70+ I get 45mpg......

 

[60/40GUY]

Thanks for the info. Now I have a few more questions.

How did you pick 2100 rpm? Is that peak hp or torque? Would a turbo change that 2100 rpm "sweet spot"?

Thanks again.

Frank

 

[HZJ60 Guy]

Look here:

http://www.airpowersystems.com.au/safari/turbo/1hz_80/torque_turbo.jpg

That is the torque curve for a 1HZ out of an HZJ75. You can see that with a Safari turbo the peak torque is between 1800/1850 and 2600 RPM. The lower you can achieve normal driving conditions within that torque curve the less fuel youre going to use. You'll still get maximum torque, and youll be operating at the lowest RPM that achieves that torque.

If others disagree please pipe in here.

WayneO is Mr. HZ performance guy. I'd like to hear what he has to say.

 

[60/40GUY]

Using the graph from Safri, peak torque is at 2000 rpms. So using your math equations it looks like I should stick with the 3.70 gears for highway use.

It's all clear as mud now.

Thanks

Frank

 

[HZJ60 Guy]

Youre welcome Frank. Im targeting mine a little on the lower (gears) side for torque.