Yet another gear thread...

[01cobravortech]

Well, believe it or not, I "worried" about fuel mileage, and drove my car Cobra to work and back every day. I was very proud of my 4.56 geared 99 Cobra getting 26 mpg+ on the highway. Good mpg is an indicator of efficiency.

All that said, 4.56s are a probably a good choice. I could not keep the 4.88s quiet, and unless it is an almost pure track car, I think they're too much.

Best of luck with whatever you go with.

I seriously don't understand why I can't get better than 18-19 mpg with my 4.56s on my 5 speed. :nonono:

I do all the maintenance. Cleaning the MAF, IAC, etc. Good tune and everything. I've even shed a lot of weight off the car. Full aftermarket exhaust and I don't understand why its so poor on fuel economy.

 

[Double"O"]

I seriously don't understand why I can't get better than 18-19 mpg with my 4.56s on my 5 speed. :nonono:

I do all the maintenance. Cleaning the MAF, IAC, etc. Good tune and everything. I've even shed a lot of weight off the car. Full aftermarket exhaust and I don't understand why its so poor on fuel economy.

perhaps if you kept the rpms below 6k you would get better mpg:-D

 

[01cobravortech]

perhaps if you kept the rpms below 6k you would get better mpg:-D

All I here you guys talk about in here is that these mod motors love to rev and run more efficiently in the top end. ;-)

 

[slonech]

All I here you guys talk about in here is that these mod motors love to rev and run more efficiently in the top end. ;-)

haha.. nice come back

 

[Double"O"]

All I here you guys talk about in here is that these mod motors love to rev and run more efficiently in the top end. ;-)

well good power and good mpg don't usually go hand in hand

 

[Bob Cosby]

There are a lot of factors that go into gas mileage. When my 99 Cobra was a "bolt-on" car and had the T45 and 4.56s, I would get 24-25 mpg highway. However, I kept the speed down to a steady 70 mph, and used a slightly taller back tire to help kill the gear.

With the NMRA F/S motor, I got as much as 28 mpg on one trip - still with the 4.56s. I did this with a T5 that had a .59 5th gear, and again with tires that were somewhat taller. And of course, like above, I kept my highway speed down to 70 mph, which is probably the single biggest contributor of good mileage (60-65 would be even better, but I can't drive THAT slow...LOL).

Looking at your username, I assume you have a Vortech on your car? All by itself, that will hurt mileage, as you're always turning the compressor, even if you're not building boost.

Bob

 

[01cobravortech]

There are a lot of factors that go into gas mileage. When my 99 Cobra was a "bolt-on" car and had the T45 and 4.56s, I would get 24-25 mpg highway. However, I kept the speed down to a steady 70 mph, and used a slightly taller back tire to help kill the gear.

With the NMRA F/S motor, I got as much as 28 mpg on one trip - still with the 4.56s. I did this with a T5 that had a .59 5th gear, and again with tires that were somewhat taller. And of course, like above, I kept my highway speed down to 70 mph, which is probably the single biggest contributor of good mileage (60-65 would be even better, but I can't drive THAT slow...LOL).

Looking at your username, I assume you have a Vortech on your car? All by itself, that will hurt mileage, as you're always turning the compressor, even if you're not building boost.

Bob

Bob, the Voretch is off and sitting around. I almost want to say that the old motor had slightly better mileage with the Vortech on and 4.10s. I don't believe that the current stock motor has a 'health isues.' No real unusual oil consumption and all the basic bolt ons that help it breath better. I do remember that between the first 25,000 miles the motor was getting about 320 miels per tank and that was with 4.10s. BUT...I also had a C&L meter on the car then, along with the previous custom tune. Stupid me went to get the car dynoed at around 25k and thats when I found the A/F ranging from 13.0 - 17.0 throughout the power band. Needless to say, thats when JLT entered my life. But its interesting to note that while running lean I was seeing my best fuel economy. Of course that wasn't healthy, but maybe I have a fuel economy dud compared to the rest of you guys.

I'm going to try these WeaponX COPs. Maybe I have something as subtle as that affecting my fuel economy. Its worth a shot because I've troubleshooted the heck of this fuel economy issue with no luck.

 

[omj]

I get around 19-20mpg no matter how I drive. But I do have a heavy convertible with wide tires.

 

[mlccar]

Gear Change Rumble

If 4.56s produced more significant gear whine, you would be able to find some posts about that in the millions of gear threads out there. I have 4.56s. They produce a 'helicopter like' whisper as I hop on it over 3000rpm. Its subtle, louder than 4.10s. If you have an aftermarket exhaust...then this additional noise should not be a contributing factor in making this simple decision. You want more acceleration, then do the 4.56. But your fuel economy will take a dive since you will find yourself having a problem keeping your foot off the car. Its perfect for a DD. If I had a Viper spec T56, I would seriously consider stepping down to 4.88s.

This decision is simple. No blower in the near future, then 4.56s. Near future to me is the next 6 months and you actually are setting aside money to purchase it. Near futures is not, "Yea, I'm thinking about a blower in the future after I get the FR500 head kit and put in the coilovers and k member, blah blah blah."

I just posted on the main thread about my gear change rumble. Went to 4:10's and have a rumble after 80 mph. Got the driveshaft rebalanced but it is still there. Do I have to live with this? Does anyone have a definitive fix to this? I am beginning to think I will have to dump thousands into this to get it fixed.

 

[Double"O"]

I just posted on the main thread about my gear change rumble. Went to 4:10's and have a rumble after 80 mph. Got the driveshaft rebalanced but it is still there. Do I have to live with this? Does anyone have a definitive fix to this? I am beginning to think I will have to dump thousands into this to get it fixed.

who did your gears bill???

have someone check the wear pattern

 

[01cobravortech]

I just posted on the main thread about my gear change rumble. Went to 4:10's and have a rumble after 80 mph. Got the driveshaft rebalanced but it is still there. Do I have to live with this? Does anyone have a definitive fix to this? I am beginning to think I will have to dump thousands into this to get it fixed.

It definitely can be eliminated. If the install was precise, its a pinion angle type problem. I had precisely the same issue when I dropped the subframe to install the subframe bushings. You may have to get a differential kit with more adjustment. I had to go this route with the MM aluminum diff. kit to get the pinion angle adjusted properly.

 

[mlccar]

Discouraged

Thanks. I am getting discouraged. So pinion angle or adjustment will cure this? I think you already posted this someplace - is there a kit to mount to the pumpkin to increase the range of angle/adjustment?

 

[01cobravortech]

Thanks. I am getting discouraged. So pinion angle or adjustment will cure this? I think you already posted this someplace - is there a kit to mount to the pumpkin to increase the range of angle/adjustment?

http://www.maximummotorsports.com/i...=product_info&cPath=21_88_194&products_id=484

Unfortunately, I couldn't get the pinion angle where it had to be with the poly diff. bushings so I had to upgrade to the alum kit. It may be pricey, but you will rid yourself of the vibe. You'll probably hear a little whispering coming from the rear end, but nothing annoying or nothing that you won't get used to in time. This kit is flawless.

 

[Bob Cosby]

I just posted on the main thread about my gear change rumble. Went to 4:10's and have a rumble after 80 mph. Got the driveshaft rebalanced but it is still there. Do I have to live with this? Does anyone have a definitive fix to this? I am beginning to think I will have to dump thousands into this to get it fixed.

You might also try clocking the driveshaft:

- Remove the 4 bolts that hold the DS to the differential yoke.
- Turn the drive shaft 180 deg.
- Drive the car and note if condition is better, worse, or unchanged.
- If unchanged or worse, remove the 4 bolts and turn the DS 90 deg in either direction.
- Drive the car and note if condition is better, worse, or unchanged.
- If worse, turn the DS 180 deg. Drive again.

You've now hit all 4 position that the DS can be bolted in to. Go with whichever position gives you the least vibration.

Bob

 

[Bob Cosby]

I get around 19-20mpg no matter how I drive. But I do have a heavy convertible with wide tires.

Wide tires will hurt mileage. I lost a consistent 2 mpg going from stock 99 tires/rims to 17x9 315s & 17x8 275s.

Bob

 

[viperbluelx]

who did your gears bill???

have someone check the wear pattern

That's not a simple task with an IRS, the installer should have checked wear pattern anyway when it was installed, if he knew what he was doing.

 

[viperbluelx]

But its interesting to note that while running lean I was seeing my best fuel economy.

Ya think? :idea:

 

[01cobravortech]

Wide tires will hurt mileage. I lost a consistent 2 mpg going from stock 99 tires/rims to 17x9 315s & 17x8 275s.

Bob

2mpg? Wow. That is a bit comforting considering I have 275/35/18s all around. I never gave much thought to this causing that drastic of a reduction in fuel economy.

 

[01cobravortech]

Ya think? :idea:

:lol: