The assumption is if he had a 3.7 rear gear and ran the car in OD or 4 gear it would act and drive the same as his current car being driven in 3rd gear with the 2.59 rear gear. So if he wants the same feel of his current car being driven in 3rd add a 3.7 rear gear and let the car use 4th gear (OD).

 

Wow, 30mpg! I do not see how I could get from 20 to 30. I would feel fortunate to get to 24 on the highway much less combined. I do not foresee a time when I could do a whole tank on the highway. What were the driving conditions when you got 30? I assume it was all highway, fairly flat, level, and the time of year was September. What state and area do you live in? I assume that as your write-up says the car was "bone stock" at the time.

 

Yes, that is what I meant. Sorry, I did not make that clear.

 

Again, I would feel very fortunate if my car could get 25-27 on the highway! Not sure if I every will get the chance to run a tank on the highway. Most of my driving is combined.

 

Mods to the car then (and now) include K&N filter, Koni shocks, C5 Z06 wheels, C5 front brakes w/HP+ pads, & upgraded stereo. None having any positive effect on gas mileage.

Drove from Telluride, CO to Las Vegas and back -definitely not flat. I think it was in May. Full STORY HERE.

 

It is combined, but mostly highway. It helps somewhat (depending on point of view) that all the stores worth shopping at but one are 30 miles away. The only time I have seen RPM make a difference in economy is with a carburetor.

 

I'm impressed! I will be very happy if I can get 25-27 combined mileage. I do not think I will ever be able to run a whole tank on the highway, so I'll probably never see anything above 27.

The 5 hour trip I took was 80% highway and 20% city and I got 20 mpg and that was using 4th gear on the auto. I drove with a light foot because the wife was with me, so I just do not see how I will ever get 24/25 combined. I cruised at 60 and 70 mph by the way with the AC on. And if I want to take trips like this in the future I will need to get the car quieter. Even with resonators, H-pipe in front of mufflers, and new MasterFlow mufflers it drones cruising and pops when decelerating and the wife does not like it! I am thinking of putting a X-pipe up front in the exhaust.

To make sure we are comparing apples to apples... My 94 is stock with 255/45ZR17 front and 285/40ZR17 rear.

 

My car has about a 2003 5.3 LS, 6 speed, 275/40R17 for all 4 tires, and (now) a dyno tune.

 

I think something may be amiss with your car, or driving style. Driving style makes a HUGE difference. Huge.

 

Well, I knew this would be questioned. I am a senior citizen, I drive conserative, and with my wife wife in the car on this trip I drove like a raw egg was between my foot and the accelerator. Cruising at 60/70, the cruise control was on and on the total 5 hour round trip the trans was in 4th gear. And I got 20 mpg. So, you can understand my shock when you guys are getting 25/27 combined and as high as 30 mpg on the highway!? And on another trip that was 75% city and 25% highway in 3rd gear I get 22mpg!

I am by no means saying that I do not believe the two of you, but I just do not see how I get to 25/27 combined! The car may need a tune-up, but it is not running that bad.

 

I hear you. I can't understand how you're only getting low 20's -I do that well with my '96 Silverado on the highway. It sounds like you're driving OK, and if cruise is on, there isn't a lot more you can do there. When ever I see people post results like yours, I just want to drive their car myself and see what is going on.

 

I understand, I feel the same way. Let me drive some more and get some more data, and then see what we have. Thanks much for your and others input. I really appreciate it!

 

Anytime...and good luck with it!

 

Yeah...like whether there's a problem with electronics, injectors, etc... As pointed out, the original question about TQ near hwy cruise speed was misunderstood. What he really needed to know was the minimum power need to keep the vehicle moving to overcome: gravity, incline, air resistance, yada-yada.

If the car runs better at higher rpms, my first thought goes to injectors and sensor monitoring. If the 20-yr-old injectors are still installed, how well are they atomizing at the lowest throttle openings? The car MIGHT feel smooth even though it's lost a lot of efficiency? And, what's the trim doing to his mpgs?

If it hasn't already happened, swapping to new injectors, tune-up, and 02 sensor(s) is easy/cheap to do on an LT car. I'd start there before considering something illogical -- like swapping to 3:45 "sport" gearing.

Or maybe do both!




Oh yeah: I want to be sure "afgunn" realizes the two mileage examples [posted in this thread] were on a manual-equipped car. M6 refers to a 6-spd (which I also have). In a ZF-6, 6th gear has only a .5 gear ratio. 5th is also an "overdrive" on these transmissions -- coming in with a .75 ratio. That means our 5th is very close to the automatic's 4th gear. M6 transmission have TWO overdrive gears! If it were "too hard" to pull the car at even lower rpms, how/why do you think an M6 was installed AND that they get even better hwy MPG's than automatics?

C4's have VERY low drag. I've read and experienced a sweet spot in MPG's around 70mph. At that speed -- in 6th -- the motor is turning a mere 1450 rpms. The motor has the ability to do that. There is enough "torque". But, the motor has to be running correctly to take advantage of those low rpms and gearing.

30mpg is what mine got back in the early 2000's. As gasoline has migrated to higher levels of ethanol, that potential has dropped. "Gasahol" is less efficient and gets worse mileage. Because it's "cheaper", there should be a wash. Even with my converted 383 stroker, I'm still walking all over your 20mpg. Mine is still above 25mpg...at around 27. If pure gasoline were still being run in my 383, I'd probably be able to lean on 30mpg's. The extra torque of a 383 would require a bit less throttle opening compared to a 350. Again, you should have a wash.

A lot of what goes into MPG's is the weight and drag of your car. Wind and weather play a factor too. That's why individual trips make it hard to decipher results. Of course, if you're a human calculator like those guys on the new TV show, Scorpion, you could probably sift through all the variables in your head! LOL

 

Hey, on your trip that you got ~30mpg do you remember your cruising speeds? Every piece of data helps!

 

D44 with 3.45 was one of my first mods, it didn't help mileage however, it didn't hurt much either. Now the 23X 23X .615 lift cam did more damage to the mileage than the gears!.

 

Hey, great info and all worth serious consideration! It will take me some time to check everything out.

One other item, maybe you guys can help. Currently the car has complete dual exhaust from the exhaust manifolds back - 3" after cats into the resonators. The original exhaust was single back to the mufflers was it not? Also, best I can tell only 2 O2 sensors are connected (1 each in front of each cat) and I believe originally there were 3 O2 sensors - one each in front of the cats and 1 after the pipes from the cats had joined into one. Right or wrong? If this has been modified from single exhaust to dual and the delete of the after O2 sensor, this would definitely affect mileage (most likely running rich) and would need to be tuned.


This and the other things you mentioned may be the reason for 20mpg.

 

It is my understanding that there were no O2 sensors after the cats until (94 is OBD1) OBD2, and that they are used to verify that the cats are functioning correctly.

 

Exhaust mods could affect things positively or negatively. If it's true duals WITHOUT an x-over AND a slightly larger catback, it's possible that could hurt your lowest throttle positions. Ideal design is to slightly reduce piping as you go back. That and an x-pipe create a bit of scavenging for efficiency.

 

There was not O2 post cat on any OBD I Corvette, so you just have the two O2's.


I had the cruise set on 84 mph.