MPG Change
Thread Starter
Intermediate
Joined: Nov 2008
Posts: 30
Likes: 0
MPG Change
I'm trying to decide on making the criminal swap from 3.15 to 2.73s. I travel long distance (2,400 mi) and at my target speed (75) this would lower the rpms by about 300. Besides lowering my rpm this would also pull me out of the start of the "boost window" of the supercharger which starts around 1800 rmp. What do you think the mpg increase will be?
Race Director
Joined: Jul 2007
Posts: 18,681
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From: Reno is so close to Hell you can see Sparks , State Of Confusion
St. Jude Donor '12-'13-'14
After having 3.15 gears there is no way I could go to 2.73 gears, It isn't worth the mile more per gal. You might even fing you will lose mileage.
Melting Slicks
Joined: Nov 2007
Posts: 3,475
Likes: 10
From: Wake Forest NC
Swapping from 3.15 to 2.73s should boost your overall top speed, but it might take longer to get there. Given the dual purpose nature of your car, (commuter with a "kick," so to speak,) that may not be a problem.
And given your stats, (2400 mi, 75 MPH, 300 RPM,) that's 576,000 less revolutions. I gotta think there has to be some gas savings in there someplace.
s'noJob
And given your stats, (2400 mi, 75 MPH, 300 RPM,) that's 576,000 less revolutions. I gotta think there has to be some gas savings in there someplace.
s'noJob
Melting Slicks
Joined: Aug 2006
Posts: 2,866
Likes: 45
From: Tampa Bay, Go BUCS!!!Go Rays!!!Go Lightning!!!
St. Jude Donor '12-'13-'14
You should be able to get a close guesstimate simply by slowing down now. Slow to about 1800 for about ten miles and watch your average MPG (reset it of course). There will be some wind resistance benefit of course but I'd bet it is minimal. I'd expect about half of the improved MPG to remain.
Now think of this,,,, If it costs you $1200 to swap the rear end how long will you have to drive to get that $1200 back? Even if you drive a lot of miles I'd expect it to be about two years (too lazy right now to do the math).
Good luck. Hope you make the right choice.
Now think of this,,,, If it costs you $1200 to swap the rear end how long will you have to drive to get that $1200 back? Even if you drive a lot of miles I'd expect it to be about two years (too lazy right now to do the math).
Good luck. Hope you make the right choice.
Drifting
Joined: Apr 2007
Posts: 1,760
Likes: 4
From: Broward County Florida
Putting a supercharger on a car that you're planning on using for a fuel efficient commuter car makes no sense. I understand you want your cake and eat it too, but that's not the reality here.
BTW, I noticed quite an increase in MPG if you utilize the cruise control.
Last edited by DevilMan02; Nov 15, 2008 at 09:28 AM.
Melting Slicks
Joined: Nov 2007
Posts: 3,475
Likes: 10
From: Wake Forest NC
Givens:
- 27.5 mpg (10% greater than 25 mpg)
- $3.00 per gallon premium gas
- 2400 miles commute
- $1200 gear change cost
It would take $288 to go 2400 miles @ 25 mpg and $3.00 gas. The same distance would cost about $262 with a 10% MPG improvement, a savings of about $26 per trip. If it costs $1200 to change the gears, the switch would start paying off after about forty-six trips, or after about 110,400 miles. Of course, there are a lot of assumptions here, but the figures can represent a starting point.
s'noJob
Race Director
Joined: Nov 2004
Posts: 11,069
Likes: 1,290
From: Central Ma
I was thinking the same thing, so I did the math. My '04 A4 with the 3.15 gears gets about 25 MPG on the Hwy. (Your mileage may vary.) Here's what I found.
Givens:
It would take $288 to go 2400 miles @ 25 mpg and $3.00 gas. The same distance would cost about $262 with a 10% MPG improvement, a savings of about $26 per trip. If it costs $1200 to change the gears, the switch would start paying off after about forty-six trips, or after about 110,400 miles. Of course, there are a lot of assumptions here, but the figures can represent a starting point.
s'noJob
Givens:
- 27.5 mpg (10% greater than 25 mpg)
- $3.00 per gallon premium gas
- 2400 miles commute
- $1200 gear change cost
It would take $288 to go 2400 miles @ 25 mpg and $3.00 gas. The same distance would cost about $262 with a 10% MPG improvement, a savings of about $26 per trip. If it costs $1200 to change the gears, the switch would start paying off after about forty-six trips, or after about 110,400 miles. Of course, there are a lot of assumptions here, but the figures can represent a starting point.
s'noJob
Your math should be good enough to convince the op not to bother.
Drifting
Joined: Feb 2004
Posts: 1,829
Likes: 45
From: Las Vegas Nevada
I don't see a reasonable pay back with the gear change. Why boost the ls1 and then back down the rear cog?
Besides that, 3.15's with the modded motor would be a kick in the
butt. Hit the cruise control and wave.
Besides that, 3.15's with the modded motor would be a kick in the
butt. Hit the cruise control and wave.
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Race Director
Joined: Jun 2004
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From: Schaumburg IL
CI 7-8-9-11 Veteran
St. Jude Donor '06-'10, '13
What's the point with a boosted car. You are probably arleady running larger injectors anyway which has you sucking up more fuel. With the time and money it takes to swap gears, not worth it. You might pick up 1 or 2mpg. Now if you were running 410 or something and wanted to back down to 273 or 315 that might be a different story.IMO 315 and 342 are the perfect gear for Big HP cars.
Thread Starter
Intermediate
Joined: Nov 2008
Posts: 30
Likes: 0
Thanks for the input. Pre-supercharger my highway avg was around 26 with a high of 28. Now I see 21-22 and I am a cruise control devotee (keeps me from speeding, well, too much anyway). Last trip (granted, pulling a custom trailer, que the eye rolls & groans - it's the cost of not having to drive the minivan) I averaged 19.8 per DIC (math was 19.6). What causes the delima is I can have a swap done for near free. For now I think I'll wait and see what the header install does. I'm thinking once I get the back pressure down (I'm seeing several #s of boost I shouldn't) it may help in the mpg department when cruising. Again, thanks for the replies.
