Can I squeeze any better MPG from a 3800sc? (Page 1/1)
Additivewalnut JAN 12, 01:17 PM
Light searching has shown that 4spd 3800 cars should be getting something like 25-30 MPG on average but for the last 2 tanks of almost only highway driving, I'm getting about 170 miles to the (1/4) tank or 19-21mpg. That's the same as my big heavy AWD automatic infiniti so something seems wrong here... or maybe it's normal. Could be my stock front brakes are dragging a little or maybe it's just my reduced capacity since the car likes to die with 1.5 gallons still in it, the car does have 215-40-17 tires on it so maybe that makes a big difference?
olejoedad JAN 12, 01:37 PM
The engine will be turning a lot of rpm at highway speeds with the 4 speed. The small diameter back tires just add to the problem.
That fuel mileage sounds about right for your combo.
fieroguru JAN 12, 01:55 PM
Also check to see if the boost bypass valve is working. At cruise on the highway, you should not be building boost. If you are, then that will impact fuel economy.
Additivewalnut JAN 12, 01:58 PM

quote
Originally posted by olejoedad:

The engine will be turning a lot of rpm at highway speeds with the 4 speed. The small diameter back tires just add to the problem.
That fuel mileage sounds about right for your combo.



Yeah I kinda figured that wasn't helping. I need to get a coilover conversion so I can run a 17 with some sidewall. The wheels I got don't have much offset so I can't run a anything bigger than a 45 sidewall and even that is pushing it. That on top of the almost 3k rpm to do 70mph is a little ridiculous, but it does make it peppy during autocross LOL
Additivewalnut JAN 12, 02:00 PM

quote
Originally posted by fieroguru:

Also check to see if the boost bypass valve is working. At cruise on the highway, you should not be building boost. If you are, then that will impact fuel economy.



Gauge says no boost! Speaking of which, I know that's not the topic of this thread, but is ~6psi normal on a stock pulley? Can't ever seem to get a clean answer from the 3800pro forum
jelly2m8 JAN 12, 02:27 PM
That's a bit low. I just watched a video I took, travelling at 60 mph I'm pulling 1800-1900 RPM with the TCC locked. My Combo is an 02 engine, GenV blower on a stock pulley, an 03 4T65EHD. BOV vacuum controlled as per Sinister, 235/45R17 rears and a Tune from Sinister. I did request a basicially stock engine tune plus whatever magic he does to make that combo work. This was with AC on.
cvxjet JAN 12, 02:27 PM
I actually went with 16 x 7 wheels with 205/55 and 225/55 tires....Those size tires were very popular in the 90s so there are a lot of options/manufacturers. I have 38 mm offset which helped up front with the steering effort and kickback. My front tires are 24.8" diameter and the rears are 25.7" diameter (I also have a 5 spd- but only a 3.4)

My VOXX Monza wheels are no longer available....they only weigh 13.5 lbs- eliminating unsprung/rotational weight will really improve performance but also improve the feel...

[This message has been edited by cvxjet (edited 01-12-2024).]