Larger exhaust housing= less mpg.

[RonA]

Have been noticing over the last year and a half that the slower it spools, the more fuel it uses. I sarted keeping closer tabs on my fuel mileage with a 62/. 7 secondary charger. Average was running 19. 2. 21 on longer fwy trips. Switched to a 62/. 8 seconday and the mileage dropped to 18. 2. 20. 2 on longer fwy trips. Up in the mountains it's a bit worse. The smaller housing requires less pedal to build and maintain power between corners. Towing is worse still. This is the best compromise i have found. The 1mpg will add up, but the extra top end power is nice.

Ron

 

[JStieger]

I noticed the same too. When I went from a HTB2 62/14 to a SPS 66 my mileage dropped 2 mpg around town and maybe dropped 0. 5 mpg on the interstate. Then when I switched to a SPS 62 my mileage around town increased by 1 mpg, but didn't change too much on the interstate. Blowing that black smoke while trying to build boost really eats up the fuel costs over time! Basically the bigger exhaust housings totally cancelled out any fuel mileage gains I got with the cam (~1. 5 mpg) so I really wonder what the mileage would be without the cam.

 

[RonA]

Highway mileage isn't affected as much. It is the slightly increased lag when starting to accelerate that hurts the mileage. I am running an SPS 62 also. As a secondary it is a wee bit slower to light. But like I said, it is the best compromise between towing and performance I have found. I'm happy that I only lost 1 mpg.