Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com - View Single Post - Tire size vs lean burn, gear ratios, etc. What tire size am I after?

phyregod · 07-21-2015, 10:46 PM

Ok, Big question here.. I'm building a hypermiler. So far, the build is:

1990 CRX Si
1994 Civic VX lean burn engine
1991 CRX HF transmission

I'm pondering tires size for lean burn at 75mph (my commute speed)

Stock HF tire size is 165/70-13.

Tire size I'm considering 195/55/15 (because of Konig Helium 15x6.5 wheels to save weight)

According to my calculations, the car will be at ~2200rpm at 75mph on these tires, which is well into lean burn territory if the load is low, which it should be? (Total guess here) given the .29 stock coefficient of drag.

My question is, would it be better to go with a smaller tire diameter to get the engine speed up a bit and reduce the load on the motor in order to keep lean burn active? Or would lean burn be active anyway? Would that produce better or worse mpg? From my research, I need >4 in hg manifold pressure and under 2250rpm for "high load", under 3000rpm for "low load".

I commute 200 miles a day on a pretty flat highway and I'm looking to do 75mph on the commute. Every 5mph lost is an additional 17 minutes per day, hence the "high speed" (legal speed limit) requirement.

07-21-2015, 10:46 PM	#1 (permalink)
phyregod !!ytinasnI Join Date: Jun 2009 Location: Central Texas Posts: 11 PhY-49 - '90 Honda CRX Si Thanks: 1 Thanked 0 Times in 0 Posts	Tire size vs lean burn, gear ratios, etc. What tire size am I after? Ok, Big question here.. I'm building a hypermiler. So far, the build is: 1990 CRX Si 1994 Civic VX lean burn engine 1991 CRX HF transmission I'm pondering tires size for lean burn at 75mph (my commute speed) Stock HF tire size is 165/70-13. Tire size I'm considering 195/55/15 (because of Konig Helium 15x6.5 wheels to save weight) According to my calculations, the car will be at ~2200rpm at 75mph on these tires, which is well into lean burn territory if the load is low, which it should be? (Total guess here) given the .29 stock coefficient of drag. My question is, would it be better to go with a smaller tire diameter to get the engine speed up a bit and reduce the load on the motor in order to keep lean burn active? Or would lean burn be active anyway? Would that produce better or worse mpg? From my research, I need >4 in hg manifold pressure and under 2250rpm for "high load", under 3000rpm for "low load". I commute 200 miles a day on a pretty flat highway and I'm looking to do 75mph on the commute. Every 5mph lost is an additional 17 minutes per day, hence the "high speed" (legal speed limit) requirement.