Tire size & gear ratio
Thread Starter
Teching In
Joined: Nov 2007
Posts: 16
Likes: 0
From: Dallas
Tire size & gear ratio I have aquired a 71 Chevy 12 bolt, sandblasted & POR15ed. It will go in my 69Cutlass Convertable w/ LS1/auto from a 99 Firebird. The Cutlass had 245 60R14s that measure about 25" dia. What size tires would have been on the Firebird and what ratio gears? The Cutlass will be a little heavier so I want pick the best all around gear set for street driving w/ plenty of pick up. I hope to include a Maggie if theres enough $$. I have a local guy to build the 12 bolt. We are talking about a Detroit Locker w/ 373 gears. Any comments appreciated. Anyoue used that new Eaton ECTED unit?
Dave in Dallas
david@coolingpro.net
Dave in Dallas
david@coolingpro.net
TECH Resident
Joined: Oct 2004
Posts: 800
Likes: 1
From: Simi VAlley,CA
I had 3.70 gears with a 29 inch tire which worked perfectly, excellent performance and effortless cruising along with surprising economy. The car weighs about 3300 lbs.. I made the mistake of changing to 3.25 gears in anticipation of making some long road trips and what a bad move that was! The car feels "lazy" now, shifts like crap (even though we have juggled shift points/pressures etc) and overall it just drives shitty.
I have an LS1/4L60E combo and am running 3.80 gears with a 29" tall tire. Works out to 2000 RPM at 65 mph. Gear spacing seems to be good for driving and acceleration. You can calculate you results using this formula
GR*Spd*168/(TD/2)*OD = RPM
GR= Gear Ratio
Spd = Speed you want to know the RPM for
TD = Tire Diameter
OD = Overdrive ratio, for 4L60E is it .7 for the 6 spd it is .5 for top gear.
RPM = RPM
So for your case: 3.73 * 65 * 168 / (26/2) * .7 = 2193.24 RPM
If you want to know your RPM in 3rd gear, just don't do the final caculation or use 1 for your OD value. I built a simple spreadsheet in Excel that enables me to experiment with different values.
Hope that helps
Pat
GR*Spd*168/(TD/2)*OD = RPM
GR= Gear Ratio
Spd = Speed you want to know the RPM for
TD = Tire Diameter
OD = Overdrive ratio, for 4L60E is it .7 for the 6 spd it is .5 for top gear.
RPM = RPM
So for your case: 3.73 * 65 * 168 / (26/2) * .7 = 2193.24 RPM
If you want to know your RPM in 3rd gear, just don't do the final caculation or use 1 for your OD value. I built a simple spreadsheet in Excel that enables me to experiment with different values.
Hope that helps
Pat
Last edited by Stu Cool; Jan 9, 2008 at 10:48 AM.
TECH Resident
Joined: Oct 2004
Posts: 800
Likes: 1
From: Simi VAlley,CA
I switched from a Ford Trac-Loc to a Detroit Tru-Trac (gear type) differential while I was in there. That Trac-Loc was noisy, grabby and just not very friendly on the street for some reason......in spite of all the different lubes and additives I tried. Now it's absolutely silent.
