GM Truck Club Forum banner

1 - 6 of 6 Posts

·
Registered
Joined
·
17 Posts
Discussion Starter #1
i have 05' suburban 1500 4wd,5.3 w/4.10 gears. @ 2200rpm =70 mph ,with 30.7" tires. i put on new firestone destination a t's.285 70 17's. does anyone know my new ratio? i was trying to lower hgwy.rpm @ cruise. are my gauges still accurate (tach, speedo )? should my computer stiill be accurate ( on truck ). thanks len
 

·
Registered
Joined
·
7,979 Posts
Alright not to sound like a dick but did you pull the RPO codes to get the 4.10 ratio?? your speedo is probably closer to accurate now than before thanks to the fact that GM speedos are usually off by about 3-5mph. As to everything else yes it is still reading correctly.
 

·
Registered
Joined
·
2,460 Posts
with the 285/70/17's you increased tire diameter by 1 inch, so yes your speedo will be off some, about 2 miles per hour (reading faster than you are actually travelling) your tach will still be accurate. should reduce RPMs about 100 rpms (at 60 with a 1:1 ratio)

---------- Post added at 11:09 AM ---------- Previous post was at 11:08 AM ----------

was a little surprised to see the 4.10 in the 1500 as well

---------- Post added at 11:09 AM ---------- Previous post was at 11:08 AM ----------


Here is a formula for the effective gear ratio change (old tire diameter/new tire diameter)x old gear ratio= new gear ratio

ie (30.7/31.7)x4.1=3.97
(30.7/31.7)x3.73= 3.61
 

·
Registered
Joined
·
1,218 Posts
Awww...come on guys cut him some slack..my 08 1500 has 4.11's.........ok ok it had 3.23's originally.
 

·
Registered
Joined
·
17 Posts
Discussion Starter #5
i have original sticker. 4:10 was an option.thanks for the help. i just added a gm performance exhaust, not real happy. it drones @highway speed. seems to have lost some power and definitely about 2 mpg,s.?
 

·
Registered
Joined
·
2,460 Posts
i have original sticker. 4:10 was an option.thanks for the help. i just added a gm performance exhaust, not real happy. it drones @highway speed. seems to have lost some power and definitely about 2 mpg,s.?
sorry to hear about not liking the gm exhaust. Then my first post had the right info on the effective gear ratio, speedo etc. a 3.97 effective gear ratio, should read 2 mph faster than actual. so should show better mpg's but not actual. How big is the exhaust pipe on the new exhaust. if you go too big it gets rid of too much back pressure and can effect power. If its a true dual 2.5-3" diameter is about the largest you want to go
 
1 - 6 of 6 Posts
Top