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gear ratio?

Discussion in 'General Chevy & GM Tech Questions' started by lenb, Jul 1, 2011.

  1. lenb

    lenb New Member

    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
  2. murdog94

    murdog94 Epic Member 5+ Years 5000 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.
  3. dsfloyd

    dsfloyd Epic Member 5+ Years ROTM Winner Gold Member 1000 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
    Last edited: Jul 1, 2011
  4. sstoner911

    sstoner911 Epic Member 5+ Years 1000 Posts

    Awww...come on guys cut him some 08 1500 has 4.11's.........ok ok it had 3.23's originally.
  5. lenb

    lenb New Member

    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.?
  6. dsfloyd

    dsfloyd Epic Member 5+ Years ROTM Winner Gold Member 1000 Posts

    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

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