# Math problemo.........tire size vs stock

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by BurbinOR, Aug 19, 2004.

1. ### BurbinOR3/4 ton status

Joined:
Feb 17, 2000
Posts:
5,829
0
Location:
Oregon
Say a factory tire is supposed to be 29" and you're running 35's............what is the formula to calculate the % odometer/speedometer difference for gas mileage calculations? /forums/images/graemlins/confused.gif

Probably simple but I just cracked open a bottle of Yukon to cut it with lime juice (mmmmmmmmm Snakebite) and *had* to drink the two shots out of the bottle to fit the right amount of lime juice in. /forums/images/graemlins/smile.gif

2. ### jekquistk5Weld nekidPremium MemberGMOTM Winner

Joined:
Jan 18, 2004
Posts:
12,097
2,247
Location:
Pinckney, MI
Build(s):
3
Easy just devide 35/29 you get 1.2! Whatever your spedo reads times that by 1.2 and you have your speed!

Whoa just realized you want something else, need to put down the JD and go to bed

Good luck

Joined:
May 27, 2004
Posts:
916
0
4. ### dyeager5351 ton statusPremium Member

Joined:
Dec 13, 2000
Posts:
27,834
434
Location:
Roy WA
Best idea is to figure out how far off %-wise your speedo is reading (GPS, speedometer check station/stopwatch) and calculate from there.

Actual tire size, growth, actual gear ratio can all play into what you are actually off. Perhaps try both methods (timed distance and calculator), but the ratios GM used for stock speedometer gears/tires are almost never what the online calculators (or sites) tell you should have been used. Might get you close, but can't be as close as timing your vehicle for a given distance with YOUR tires. We all know advertised and actual tire size is almost never right.

Stolen from someone that has dealt with this more than I... "so change gear ratio of the speedo gears by (example) 15% to slow them down (add to driven, reduce drive)"

Joined:
Nov 17, 2003
Posts:
1,556