TIRE SIZE HELP? and speedo correction is it off?

[mustangmanjeff]

tire size help

I bought my 95 mustang with 265 45 zr17 inch tires on it out back with 17 inch rims with stock transmission and 3.73 gears, I changed out to 255 35 zr18 inch tires outback and now have a aftermarket tremec tko500 with 3.73 gears, So I did a measurement and thats a 25'' inch tall tire? with 3.73 gears what does that = to ? I read there is a plastic little speedo tooth gear that is diff for gears, when the dealer installed my transmission I'm thinking they might of installed the gear for stock gears for my year which would be 2.73's or 3.08's. and I run aftermarket 3.73's. I'm trying to see how far off my speedo is and how to fix it or test? and if the tire size and gear ratio is good.

Reason I ask I feel the faster I go my speedo is way off when I have my GPS on the speeds dont match up , and what the speedo says and how fast i feel im going dont add up. I kno tire height / gear size etc makes a huge diff esp if you change it. I know the newer cars with digital odometers they sell a "speed cal" to adjust this problem, but are older years have the mecanical speedo so what can I do to make it accurate, and with that tire size what is the gears = to . I might step up my rear tire size being I need new tires soon, I might buy some 265 or 275 rear tires.

Edited December 21, 2013 by mustangmanjeff

[Steve-Oh]

The width of the tire shouldn't change anything, your gonna want to pay attention to sidewall size in regards to this. Since you stepped down in sidewall size everything should stay relatively the same. [MENTION=7]OnyxCobra[/MENTION] is pretty good with tire size calculations.

When I had the 4.10's put in I had them change over the speedo gear in mine and it did nothing. Whether or not they just dicked me and never changed it is another story but my speedo hasn't worked correctly in years. In theory though yes you should be able to change the speedo to match what rear gear you're running.

[OnyxCobra]

well in this case the tires are shorter than stock but the gears are probably having way more of an effect than the tires. You said you use a GPS, how much off does your speedo read at a given speed? I'm not sure they make a speedo gear to correct for 3.73s but they do make speed cals for our cars that allow you to dial in different tire sizes and gear ratios.

check out these two

http://www.latemodelrestoration.com/item/M4209ADPTAC/94-10-Mustang-Speed-Dial-Speedometer-Adjuster-M-4209ADPT-AC

http://www.dallasmustang.com/mustang-parts/product/Speed-Cal-1994-2004-5-Speed-Cars/7626

[mustangmanjeff]

thanks!

[scottydsntknow]

The width of the tire shouldn't change anything, your gonna want to pay attention to sidewall size in regards to this. Since you stepped down in sidewall size everything should stay relatively the same. [MENTION=7]OnyxCobra[/MENTION] is pretty good with tire size calculations.

When I had the 4.10's put in I had them change over the speedo gear in mine and it did nothing. Whether or not they just dicked me and never changed it is another story but my speedo hasn't worked correctly in years. In theory though yes you should be able to change the speedo to match what rear gear you're running.

 

Actually the sidewall is a percentage of the total overall width of the tire so it is not going to stay the same.  If its close like 265/285 it won't be off TOO much but when ppl think they can run a 245/35/18 up front and a 315/35/18 out back and have the same diameters...  The 35 in that number for example is 35% of 245mm and 35% of 315mm.  Definitely not the same.  

 

well in this case the tires are shorter than stock but the gears are probably having way more of an effect than the tires. You said you use a GPS, how much off does your speedo read at a given speed? I'm not sure they make a speedo gear to correct for 3.73s but they do make speed cals for our cars that allow you to dial in different tire sizes and gear ratios.

check out these two

http://www.latemodelrestoration.com/item/M4209ADPTAC/94-10-Mustang-Speed-Dial-Speedometer-Adjuster-M-4209ADPT-AC

http://www.dallasmustang.com/mustang-parts/product/Speed-Cal-1994-2004-5-Speed-Cars/7626

 

^ This, if you have a 94-up car, get a speedcal.  Don't mess with speedo gears or anything like that.  You can plug in your specs and it'll calibrate the VSS signal going to the speedometer for your correct speed.  

 

Also OP, your 255/35/18 tires are an inch smaller in diameter than stock.  Your 265/45/17s were much closer.  Stock size on these cars is a 245/45/17 (or 255 if you had a 5.0 Cobra) or 26" total.  your 255/35/18s are 25" tall.  I mean its not going to hurt anything really besides a harsh ride from the short sidewalls.