Speedometer Question???

[pro-street dart]

I now have 31" tires with 4.88 gears on my dart - which will no doubt throw off the speedometer, what speedometer gear would I have to get in order to get it to work close to what it did originally?? Any help appreciated.

[ehostler]

If I've done the math right, you will need the 2538941 (41 tooth) gear.

[ehostler]

I have redone my math and confirmed that you would need approx 41.426 teeth on the gear, to be accurate. If you go with 41 teeth, your displayed speed will be just a little faster than actual.

My base numbers are a 4.56 gear with a 30" tire uses a 40 tooth gear. I got this from http://www.moparts.com/moparts/tech/pages/speedo.html

[pro-street dart]

Thanks for the info!!

[TOOL MAN]

Eholster,

thanks for the info showing all the #'s for the ratios. i asked this question some time ago and the only answer i got was to call mopar performance. this helps

thanks again

tool man

[ehostler]

Glad to have been of help.

I may have seen the earlier request, however I forgot that I had this bookmarked, until I recently saw it, while skimming my bookmarks.

[ehostler]

For any of you that want the math, to calculate the correct speedo pinion, here it is -

((63360/tire circumference)*gear ratio)/76.3 = number of speedometer gear teeth

tire circumference = tire diameter * PI(3.14159)

you should round the number of teeth to the nearest whole number. If you round down, the speedo will read a little fast. If you round up, the speed will read a little slow.

PN for speedo gears are 25389XX (where XX=number of teeth)

I set this up in a spreadsheet and I just have to punch in the rear ratio and the tire diameter. it then gives me the correct number of teeth.