Reference info for speedometer gear sizing

[Dakota]

Just posting some reference links that I used for my car:

tire size calculator (this info is needed for the next step):   https://tiresize.com/calculator/

speedometer gear selection: http://www.mymopar.com/speedometer.htm

[RUNCHARGER]

Ha, ha: I always use about a 27 inch tire and I always use a 35 for 3.54, 41 for 4.10 and so on. It works great.

[nsmall]

Thanks for the info.  I changed my gears to 355 with a 28 inch tire so I chose the yellow.  Its been installed.

  HOWEVER I changed my tire to a 26.7 so any guess how "off" I am on my MPH?

Thanks, I dont have a GPS so I guess I could drive next to someone to try to figure out how "off" I am.

[73440]

After installing a new speedo cable, I checked it with a phone app called Ulysse Speedometer .

[Dakota]

Thanks for the info.  I changed my gears to 355 with a 28 inch tire so I chose the yellow.  Its been installed.

  HOWEVER I changed my tire to a 26.7 so any guess how "off" I am on my MPH?

Thanks, I dont have a GPS so I guess I could drive next to someone to try to figure out how "off" I am.

I really shouldn't do math when operating on just one cup of coffee, but here goes anyways:

Moving from a 28.0 inch diameter tire to a 26.7 inch diameter tire with the same speedo pinion gear and differential gearing would cause your actual speed to be about 5% lower than your prior indicated speed, so if your indicated speed was accurate at 60 mph with 28 inch tires, you'd be travelling about 57 mph with the smaller tires.     

I don't think it's worth changing your speedo pinion gear to correct this situation.   With the same tire size and differential ratio, moving from a yellow (33 tooth) speedo pinion gear to a red (36 tooth) would mean your actual speed would be about 9% faster than your indicated speed (indicated speed of 60 mph, actual speed of almost 66 mph).   It's likely you're going to be a little "wrong" with a mechanical speedo no matter what tire and speedo gear combination you run.  In addition, my understanding is that legally you're better off having your speedometer indicate that you're traveling faster than you really are rather than the other way around (this probably has something to do with keeping the odometer reading from being understated).

[RUNCHARGER]

Agreed: That's why I do the 35=3.54 thing. Even new car speedos aren't accurate all the way up. If one is right at say 60MPH chances are it is wrong ast 30MPH. When you look at the percentage of say 2 teeth it isn't very much. I like them to be within 3 or so MPH at 60MPH if possible.

[nsmall]

Thanks Dakota, I appreciate the math.  I was just heading in to make a cup of coffee myself.

Glad to hear I am close as I spent $280ish to have my speedo rebuilt.  I thought it was reading fast.  I will confirm by driving nexAT

[73440]

so if going from 28 dia to 26.7 dia and driving the same speed it should take you longer time to go the same distance , is that right ?

[Dakota]

so if going from 28 dia to 26.7 dia and driving the same speed it should take you longer time to go the same distance , is that right ?

If by "speed" you mean what's showing on the speedometer and the only thing that changed was your tire size, you're correct.   For a given indicated speed on the speedometer while traveling in a straight line, the drive shaft and axle(s) are rotating at exactly the same rate regardless of the tire size. A 28 inch tire has to rotate about 721 times to travel 1 mile; a 26.7 inch tire has to rotate about 756 times to travel a mile.  With the smaller tires installed, your actual speed would be less than the speedometer reading when you had the larger tires on, so it will take you a little longer to travel the same distance.   

[73440]

Thanks @Dakota , sometimes the more I think about things the less sure I am of it.

[CudaMoparRay]

Great information for those wishing to remain original in every aspect including that which is not seen.
They offer gps that can run the original speedometer gear and of course also aftermarket gps speedometers which I installed eliminating,at least for me,  never again worrying about changing to any size tire ,wheel ,rear gear combination possible.