Speedo tech question

[Occhty]

Setup is
Aspire motor and 5 speed trans,
Festy body and speedo,
Tires are 205/60-13

Trying to double check the speedo,
What size tire did the aspire have?
I checked it with an iPhone gps mph app
and it seems dead on, but just doesn't
Feel right.
Any way to figure this out with math to double check?

[freshtiva]

Know anyone with a GPS ?

[FestYboy]

stock aspire tire is 155-80-13. 205-60-15 is WAY to big, you need a tire between 21.7" and 22.4" tall. your 205s are 24.7" tall. you should drop to a 205-45-15 to get it right. or better yet, a 195-50-15 as that size is within .1" of factory Aspire. or even 165-60-15 (exact same size).

[GenevaDirt]

I do not think he has an Aspire.....I think he took an Aspire engine and tranny and put it in his Festy. If that is the case I believe the gearing in the Festy speedo/tach takes care of gearing differences and makes it 155/80/12's as the right size tire to make the speedo read right.

[Occhty]

Thanks!

I like the biggies, I'll just compensate 

Yes, aspire motor and tranny.

Festy body and speedo cluster.

Fetyboy I must have did an edit while you were replying
tire size I'm running is 205/60-13.

Sorry

[FestYboy]

ah that makes a BIG differance lol. that's the equivilant to 185-60-14 that i run on Scrappy, and he's dead accurate (speedo - GPS - radar signs). so just keep doing what you're doing!

[Occhty]

My thoughts are
Aspire 155/80-13, 155/25.4*0.80 = 4.88*2+13= 22.76 in
Tires 205/60-13, 205/25.4*0.60 = 4.84*2+13= 22.68 in
.76-.68= .08 inch difference.

Is this the correct formula?

[GenevaDirt]

Should be a little different....I believe I had the same size on a car once and they were off by a couple mph....

Tire Size Comparison

Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
155/80-12 4.9in 10.9in 21.8in 68.4in 927 0.0%
185/60-14 4.4in 11.4in 22.7in 71.4in 887 4.5%

[GenevaDirt]

My thoughts are
Aspire 155/80-13, 155/25.4*0.80 = 4.88*2+13= 22.76 in
Tires 205/60-13, 205/25.4*0.60 = 4.84*2+13= 22.68 in
.76-.68= .08 inch difference.

Is this the correct formula?

Tire Size Comparison

Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
155/80-12 4.9in 10.9in 21.8in 68.4in 927 0.0%
205/60-13 4.8in 11.3in 22.7in 71.3in 889 4.2%
about the same as Arty

[FestYboy]

^ yeah Willie i've seen those numbers before, and i can attest to the factory tires being too small via GPS. just bought a rust free running Auto for $30 (yes that's thirty) and on my trip to Sandman's place i observed a 4mph differance at 60 mph (GPS said 60, speedo said 64).

[GenevaDirt]

^ yeah Willie i've seen those numbers before, and i can attest to the factory tires being too small via GPS. just bought a rust free running Auto for $30 (yes that's thirty) and on my trip to Sandman's place i observed a 4mph differance at 60 mph (GPS said 60, speedo said 64).

I think we, not you but others, have discussed this and thought that maybe the year made a difference. I have had 3 90's cars with the 12's on them and had the gps in each and at any given speed the gps and speedo was exactly the same. Not sure where all the differences lie but it is only a couple miles an hour we are talking and does not really make that big of a difference. I think it best for anyone to check there own set up and use the info accordingly when calculating mpg and mph.

[Occhty]

Okay, but wouldn't the aspire trans be setup for
13s ? The only festy part would be the speedo,
Soo, would I have to calc the speedo relative to the
Tranny and then the tires?  lol

[GenevaDirt]

Okay, but wouldn't the aspire trans be setup for
13s ? The only festy part would be the speedo,
Soo, would I have to calc the speedo relative to the
Tranny and then the tires?  lol

no....the trans are the same except for actual gearing. The speedo takes care of the difference for the car. When it is in a Festy the trans will read correct with the 12's. When it is an Aspire the Aspire speedo makes up for the 13's.

[FestYboy]

yeah, speedo gear ring on the diff is the same for both cars, so go with the tires that match the cluster you are using. and test test test!