trading up to 13" tires

[milehighbear]

Question: If i go to 13" rims and wheels, how does that effect the reading of speedometer

[worfdog]

I put 155-80-13's on mine and by the GPS , i'm 1 MPH off on my speedo.

[mattswabb]

try this calculator

http://www.1010tires.com/TireSizeCalculator.asp?action=submit

[dahodges]

Putting 155r80/13s gives me a 5mph error per several Radars.

David

[GenevaDirt]

well according to the calculator that Matt posted....

Actual Speed:60 mph 100 km/h , the differance in the two Speedometer1: 57.3 95.6 Speedometer Difference: 4.596% too slow Diameter Difference: -Diameter Difference: 4.4%

[FB71]

^^^you've got something else wrong. The rolling diameter is only 1/2 inch different (vs 155R12), which should yield only a 2-3 mph error at 60 mph. Considering the Festy (as well as most other vehicles) has a built in error of positive 2-3 mph (reads 2-3 mph faster than actual) at 60 mph, the 155/80/13 should correct the speedo to within 1-2 mph. If you're getting nailed for 5mph over you either have a significant error in the speedo head, or the tires are mis-manucatured/mislabeled size.

[batstiva]

It's a bit of a moot point, but Festivas originally came with 145/r12s, so everyone rolling on 155/r12s would already be off on both the speed and the distance. I believe when I had my 14s on the Batstiva, that calculator said I was off about 6 or 7 mph. Something to think about when I rolled past the cops doing "60 in a 55"

[GenevaDirt]

I have the 155's on my car and according to my GPS...it is exact on...I have watched it since I owned the GPS and if I gain or slow by 1 mile per hour....so does the GPS.

Just my observation.

[FB71]

It's a bit of a moot point, but Festivas originally came with 145/r12s, so everyone rolling on 155/r12s would already be off on both the speed and the distance. I believe when I had my 14s on the Batstiva, that calculator said I was off about 6 or 7 mph. Something to think about when I rolled past the cops doing "60 in a 55"

They also had optional 165/70/R12s

[batstiva]

True, but if I remember correctly the sticker inside the door on mine called for 145s, which, for the record, were never on the car the whole time I owned it.