73-87chevytrucks.com
73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: Smitty31 on October 13, 2017, 06:35:34 PM
-
My truck is a 1986 custom deluxe 4.3L 4 BBL 400 turbo trans. 2wd. My speedometer gear is a medium green i believe it had 42 teeth on it. Because of tire size my speedometer is about 5 mph off. So when it shows say 45 its actually running 50. My question is there a gear that will correct that ? I know you can get different gears just not sure which one.
-
What's the tire size and axle ratio?
-
275/60 15 in. i dont know about the axle ratio and don't know how to find out.
-
Look at the SPID label in the glovebox.
-
2.75 ratio Is that a good ratio for gas mileage can you tell me how it stacks up against other ratios in rear-ins? I know less than than nothing about rear-ins lol.
-
Is that the info you was looking for Bd
-
I misread the SPID label the other day the gear ratio is not 2.75 but is 2.73 sorry guys for the wrong info. Also it saids its a 1Gu2 model if that helps at all. So again tire size 275/60, ratio 2.73 and I have the medium green speedometer gear. Presently my speedometer is off 5 mph at 45mph it is really 50mph due to tire size. So I was wondering if there was a different speedometer gear that would correct this.
-
This is over my head big time any help would be great guys.
-
here is a chart based on some know parameters
-
I've been looking at that chart for several days off-and-on in spare time....
(http://forum.73-87chevytrucks.com/smforum/index.php?action=dlattach;topic=35750.0;attach=37734;image)
The gear combination you currently have in the truck suggests the original tire size was P195/75R15 (26.5" diameter). The P275/60R15 tire you presently are running has a diameter of 28". Using the gear combination specified for a P225/75R15 (28.5" diameter) would shift the speedometer error to ~1.5 MPH fast, instead of the current 5 MPH slow. Or, take the vehicle to a local speedometer shop for retrofit of a ratio adapter that will calibrate your speedometer exactly.
-
I may have misspoke it is running 5 mph faster than it shows according to the gps so at 50 your really going 55
-
Correct. Your speedometer is indicating 5 MPH slow @ 55 MPH actual, running a tire that is 1.5" oversized for the original factory speedometer calibration. So, if you change the factory calibration to a tire diameter that is 28.5" tall compared to the 28" tall P275/60R15, the speedometer will be slightly off in the opposite direction using the new gearing. This is because, after recalibration, the P275/60R15 will be 0.5" undersized for the new speedometer gear combination. The recalibrated speedometer reading should be ~1.5 MPH too fast, indicating 56.5 MPH @ 55 MPH actual.