Joe Flanagan Posted August 11, 2015 Report Posted August 11, 2015 My speedometer reads 7 to 9 MPH faster than my actual speed. Someone suggested to me that this might be because the tires I have on my car are too small. They are P20575R15. I will be getting new tires for the front and wanted to know what size I ought to get. I won't be changing out the rears since they are in good condition but I figure once I do have to change them out, I'll get the correct size on the back as well. Quote
Don Coatney Posted August 11, 2015 Report Posted August 11, 2015 Need more information. 7-9 MPH faster at what speed? Current tire diameter? Differential ratio? You can use the chart below and the linked speed calculator to figure it out. http://www.ply33.com/Misc/speed Description Tread width Overall diameter Rev / mile Rim Width 135-75R15 3.5 23.0 878 145-75R15 3.8 23.6 856 155-75R15 4.0 24.2 835 165-75R15 4.3 24.7 815 185-75R15 4.8 25.9 778 195-75R15 5.1 26.5 761 205-75R15 5.3 27.1 744 215-75R15 5.6 27.7 728 225-75R15 5.9 28.3 713 235-75R15 6.1 28.9 Quote
desoto1939 Posted August 11, 2015 Report Posted August 11, 2015 Charge up your GPS and then use the battery that is included in the GPS. Set it up in your old car and then go for a ride. The GPS willthen tell you the speed limit for your road and also the current speed that you are doing. This way you can check the accuracy of the speedometer at different speeds based on the reading from your speedometer. This will provide a more accurate reading based on real driving conditions Rich Hartung Quote
Joe Flanagan Posted August 11, 2015 Author Report Posted August 11, 2015 Let me have a look at the calculator Don posted. Thanks, Don. I used a GPS device while driving down the road at what my speedometer said was 50 MPH. The GPS indicated I was actually only going 41 to 43 (the readout kept fluctuating). My wheels are 15 inches in diameter, if that's worth anything. I will check the calculator and see what I get, though. Thanks. Quote
Plymouthy Adams Posted August 11, 2015 Report Posted August 11, 2015 finding a very suitable tire in 15 that will give you the height you need may be a challenge in modern radial as the aspect ratio on modern tires even at the 75 series is significantly less than what was stock in the day.. modern rims of 16 and 17 is available but not without some backset issues...you can look to a modern 15 passenger van, Ford Econoline is one, (sm. buss) tire...they have a better side profile.. there are many various speedometer correctors out there that will allow you to adjust for the MPH difference if you have the size tire and look you are satisfied with... Quote
TodFitch Posted August 11, 2015 Report Posted August 11, 2015 You might want to check your odometer. . . If the tire size and/or rear end ratio are different than built then both the odometer and speedometer should be off by the same amount. If the odometer is correct then the speedo could just be miscalibrated. 1 Quote
Frank Elder Posted August 11, 2015 Report Posted August 11, 2015 My speedometer reads 7 to 9 MPH faster than my actual speed. Someone suggested to me that this might be because the tires I have on my car are too small. They are P20575R15. I will be getting new tires for the front and wanted to know what size I ought to get. I won't be changing out the rears since they are in good condition but I figure once I do have to change them out, I'll get the correct size on the back as well. So put your old incorrect rear tires on the front...........and put your new correct tires on the back so your speedo will read correctly..... 1 Quote
Joe Flanagan Posted August 11, 2015 Author Report Posted August 11, 2015 RIght, I was thinking the same thing. I just need to find out what the correct size is. According to my manual, it was originally 6.70 x 15, whatever that means. Also, to answer an earlier question, I'm using the original rear end on the car. Quote
greg g Posted August 11, 2015 Report Posted August 11, 2015 (edited) put the rears on the front and get you some 225 75's for the rear. They're what I run, my speedo at 55 is 53 on GPS 65=62. Close as you can get when subbing out modern tires for the old sizes I think. I run the 205 on the front also. Edited August 11, 2015 by greg g 1 Quote
Frank Elder Posted August 11, 2015 Report Posted August 11, 2015 put the rears on the front and get you some 225 75's for the rear. They're what I run, my speedo at 55 is 53 on GPS 65=62. Close as you can get when subbing out modern tires for the old sizes I think. I run the 205 on the front also. Rubber rake! Quote
deathbound Posted August 11, 2015 Report Posted August 11, 2015 If the original size tire for your car was 6.70-15, use the following link and attached chart (2 different sources) to get a radial tire size close, maybe not dead nuts, but close & your speedo should be close to accurate. According to your original post, your tire existing tire size 205/75R15 should be a very close match. Maybe your speedo needs calibration? http://www.teufert.net/speedo/tires.htm Quote
Frank Elder Posted August 12, 2015 Report Posted August 12, 2015 (edited) Joe, you don't have a D-24 but I'm pretty sure this applies to your car too. Depending on your rear gear ratio, is it stock? Or did you swap it out? Anyway say you have x amount of rear gear that means your speedometer pinion gear...goes in your tranny.....should have x amount of teeth. Mine is a 3.90 so my pinion has 17 teeth. A 4.10 uses 18 teeth, and a 3.73 uses 16 teeth...etc. Without the correct pinion gear your speedo will never be correct no matter what size tires you buy. Edited August 12, 2015 by Frank Elder Quote
greg g Posted August 12, 2015 Report Posted August 12, 2015 not a noticeable difference appearance wise. Quote
plyroadking Posted August 12, 2015 Report Posted August 12, 2015 Is there a potential that your speedometer drive gear was changed at some point? It's been my experience that a single tooth difference adds or subtracts around 7-10 mph to the speedometer. I've never checked the odometer to see if my speedometer was out of calibration I'm sure someone here can give the factory specs for drive gears and calibration. I always seem to 17 and 18 tooth gears, I'd like to find a 15 tooth as my speedometer currently reads 9ish mph slow with a 16 tooth. Quote
Frank Elder Posted August 12, 2015 Report Posted August 12, 2015 Is there a potential that your speedometer drive gear was changed at some point? It's been my experience that a single tooth difference adds or subtracts around 7-10 mph to the speedometer. I've never checked the odometer to see if my speedometer was out of calibration I'm sure someone here can give the factory specs for drive gears and calibration. I always seem to 17 and 18 tooth gears, I'd like to find a 15 tooth as my speedometer currently reads 9ish mph slow with a 16 tooth. Read post 12 Quote
plyroadking Posted August 12, 2015 Report Posted August 12, 2015 Another instance of my smart phone failing to keep up it's end of the bargain.... Quote
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