Hello, I'm trying to find out which aspect ratio was on the maverick as standard. It's important for me because it but for an incorrect value is a deviation due to the wrong speedometer circumference. Regards Nils sorry as always google translate
My first Comet still had an original tire still in the trunk and it was comically small. It was a numeric size too .... 6.45-14, if I am recalling this correctly. At the time, translating that into a letter size would have made it a B78-14. That size converts to a 175/75/14 or a 165/80/14. Are you concerned with a stock appearance .... restoring it to look original? Or are you more worried about the speedometer calibration? Tire height has the most to do with how accurate the speedo will be and you have a fair amount of range there. These cars also came with D70-14 tires as an option... 185/70/14 is close. I am running a much bigger back tire 265/50/15, and found that the speedometer was roughly 7-8% slower than actual speed with the stock 3:00 gear. Still not much difference ... about 5 mph at 60. If you want to run a bigger tire, you will get a little better gas mileage, and you can correct the speedometer by changing the drive gear. This is located where the speedo cable goes into the transmission. Different tooth count gears are available.
Hello Jeff, thanks a lot! Yes it is about the speedometer calibration. I would like to drive 215 R15 with white letter. Aspect ratio 60, 65, 70 or 75. I can not decide. Regards Nils
measure the height of a stock tire...then find a replacement tire the same height but a different width if you like...
I am assuming you guys are referring to this statement ...."That size converts to a 175/75/14 or a 165/80/14." Those sizes are the later P-metric sizes that didn't come about until the early 1980s. These are sizes that the original numeric size tires or the letter sizes would convert to in modern size tires, working with two aspect ratios, 75 and 80. They would be the closest. IIRC, numeric size tires were all assuming an aspect ratio of 80. The original tire I found in the trunk as a spare in my first Comet (it was a 71) was a numeric 6.45-14. It had relatively straight tread and "pie crust" cleats around the outer sidewall. Looked a bit like a small tractor tire. Letter size tires were just getting more popular in the early 70s. Seemed like the bigger cars all had them first, and our humble little economy cars were using up what was left over before switching. They were a bit of an odd size. Realistically, there is a pretty wide range of tire size that can be used, and the speedo will be close to right, before you need to jump to another gear in the trans to recalibrate it.
Per a speedo cal shop I used after getting a ticket, each tooth removed on driven gear will increase reading approx 3mph... Sooo if this is correct, loosing two teeth(say going from a 19 to 17 tooth gear) should put yours just about right... AFAIK a 16 tooth gear is as short as they come(21 largest, though there is a 23 tooth aftermarket [actually a Chrysler gear]), so if you already have a 17, it won't be totally correctable...
I appreciate the info, and I am sure it will help others. However, I have now switched to a 3:55 gear and have the matching trans gear for a Mustang II that I got the pumpkin out of. That likely will not be the correct one to use with my combo, especially due to tire size ... but it may make the stock one that is still in there the proper one. I'll have to try it out when the car is running again, and see how far off it is. BTW, in between the 3:00 and the 3:55, I had a 4:62 gear in it. That meant that 45 mph read 70 or so on the speedo .... It was 5 mpg big stupid fun ( for a while .... then it got old). I was bracket racing the Comet a lot then, and it was an 1/8th mile track. The car was out of breath about 100 feet from the finish line ... but, it did make it super-consistent. I will bet that with the 3:55, the car will ET close to the same, but MPH better than the big gear. I know I will want to drive it a lot more.
Sounds like fun, I beat on a 5.0 T-Bird at a 1/8th mile track... Also I keep a variety of gears around to try, as I despise having a speedo that's way off cal... Depending on the number of teeth on the drive gear(one on output shaft)will determine just how much change each tooth in the driven gear will make, of course tire size can play a large part as well... A large tire using the 3.55 gear may be close using the orig gear...
According to the Tire and Rim Assoc. a 6.00x13, which translates to A78-13 (base tire size in 70) was 24.16 inch's tall. The 6.45x14 (which translates to B78) was 24.7 inch's tall. The C78-14 was 25.24 inch's tall. The D70-14 were 25.32 inch's tall. In radials, the BR78-14 was 24.54 inch's tall. The CR78-14 was 25.00 inch's tall. The DR78-14 was 25.4 inch's tall. And the DR70-14's were 25.24 inch's tall. They also said the manufacturers were allowed a 7% variation in these dimensions due to manufacturing tolerances.
