The question of tire sizes and speedometer drive gears comes up every now & then on the Jensen-cars email list. Here's an article by our esteemed David Crowne from April 1999: ----- Ed Campbell is right in suggesting that you should consult your local Mopar dealer before you rush into changing your speedometer drive pinion. The factory supplied pinions with 31 teeth in cars with 3.07:1 rear axle ratios (most 440s from 1972 onwards) and 29 teeth in cars with 2.88:1 rear axle ratios (early Interceptor IIIs with 383s and almost all Interceptor Is and IIs). If your speedo drive pinion has 26 teeth, I'm not surprised that your speedometer is inaccurate. The 29 and 31 tooth pinions should produce correct speed (and distance) readings with 205/70 15 tires, since those were the size the cars came with from the factory. A moment with my pocket calculator tells me that the diameter of that--the standard tire--is 26.426". Bill Spohn surmises that his 235/60 15 tires are about the same diameter, and he is right: 26.228". The nice Pirelli p4000 215/70 15 tires that Bob Adams and I just bought have a diameter of 26.976"--a little bigger, so our speedos will read a tad slow. Your 225/70 15 tires are 27.528" in diameter, so even with the correct drive pinion your speedo will read slower than mine, but would probably be just about on the button with a 30-tooth pinion. Anyone who cares to spend a few minutes with HIS (or her) pocket calculator can figure the percentage of difference on the circumference of these tires and figure that in relation to the percentage of difference, tooth by tooth, on the speedo drive pinions. For me, however, it's been a longish day and I'm going to bed. Before I turn in, however, let me assure anyone who is interested that speedometer drive pinions in the whole range of teeth suitable for Interceptors are still readily available at Mopar dealers for less than $10. If anyone in Britain or in Oz or elsewhere needs one, let me know and I'll see if I can work out the logistics of getting it to you. Changing the thing is a five-minute job if you've done it before, ten if you haven't. David Crowne in San Diego 71 Interceptor III 133/5474 ----- In addition to the Mopar dealers, K&D Enterpises (www.interceptor.org) also carry the speedo drive gears. There are several web-based calculators for tire sizes, here are some: http://www.powerdog.com/tiresize.cgi http://www.miata.net/garage/tirecalc.html Lastly, in an August 1999 message to the list, Craig Hanson wrote some general information about tire sizes: ----- Since it keeps coming up here's some basic tire and wheel stuff: Wheels: Widths are measured bead seat to bead seat, not flange to flange. Same for diameter. A wheel is considered to have positive offset if the backspace is greater than the centerline dimension of the wheel. Negative offset if less than half of the wheel width. (pretty easy so far folks!) A wheel is considered to be hubcentric if the center bore precisely fits the hub of the axle, therefore carrying the load to the center rather than on the wheel studs. (most aftermarket wheels are not btw) Bolt circles for 5 bolt wheels are measured center of stud to center of stud in a star pattern, not to the adjacent stud. 4 bolt wheels are measured to the furthest stud centerline. 4 on 4 is NOT 4 on 100m! Yes it will go on but yes it will fall off!! Don't do it! Mag style lug nuts should never be used in place of conical seat lug nuts or vis versa. Wheel runout should be no more than 1/8" radial or lateral. Aluminum wheels generally are lighter (not always) than steel and dissipate brake heat better. Aluminum wheels can be as malleable as steel or as fragile as an egg depending upon the tensile strength of the material. A well constructed aluminum wheel will carry the same loads as a steel wheel. Tires Modern tire sizing is metric. For example an unladed inflated 215-60R15 is 215mm shoulder to shoulder with a sidewall height (aspect ratio) that is 60% of the width of the tire. Tread wear index shows the % under or over an expected tire life of 30,000 miles, i.e.. a tire with an index of 180 has an expected life of 54,000 miles. It is not recommended to fit a tire with a speed rating more than one grade lower than the car's factory spec. Any attorney would make the case that any downgrading of the speed rating of the new tire would be libelous. Reality is that high speed rated tires are as much of a marketing gimmick as they are a necessity on some cars. Other cars are totally dependent on a particular size and construction. Your car may not care. But VR rated (150 mph) tires on a Land Rover??? You couldn't get one going that fast unless you threw it out of a plane! Even though you may not feel you need a 150 mph tire, (99% of us don't) the suspension and brakes of the car might be designed with the assumption that its there. Better to be safe and stay with the factory spec as a minimum. A multi ply rating does NOT necessarily mean that the tire has that many plies, just that the manufacturer has tested the tire to that load capacity. A tire's rolling diameter can vary as much as 1" in it's lifespan. (1/2" tread depth) Hope these help someone! Craig -----