Posted by Charlie on Wednesday, November 22, 2000 at 04:05:41:
In Reply to: Large diameter tires...is this the best way to go? posted by Roy Sanders on Tuesday, November 21, 2000 at 07:03:49:
Certainly even a 230 flathead if in good condition will not have any problem with any "reasonable" sized tire, anything that'll fit on the stock rims. We've all seen M37 with flatheads on DC3 tires etc., usually permanently in low range as tundra buggies. For part-time highway use, consider that tires 10% taller (35" NDs going to 38.5" Boggers or Michelins) WILL cause a 9% loss in effective rear wheel torque. You will feel it a bit with a flathead. Probably it won't be noticeable with a V8. The issues with the V8s and diesels (being repetitive) are reducing rpms in the highest gear sufficiently so that the vehicle can be driven at 65 mph in an rpm range that is economical and not harmful to the engine. That means about 24-2600 for all the motors, roughly. That means a "step-down" of about 35%, or an overall ratio of about 0.65.
The tallest tires that can be gotten on the 16" rims are 38.7". That gets you 0.91.
4.89s give a multiplier of 0.84, for a total now of 0.76. We're still a bit high, we'll be turning 2850 rpm at 65 mph. Looks we'll need an overdrive of some sort also, with a ratio anywhere from 0.85 down to 0.73, depending on whatever. In marginal cases like for instance certain diesels, all the changes may be too much. A less powerful gas motor like a 4.3L V6 is capable of turning over 4000 rpm and it might be happier with a 5 spd and big tires, but 5.83s. And so on.
JUST the big tires will only be a big improvement for flatheads. Naturally, if you put a 318 or 360 in and keep 5.83s because application will be mainly offroad, big tires will drop road rpms a bit and probably more important tremendously help offroad performance.
Charlie