Optimum RPM at Highway Speeds?

**Rande** · 02-11-2009, 09:01 PM

Haven't found any info on this subject.

What is the optimum (or thereabouts) rpm range for a 263 at highway speeds? It'll be going into my '37 Chevy pickup. Should be much lighter than an old Buick.

I know this is going to be a guess or (hopefully) an educated guess. I am using a T5 with a .72 overdrive. I am also using 32" tall tires. I need to make a decision about axle gears. To that end, I need a target rpm for the engine. For now, the engine will be run mostly stock, just a homemade header, low restriction exhaust and likely a carb upgrade to something more modern (leaning towards an Autolite 2100) and perhaps an electronic ignition of some sort.

**eddie1948** · 02-11-2009, 09:35 PM

where did u get the adapter for the t5 I would use 3.73 at 75 u should be at under 3000 rpm

**Spud** · 02-11-2009, 11:15 PM

I would like to know how you got the t5 on the back of the eight also.

**eddie1948** · 02-12-2009, 09:45 AM

hey spud how u been? I know i shouldn't open another topic but I got to ask Ive been talking to Jim Carmichael he states u could bolt on a 3rd member of a 62 on up into a 55 rear end sounds good but what about the 48 super can u do the same

**Jyrki** · 02-12-2009, 09:52 AM

Rande, shoot for 2000 RPM at 60 mph

**eddie1948** · 02-12-2009, 09:59 AM

jyrky is right that will keep your engine healthy those old gears put some much strain on your motor I know im hurting and pissing people off on the hwy jyrky can u help me with that answer above

**Spud** · 02-12-2009, 12:34 PM

Originally Posted by eddie1948

hey spud how u been? I know i shouldn't open another topic but I got to ask Ive been talking to Jim Carmichael he states u could bolt on a 3rd member of a 62 on up into a 55 rear end sounds good but what about the 48 super can u do the same

I got a hollander interchange book, it says 38 -48 interchange, I also got two articles that show how to put a 55 3rd member in a 38 buick, they wanted to keep the torque tube, so I would think if that would work a newer one would to. You have to machine the flange down on the 3rd member so the axles line up, change side and spider gears so you can keep your axles they interchange. If I can find out how to get the article's scanned and printed I'll do it. Sorry it is so long, there is more let me know.

**Rande** · 02-12-2009, 07:27 PM

Thanks guys. I'm looking to keep the engine in a good operating rpm range. Too low and it lugs and gas mleage drops. Too high, gas mileage suffers and engine gets stressed.

Over the years, I've read 2200 rpm at 70 is about average but thats always been related to V8 engines. I have no idea where these old Buicks like to run.

Being the rear gear set is an expensive purchase and install, I was hoping to have more to go on than my own guesses.

I have a calculator here and will start plugging in numbers and use the 2200rpm at 70 as my target.

**BigRivy** · 02-13-2009, 01:36 AM

The standard rule for gearing is to put it within 10% of your torque peak rpm. If your engine is stock use the factory rating. If it says x amount of torque at say 2200 rpm, make sure you gear it to run within 220 rpm at what you think will be your most used cruise speed. So 2000 to 2400 is what you would aim for for a 2200 rpm torque peak. Stick to this and you should have the best combination of mileage and performance. Adding better intake and exhaust and ignition will add power, but usually don't move the torque peak very much. Changing cams is what moves it the most. More duration almost always moves the torque peak higher in the rpm range. So in this application I'd look up whatever the factory torque rating was for your year of straight 8 and go from there. If you do install a hotter cam your cam grinder should be able to recommend proper gearing changes to match the cam profile.

**39CENT** · 02-13-2009, 10:19 AM

Originally Posted by Spud

I got a hollander interchange book, it says 38 -48 interchange, I also got two articles that show how to put a 55 3rd member in a 38 buick, they wanted to keep the torque tube, so I would think if that would work a newer one would to. You have to machine the flange down on the 3rd member so the axles line up, change side and spider gears so you can keep your axles they interchange. If I can find out how to get the article's scanned and printed I'll do it. Sorry it is so long, there is more let me know.

yes 40 t0 55 buicks will interchange as is. 38 and 39 will if the diff. face is milled about 1/8 ", to line up to the axle centerline. and open driv diff. from 61 to 64 or so, is supposed to also fit. good info