This chart shows what wheel/tire size combos will have a similar overall diameter to the stock Dakota Sport Plus wheels, 255x55 on 15" rims. Stock RT wheel/tires, 255x55 on 17" rims, are very similar and can reference this chart as their own.

I've arranged the chart shows wheel/tire sections that are within 3% accuracy with original speedo settings. 4th column is % off of the original wheel/tire combo. To give some perspective on this percentage the last column shows your actual speed when your speedo reads 60mph. This is all mathematically done so don't blame me. Variables not taken into account are tire pressure and your vehicles quarks. < 3%

15 inch
Width	Series	Overall Dia	% off	Speed @reading 60
255	65	712.5	0.000000	60.00000
265	65	725.5	1.824561	61.09474
265	60	699.0	-1.89474	58.86316
275	60	711.0	-0.21053	59.87368
285	60	723.0	1.473684	60.88421
285	55	694.5	-2.52632	58.48421
295	55	705.5	-0.98246	59.41053
305	55	716.5	0.561404	60.33684
315	55	727.5	2.105263	61.26316
315	50	696.0	-2.31579	58.61053
325	50	706.0	-0.91228	59.45263
16 inch
Width	Series	Overall Dia	% off	Speed @reading 60
255	60	712.4	-0.01404	59.99158
265	60	724.4	1.670175	61.00211
265	55	697.9	-2.04912	58.77053
275	55	708.9	-0.50526	59.69684
285	55	719.9	1.038596	60.62316
285	50	691.4	-2.96140	58.22316
295	55	730.9	2.582456	61.54947
295	50	701.4	-1.55790	59.06526
305	50	711.4	-0.15439	59.90737
315	50	721.4	1.249123	60.74947
325	50	731.4	2.652632	61.59158
325	45	698.9	-1.90877	58.85474
17 inch
Width	Series	Overall Dia	% off	Speed @reading 60
255	55	712.3	-.028070	59.98316
265	55	723.3	1.515789	60.90947
265	50	696.8	-2.20351	58.67789
275	50	706.8	-0.80000	59.52000
285	50	716.8	0.603509	60.36211
295	50	726.8	2.007018	61.20421
295	45	697.3	-2.13333	58.72000
305	45	706.3	-0.87018	59.47789
315	45	715.3	0.392982	60.23579
325	45	724.3	1.656140	60.99368
325	40	691.8	-2.90526	58.25684
18 inch
Width	Series	Overall Dia	% off	Speed @reading 60
255	50	712.2	-0.04211	59397474
265	50	722.2	1.361404	60.81684
265	45	695.7	-2.35790	58.58526
275	50	732.2	2.764912	61.65895
275	45	704.7	-1.09474	59.34316
285	45	713.7	0.168421	60.10105
295	45	722.7	1.431579	60.85895
295	40	693.2	-2.70877	58.37474
305	45	731.7	2.694737	61.61684
305	40	701.2	-1.58597	59.04842